site investigation manual – 2013 foreword

Site Investigation Manual – 2013 Foreword

Ethiopian Roads Authority Page i

FOREWORD The road network in Ethiopia provides the dominant mode of freight and passenger transport and thus plays a vital role in the economy of the country. The network comprises a huge national asset that requires adherence to appropriate standards for design, construction and maintenance in order to provide a high level of service. As the length of the road network is increasing, appropriate choice of methods to preserve this investment becomes increasingly important.

In 2002, the Ethiopian Roads Authority (ERA) first brought out road design manuals to provide a standardized approach for the design, construction and maintenance of roads in the country. Due to technological development and change, these manuals require periodic updating. This current version of the manual has particular reference to the prevailing conditions in Ethiopia and reflects the experience gained through activities within the road sector during the last 10 years. Completion of the review and updating of the manuals was undertaken in close consultation with the federal and regional roads authorities and the stakeholders in the road sector including the contracting and consulting industry.

Most importantly, in supporting the preparation of the documents, a series of thematic peer review panels were established that comprised local experts from the public and private sector who provided guidance and review for the project team. This Manual supersedes the Site Investigation Manual part of the ERA Design Manuals of 2002. The procedures set out shall be adhered to unless otherwise directed by the concerned bodies within ERA. However, I should emphasize that careful consideration to sound engineering practice shall be observed in the use of the manual, and under no circumstances shall the manual waive professional judgment in applied engineering. For simplification in reference this manual may be cited as ERA’s Site Investigation Manual - 2013.

On behalf of the Ethiopian Roads Authority I would like to take this opportunity to thank DFID, Crown Agents and the AFCAP team for their cooperation, contribution and support in the development of the manual and supporting documents for Ethiopia. I would also like to extend my gratitude and appreciation to all of the industry stakeholders and participants who contributed their time, knowledge and effort during the development of the documents. Special thanks are extended to the members of the various Peer Review Panels whose active support and involvement guided the authors of the manual and the process. It is my sincere hope that this manual will provide all users with both a standard reference and a ready source of good practice for the design of roads, and will assist in a cost effective operation, and environmentally sustainable development of our road network.

I look forward to the practices contained in this manual being quickly adopted into our operations, thereby making a sustainable contribution to the improved infrastructure of our country. Comments and suggestions on all aspects from any concerned body, group or individual as feedback during its implementation is expected and will be highly appreciated.

Addis Ababa, 2013 Zaid Wolde Gebriel Director General, Ethiopian Roads Authority

Preface Site Investigation Manual – 2013

Page ii Ethiopian Roads Authority

PREFACE The Ethiopian Roads Authority is the custodian of the series of technical manuals, standard specifications and bidding documents that are written for the practicing engineer in Ethiopia. The series describe current and recommended practice and set out the national standards for roads and bridges. They are based on national experience and international practice and are approved by the Director General of the Ethiopian Roads Authority. This Site Investigation Manual -2013 forms part of the Ethiopian Roads Authority series of Road and Bridge Design documents. The complete series of documents, covering all roads and bridges in Ethiopia, are contained within the series: 1. Geometric Design Manual 2. Site Investigation Manual 3. Geotechnical Design Manual 4. Route Selection Manual 5. Pavement Design Manual Volume I Flexible Pavements 6. Pavement Design Manual Volume II Rigid Pavements 7. Pavement Rehabilitation and Asphalt Overlay Design Manual 8. Drainage Design Manual 9. Bridge Design Manual 10. Low Volume Roads Design Manual 11. Standard Environmental Procedures Manual 12. Standard Technical Specifications 13. Standard Detailed Drawings 14. Standard Bidding Documents for Road Work Contracts – A series of Bidding

Documents covering a full range from large scale projects unlimited in value to minor works with an upper threshold of $300,000. The higher level documents have both Local Competitive Bidding and International Competitive Bidding versions

These documents are available to registered users through the ERA website: www.era.gov.et Manual Updates Significant changes to criteria, procedures or any other relevant issues related to new policies or revised laws of the land or that is mandated by the relevant Federal Government Ministry or Agency should be incorporated into the manual from their date of effectiveness. Other minor changes that will not significantly affect the whole nature of the manual may be accumulated and made periodically. When changes are made and approved, new page(s) incorporating the revision, together with the revision date, will be issued and inserted into the relevant chapter. All suggestions to improve the draft manual should be made in accordance with the following procedures:

Site Investigation Manual – 2013 Preface

Ethiopian Roads Authority Page iii

1. Users of the manual must register on the ERA website: www.era.gov.et 2. Proposed changes should be outlined on the Manual Change Form and forwarded with

a covering letter of its need and purpose to the Director General of the Ethiopian Roads Authority.

3. Agreed changes will be approved by the Director General of the Ethiopian Roads Authority on recommendation from the Deputy Director General (Engineering Operations).

4. The release date will be notified to all registered users and authorities. Addis Ababa, 2013 Zaid Wolde Gebriel

Director General, Ethiopian Roads Authority

Preface Site Investigation Manual – 2013

Page iv Ethiopian Roads Authority

ETHIOPIAN ROADS AUTHORITY CHANGE CONTROL DESIGN MANUAL

MANUAL CHANGE This area to be completed by the ERA Director of Quality Assurance

Manual Title:____________________________

_______________________________________

CHANGE NO._____________

(SECTION NO. CHANGE NO. _________________________

Section Table Figure Page

Explanation Suggested Modification

Submitted by:

Name:____________________________________Designation:______________________________

Company/Organisation Address

____________________________________________________________________

_______________________________________email:__________________________Date:________

Manual Change Action

Authority Date Signature Recommended Action Approval

Registration

Director Quality Assurance

Deputy Director General Eng.Ops

Approval / Provisional Approval / Rejection of Change: Director General ERA:__________________________________ Date: __________________

Site Investigation Manual – 2013 Acknowledgements

Ethiopian Roads Authority Page v

ACKNOWLEDGEMENTS The Ethiopian Roads Authority (ERA) wishes to thank the UK Government’s Department for International Development (DFID) through their Africa Community Access Programme (AFCAP) for their support in developing this Site Investigation Manual – 2013. The manual will be used by all authorities and organisations responsible for the provision of roads in Ethiopia. This Site Investigation Manual-2013 is based on a review of local and international procedures and is based largely on ERA’s Site Investigation Manual – 2002 but includes improvements and extensions to deal with topics that were not included in the earlier manual. This manual also contains relevant parts of ERA’s Low Volume Roads Design Manual.

From the outset, the approach to the development of the manual was to include all sectors and stakeholders in Ethiopia. The input from the international team of experts was supplemented by our own extensive local experience and expertise. Local knowledge and experience was shared through review workshops to discuss and debate the contents of the draft manual. ERA wishes to thank all the individuals who gave their time to attend the workshops and provide valuable inputs to the compilation of the manual.

In addition to the workshops, Peer Review Groups comprising specialists drawn from within the local industry were established to provide advice and comments in their respective areas of expertise. The contribution of the Peer Group participants is gratefully acknowledged.

The final review and acceptance of the document was undertaken by an Executive Review Group. Special thanks are given to this group for their assistance in reviewing the final draft of the document. Finally, ERA would like to thank Crown Agents for their overall management of the project As with the other manuals of this series, the intent was, where possible, and in the interests of uniformity, to use those tests and specifications included in the AASHTO and/or ASTM Materials references. Where no such reference exists for tests and specifications mentioned in this document, other references are used.

Executive Review Group

No. Name Organization

1 Amare Assefa, Ato Ethiopian Roads Authority

2 Daniel Nebro, Ato Ethiopian Roads Authority

Acknowledgements Site Investigation Manual – 2013

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List of Persons Contributing to Peer Group Review

No. Name Organization

1 Abebe Asefa, Ato Ethiopian Roads Authority

2 Alemayehu Ayele, Ato Ethiopian Roads Authority

3 Asnake Haile, Ato OMEGA Consulting Engineers

4 Asrat Sewit, Ato Saba Engineering

5 Colin Gourley, Dr. ERA/DRID

6 Daniel Nebro, Ato Ethiopian Roads Authority

7 Efrem Degefu, Ato BEACON Consulting Engineers

8 Fikert Arega, W/ro Ethiopian Roads Authority

9 Muse Belew, Ato Ethiopian Roads Authority

10 Shimelis Tesfaye, Ato Spice Consult

11 Tewodros Alene, Ato Ethiopian Roads Authority

12 Zerihun Nuru, Ato Gondwana Engineering

Project Team

No. Name Organization Role

1 Bekele Negussie ERA AFCAP Coordinator for Ethiopia

2 Abdo Mohammed ERA Project Coordinator

3 Frew Bekele ERA Project Coordinator

4 Lulseged Ayalew AFCAP/Crown Agents Lead Author

5 Robert Geddes AFCAP/Crown Agents Technical Manager

6 Les Sampson AFCAP/Crown Agents Technical Director Addis Ababa, 2013

Zaid Wolde Gebriel

Director General, Ethiopian Roads Authority

Site Investigation Manual – 2013 Table of Contents

Ethiopian Roads Authority Page vii

TABLE OF CONTENTS

FOREWORD........................................................................................................................ I PREFACE .......................................................................................................................... II

ACKNOWLEDGEMENTS ..................................................................................................... V

TABLE OF CONTENTS ..................................................................................................... VII

LIST OF FIGURES ............................................................................................................. XI

LIST OF TABLES.............................................................................................................. XII

GLOSSARY OF TERMS .................................................................................................... XIV

ABBREVIATIONS AND ACRONYMS................................................................................XVIII

1 INTRODUCTION ...................................................................................................... 1-1

1.1 Background and Context .................................................................................. 1-1 1.2 Objectives ........................................................................................................ 1-2 1.3 Scope ............................................................................................................... 1-3 1.4 Stages of Site Investigation ............................................................................... 1-3 1.5 Approach .......................................................................................................... 1-4 1.6 Manual Structure .............................................................................................. 1-4 1.7 Types of road projects ...................................................................................... 1-6

1.7.1 New Construction ................................................................................... 1-6 1.7.2 Rehabilitation ......................................................................................... 1-7 1.7.3 Reconstruction (including upgrading) ..................................................... 1-7

1.8 The Site Investigation Team ............................................................................. 1-8 1.9 Other Factors .................................................................................................... 1-8

1.9.1 Health, Safety and the Environment ........................................................ 1-8 1.9.2 Site Access .............................................................................................. 1-8 1.9.3 Presence of Existing Services .................................................................. 1-8 1.9.4 Security ................................................................................................... 1-9 1.9.5 Socio-political considerations ................................................................. 1-9 1.9.6 Proximity to Existing Roads and Waterways ........................................... 1-9

2 PHYSIOGRAPHY, CLIMATE, GEOLOGY AND SOIL DISTRIBUTIONS ........................... 2-1

2.1 Introduction ...................................................................................................... 2-1 2.2 Physiography and landform .............................................................................. 2-1 2.3 Climate ............................................................................................................. 2-3

2.3.1 Climatic Zones ........................................................................................ 2-3 2.3.2 Climatic Indices ...................................................................................... 2-5

2.4 Geology ............................................................................................................ 2-6 2.5 Soil type and distribution .................................................................................. 2-8 2.6 Land cover and land use ................................................................................. 2-12

3 INVESTIGATION METHODS AND TECHNIQUES ....................................................... 3-1

3.1 Introduction ...................................................................................................... 3-1 3.2 Topographic and thematic maps ....................................................................... 3-2 3.3 Remote Sensing ................................................................................................ 3-2 3.4 Geophysical methods ........................................................................................ 3-4 3.5 Seismic refraction ............................................................................................. 3-6

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3.6 Electrical resistivity........................................................................................... 3-7 3.7 Pits and Trenches .............................................................................................. 3-7 3.8 Boring ............................................................................................................... 3-9

3 .8.1 Auger boring ......................................................................................... 3-11 3 .8.2 Wash type boring ................................................................................... 3-12 3 .8.3 Rotary wash boring ............................................................................... 3-12 3 .8.4 Drilling in rock ...................................................................................... 3-12

3.9 Pit, Trench and Boring Logs ........................................................................... 3-13 3.10 Sampling ......................................................................................................... 3-15 3.11 In-situ tests...................................................................................................... 3-17

4 SOIL AND ROCK DESCRIPTION AND CLASSIFICATION ............................................ 4-1

4.1 Introduction ...................................................................................................... 4-1 4.2 Soil description ................................................................................................. 4-1 4.3 Coarse grained soils .......................................................................................... 4-2 4.4 Fine grained soils .............................................................................................. 4-2 4.5 Soil classification .............................................................................................. 4-5 4.6 Engineering characteristics of soils ................................................................... 4-9

4 .6.1 Coarse grained soils ................................................................................ 4-9 4 .6.2 Fine grained soils .................................................................................. 4-10

4.7 Rock ............................................................................................................... 4-10 4 .7.1 Description ............................................................................................ 4-10 4 .7.2 Rock name ............................................................................................. 4-11 4 .7.3 Lithological descriptions ....................................................................... 4-11 4 .7.4 Rock colour ........................................................................................... 4-14 4 .7.5 Bedding ................................................................................................. 4-14 4 .7.6 Weathering ............................................................................................ 4-15 4 .7.7 Rock strength......................................................................................... 4-16 4 .7.8 Rock discontinuity ................................................................................. 4-17

5 SITE INVESTIGATION STAGES ................................................................................. 5-1

5.1 Introduction ...................................................................................................... 5-1 5.2 Desk study ........................................................................................................ 5-1

5 .2.1 Identifying sources of information ........................................................... 5-1 5 .2.2 Reviewing available information ............................................................. 5-3

5.3 Reconnaissance survey ...................................................................................... 5-5 5.4 Preliminary site investigation ............................................................................ 5-5 5.5 Final site investigation ...................................................................................... 5-6

6 DESIGN DATA SURVEYS.......................................................................................... 6-1

6.1 Introduction ...................................................................................................... 6-1 6.2 Sub-grade characterization ................................................................................ 6-2

6 .2.1 Location and spacing of test pits and borings .......................................... 6-2 6 .2.2 Depth of test pits and boreholes ............................................................... 6-5 6 .2.3 Laboratory testing ................................................................................... 6-5 6 .2.4 Subsurface profile ................................................................................... 6-8

6.3 Road Cuts and Embankments ............................................................................ 6-9 6 .3.1 Road cuts .............................................................................................. 6-10 6 .3.2 Embankments ........................................................................................ 6-13

6.4 River crossings................................................................................................ 6-16 6 .4.1 Bridges .................................................................................................. 6-17

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6.4.2 Subsurface investigation ....................................................................... 6-17 6.4.3 Footings................................................................................................ 6-21 6.4.4 Driven Piles .......................................................................................... 6-21 6.4.5 Drilled Shafts ........................................................................................ 6-22 6.4.6 Potential scour depth ............................................................................ 6-24 6.4.7 Inspection of existing bridges ................................................................ 6-24 6.4.8 Culverts ................................................................................................ 6-25 6.4.9 Low water crossings ............................................................................. 6-26

7 SPECIAL INVESTIGATIONS ..................................................................................... 7-1

7.1 Introduction ...................................................................................................... 7-1 7.2 Landslides ........................................................................................................ 7-1

7.2.1 Types of landslides .................................................................................. 7-2 7.2.2 Depths of landslides ................................................................................ 7-4 7.2.3 The role of groundwater.......................................................................... 7-5 7.2.4 Landslide mapping .................................................................................. 7-6 7.2.5 Exploration and sampling ....................................................................... 7-7 7.2.6 Monitoring .............................................................................................. 7-9

7.3 Expansive soils ............................................................................................... 7-10 7.3.1 Identification......................................................................................... 7-11 7.3.2 Laboratory tests .................................................................................... 7-14

7.4 Collapsible soils ............................................................................................. 7-14 7.4.1 Identification......................................................................................... 7-15 7.4.2 Strength ................................................................................................ 7-15 7.4.3 Collapse potential ................................................................................. 7-16

7.5 Dispersive soils .............................................................................................. 7-17 7.5.1 Laboratory tests .................................................................................... 7-18 7.5.2 Field identification ................................................................................ 7-19

7.6 Colluvial soils................................................................................................. 7-20 7.6.1 Exploration techniques ......................................................................... 7-21 7.6.2 Engineering characteristics .................................................................. 7-22

7.7 Lateritic soils .................................................................................................. 7-23 7.7.1 Identification......................................................................................... 7-23 7.7.2 Special properties ................................................................................. 7-24

7.8 Saline soils ..................................................................................................... 7-25 7.9 Degradable rocks ............................................................................................ 7-26 7.10 Groundwater................................................................................................... 7-28 7.11 Wetlands ........................................................................................................ 7-32 7.12 Disposal sites .................................................................................................. 7-33

8 CONSTRUCTION MATERIAL SURVEYS ................................................................... 8-1

8.1 Introduction ...................................................................................................... 8-1 8.2 Investigation procedures ................................................................................... 8-1

8.2.1 Aerial photographs ................................................................................. 8-3 8.2.2 Pits and borings ...................................................................................... 8-4

8.3 Material types ................................................................................................... 8-5 8.3.1 Common Fill ........................................................................................... 8-5 8.3.2 Sub-grade and capping layer .................................................................. 8-6 8.3.3 Unbound granular pavement materials ................................................... 8-6 8.3.4 Bitumen-Bound Granular Layers and Surfacing Aggregates ................... 8-7

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8.4 Sources of materials .......................................................................................... 8-8 8 .4.1 Borrow pits ............................................................................................. 8-9 8 .4.2 Quarry materials ................................................................................... 8-10

8.5 Laboratory tests............................................................................................... 8-11 8 .5.1 Basic engineering tests .......................................................................... 8-11 8 .5.2 Aggregate tests ...................................................................................... 8-14 8 .5.3 Chemical and petrographic tests ........................................................... 8-17

8.6 Sampling ......................................................................................................... 8-17 8.7 The Geological Background ............................................................................ 8-18

8 .7.1 Sedimentary rocks ................................................................................. 8-19 8 .7.2 Volcanic rocks ....................................................................................... 8-20 8 .7.3 Plutonic rocks ....................................................................................... 8-20 8 .7.4 Pyroclastic rocks ................................................................................... 8-21 8 .7.5 Metamorphic rocks ................................................................................ 8-21

8.8 The influence of weathering ............................................................................ 8-22 8.9 Local sources of rocks and soils ...................................................................... 8-25 8.10 Sources of sand ............................................................................................... 8-25 8.11 Sources of water.............................................................................................. 8-25

9 CONSTRUCTION REVIEW ........................................................................................ 9-1

9.1 Introduction ...................................................................................................... 9-1 9.2 Subgrade conditions .......................................................................................... 9-1 9.3 Road cuts .......................................................................................................... 9-3 9.4 Embankments ................................................................................................... 9-4 9.5 River crossings.................................................................................................. 9-5 9.6 Landslides ......................................................................................................... 9-6 9.7 Retaining walls ................................................................................................. 9-7 9.8 Construction materials ...................................................................................... 9-7 9.9 Pavement condition survey................................................................................ 9-9

10 REPORTS AND CHECKLISTS .................................................................................. 10-1

10.1 Introduction .................................................................................................... 10-1 10.2 Reports ........................................................................................................... 10-1

10.2.1 The site investigation report .................................................................. 10-1 10.2.2 Soil and materials report ....................................................................... 10-5

10.3 Checklists ....................................................................................................... 10-6

11 REFERENCES ........................................................................................................ 11-1

APPENIDIX A THE DYNAMIC CONE PENETROMETER (DCP) TEST............................... 1

APPENIDIX B SYSTEMS OF ROCK MATERIALS AND DISCONTINUITY DESCRIPTION ..... 1

APPENIDIX C SUMMARY OF GEOTECHNICAL NEEDS AND TESTING CONSIDERATIONS 1

APPENIDIX D COMMON SOIL LABORATORY TESTS ..................................................... 1

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LIST OF FIGURES Figure 1.1: Phased Approach to Site Investigation (Geotechnical aspects) ...................... 1-5 Figure 2.1: Physiographic regions of Ethiopia ................................................................ 2-2 Figure 2.2: Traditional climatic zones in Ethiopia. ......................................................... 2-4 Figure 2.3: Rainfall distribution of Ethiopia. .................................................................. 2-5 Figure 2.4: Generalized geological map of Ethiopia. ...................................................... 2-7 Figure 2.5: Agricultural soil map of Ethiopia. ................................................................ 2-8 Figure 2.6: Land cover and land use map of Ethiopia. .................................................. 2-13 Figure 3.1: Illustration of the Geophysical Seismic Refraction Method. ......................... 3-6 Figure 3.2: The Basic Installation of Electrical Resistivity Apparatus ............................. 3-7 Figure 4.1: The Unified Soil Classification System (USCS). .......................................... 4-7 Figure 6.1: Different Options of Sub-grade Locations .................................................... 6-3 Figure 6.2: Illustrations of Instability and Settlements Concerns in Embankments........ 6-13 Figure 6.3: An example of a subsurface profile at a bridge site. .................................... 6-20 Figure 7.1: Schematic Illustrations of Slope Failure - Fall and Topple. ........................... 7-2 Figure 7.2: Rotational (slump) and Translational (planar) Landslides. ............................ 7-3 Figure 7.3: Examples of flow and creep. ........................................................................ 7-4 Figure 7.4: Landslides Related to a Perched Water Table ............................................... 7-5 Figure 7.5: Illustration of Borehole Locations to Investigate a Failed Slope.................... 7-9 Figure 7.6: Distribution of Survey Stakes and Inclinometers in a Landslide.................. 7-10 Figure 7.7: Red Clays with Significant Plasticity around Bako in Wellega. .................. 7-12 Figure 7.8: Classification Chart for Swelling Potential (after Seed et al, 1962) ............. 7-13 Figure 7.9: Guide to Collapsibility and Expansion........................................................ 7-13 Figure 7.10: Collapse holes near Shashemene .............................................................. 7-15 Figure 7.11: Erosion Gulies at Roadcuts in the Rift Valley near Arsi Negele. ............... 7-18 Figure 7.12: Test for the Dispersive Nature of Soils. .................................................... 7-20 Figure 7.13: Colluvium from Basalt and Volcanoclastic Rocks - Blue Nile basin ......... 7-21 Figure 7.14: Nodular Laterite - Assossa-Kurmuk Road Project. ................................... 7-24 Figure 7.15: Salt Deposits in the Dallol Depression (Northern Afar region).................. 7-26 Figure 7.16.: Degradable Shale Underlying a Sandstone Layer - Road Cut near Kulbi. 7-27 Figure 7.17: Illustration of the Movement and Occurrence of Groundwater Near Roadways

............................................................................................................................. 7-30 Figure 8.1: The Relative Engineering and Excavation Concerns for Different Rocks .... 8-19 Figure 8.2: Schematic Illustration of a Cross Section in a Quarry ................................. 8-22

List of Tables Site Investigation Manual – 2013

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LIST OF TABLES Table 2.1: Ethiopian Climatic Zones .............................................................................. 2-3 Table 2.2: Thornthwaite Moisture Regions .................................................................... 2-6 Table 2.3: A summary of the characteristics and distribution of soils in Ethiopia ......... 2-11 Table 3.1: Common site investigation techniques........................................................... 3-1 Table 3.2: Comparison of Geophysical Methods. ........................................................... 3-5 Table 3.3: Comparison of Different Types of Test Pit and Trenching Methods .............. 3-8 Table 3.4: Soils and Soft Rock Boring Methods. .......................................................... 3-10 Table 3.5: Example of a Pit Log. ................................................................................. 3-14 Table 3.6: Example of a Standard Boring Log ............................................................. 3-15 Table 3.7: Common In-Situ Tests for Foundation Investigation ................................... 3-18 Table 4.1: Particle Size Definition for Gravels and Sands. ............................................. 4-2 Table 4.2: Field Identification Procedures for Fine Grained Soils. ................................. 4-3 Table 4.3: A Field Method to Describe Plasticity in terms of Dry Strength. ................... 4-4 Table 4.4: Additional Tests to Identify Fine Grained Soils in the Field........................... 4-5 Table 4.5: The AASHTO soil classification system. ...................................................... 4-8 Table 4.6: Rock groups and types ................................................................................ 4-13 Table 4.7: Terminology for Layer Thickness ............................................................... 4-14 Table 4.8: Terminology for Rock Mass Weathering ..................................................... 4-16 Table 4.9: Description of rock strength in the field ...................................................... 4-17 Table 4.10: Discontinuity Spacing ............................................................................... 4-18 Table 5.1: Basic Steps for a Typical Investigation to Design a Road .............................. 5-2 Table 5.2: Ethiopian Data Sources for Site Investigation ................................................ 5-4 Table 6.1: The Frequency and Depth of Investigation for Sub-grade Characterization.... 6-4 Table 6.2: The California Bearing Ratio (CBR) Test ...................................................... 6-6 Table 6.3: Options for Measuring Dry Density. ............................................................. 6-8 Table 6.4: Information Needs During the Design of Road Cuts and Embankments ........ 6-9 Table 6.5: Suggested spacing and depth of trenches and boreholes for road cuts. ......... 6-11 Table 6.6: Investigation Needs for Embankments ........................................................ 6-14 Table 6.7: Spacing and Depth of Exploration Points for Embankment Investigations. .. 6-15 Table 6.8: Information Needs for Design of Different Types of Bridge Foundations .... 6-19 Table 6.9: The Minimum Number and Depth of Exploration Points for Bridge

Foundations .......................................................................................................... 6-23 Table 6.10: Indicators of Active or Potential Scour at or around Existing Bridges ........ 6-25 Table 7.1: Classification of Expansive Soils according to US Bureau of Reclamation .. 7-14 Table 7.2: Qualitative Assessment of Collapse Potential .............................................. 7-17 Table 7.3: Relationship between the Degree of Dispersion and % of Exchangeable

Sodium. ................................................................................................................ 7-18 Table 7.4: Guide to Interpret of the Result of the Jar Slake Test ................................... 7-28 Table 7.5: The Characteristics of Rocks as Potential Sources of Seepage at Road Cuts.7-31 Table 8.1: Techniques that Assist the Investigation of Construction Materials ............... 8-2 Table 8.2: General Requirements for Fill Materials. ....................................................... 8-5 Table 8.3: General sub-grade and capping layer material requirements .......................... 8-6 Table 8.4: The requirements for unbound granular pavement materials .......................... 8-7 Table 8.5: Requirements for Bitumen-Bound and Surfacing Aggregate Materials. ......... 8-8 Table 8.6: Borrow pitting and quarrying methods .......................................................... 8-9 Table 8.7: Types of Tests required to Analyse Materials for various purposes.............. 8-12 Table 8.8: Basic Engineering Tests needed for Material Analyses ................................ 8-13 Table 8.9: Field tests useful to identify engineering properties of soils and rocks ......... 8-14

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Table 8.10: Aggregate strength and durability tests ..................................................... 8-16 Table 8.11: Sample sizes needed for different tests ...................................................... 8-18 Table 8.12: Weathering Grades for Describing and Classifying Road Construction

Materials. ............................................................................................................. 8-24 Table 8.13: The Local Distribution and Usage of Materials for Road Construction...... 8-27 Table 10.1: Site Investigation Checklist ...................................................................... 10-7

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GLOSSARY OF TERMS Aggregate Hard mineral elements of construction material mixtures, for

example: sand, gravel (crushed or uncrushed) or crushed rock.

Alluvium Loose, unconsolidated (not cemented together into a solid rock), soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting.

Basalt A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance.

Bedrock The more or less continuous body of rock that underlies the overburden.

Bench Step in a slope formed by a horizontal surface and a surface inclined at a steeper angle than that of the entire slope.

Berm A shelf that breaks the continuity of a slope.

Borrow Area An area within designated boundaries approved for the purpose of obtaining borrow material. A borrow pit is the excavated pit in the borrow area.

Borrow Material Any gravel, sand, soil, rock or ash obtained from borrow areas, dumps or sources other than cut within the road prism which is used for construction of the specified work for the project. It does not include crushed stone or sand obtained from commercial sources.

Boulder A rock fragment usually rounded by weathering or abrasion with an average dimension of 0.3 m or more.

Chippings Stones or aggregate used for thin bituminous surface dressings (treatments).

Colluvium Loose bodies of sediment that have been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity.

Compressive Strength

The load per unit area at which an unconfined cylindrical specimen of soil or rock will fail in a simple compression test.

Consolidation The gradual reduction in volume of a soil mass resulting from an increase in compressive stress.

Core A cylindrical sample rock, concrete, hardened grout or grouted deposits usually obtained from core drilling.

Culvert A structure other than bridge that provides an opening under the carriageway or median for drainage or other purposes.

Cut Cut means all excavations from the road prism including side-drains, and excavations from intersecting roads (including open drains when classified as cut).

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Faulting A fracture in the continuity of a rock formation caused by a shifting or dislodging of the earth's crust in which adjacent surfaces are displaced relative to one another and parallel to the plane of fracture.

Ferricrete gravel A mineral conglomerate consisting of surficial sand and gravel cemented into a hard mass by iron oxide derived from the oxidation of percolating solutions of iron salts.

Fill Materials from which a man-made raised structure or deposit such as an embankment is constructed. These could include soils, soil-aggregate or rock. Materials imported to replace unsuitable roadbed material are also classified as fill.

Foundation Lower part of a structure that transmits the load to the soil or rock.

Gravel Rounded or semi-rounded particles of rock that will pass a 75 mm sieve and be retained on a 4.75 mm sieve.

Groundwater That part of the subsurface water that is in the saturated zone.

Lacustrine deposits Sedimentary deposits that is laid down in the waters of a lake.

Laterite Soil types rich in iron and aluminium, formed in hot and wet tropical areas.

Massif A prominent upland usually of considerable extent.

Mountainous terrain Terrain that is rugged and very hilly with substantial restrictions in both vertical and horizontal alignment.

Physiography The study of physical features of the earth's surface (physical geography).

Project specifications

The specifications of a project that form part of the contract documentation and which contain supplementary and/or amending specifications to the standard specifications.

Quarry An area within existing boundaries approved for the purpose of obtaining rock by sawing or blasting

Quartzite A rock consisting entirely of quartz; white, very hard rock that shows little or no granular structure.

Reconstruction The process by which a new pavement is constructed, utilizing mostly new materials, to replace an existing pavement.

Rectilinear A form of drainage pattern in which the streams flow at right angles to each other, controlled by the joint pattern of the underlying rocks.

Glossary of Terms Site Investigation Manual – 2013

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Rehabilitation Work undertaken to significantly extend the service life of an existing pavement. This may include overlays and pre-overlay repairs, and may include complete removal and reconstruction of the existing pavement, or recycling of part of the existing materials.

Rhyolite An acid igneous rock of the same mineral composition as granite, but of fine grain. The fine grain is caused by rapid chilling of a lava flow and the consequent suppression of the growth of large crystals.

Roadbase A layer of material of defined thickness and width constructed on top of the sub-base, or in the absence thereof, the subgrade. A roadbase may extend to outside the carriageway.

Roadbed The natural in situ material on which the fill, or in the absence of fill, any pavement layers, are to be constructed.

Roadbed Material The material below the subgrade extending to such depth as affects the support of the pavement structure.

Roadway The area normally travelled by vehicles and consisting of one or a number of contiguous traffic lanes, including auxiliary lanes and shoulders.

Scarp A slope formed on the exposed ends of a tilted sequence of rocks. The slope is usually associated with a dip slope, which is developed on the exposed upper bed in the sequence. Scarp slopes are usually steeper and more irregular than dip slopes.

Schist A metamorphic rock characterised by a parallel arrangement of most of its constituent minerals, which are chiefly micas. Schists are usually soft, easily weathered, and easily split along the plane of weakness.

Shear Strength The maximum resistance of a soil or rock to shearing stresses.

Stability The condition of a structure or a mass of material when it is able to support the applied stress for a long time without suffering any significant deformation or movement that is not reversed by the release of stress.

Stabilisation The treatment of the materials used in the construction of the road bed material, fill or pavement layers by the addition of a cementitious binder such as lime or Portland Cement or the mechanical modification of the material through the addition of a soil binder or a bituminous binder. Concrete and asphalt shall not be considered as materials that have been stabilized.

Subbase The layer of material of specified dimensions on top of the subgrade and below the roadbase.

Subgrade The surface upon which the pavement structure and shoulders are constructed.

Site Investigation Manual – 2013 Glossary of Terms

Ethiopian Roads Authority Page xvii

Traps A common name for basaltic lava flows.

Vertisol A soil in which there is a high content of expansive clay known as montmorillonite that forms deep cracks in drier seasons or years.

Volcanism The phenomena associated with volcanic activity.

Wadi A flat-floored valley with an intermittent stream, characteristic of arid and semi-arid areas.

Wetland An area of land whose soil is saturated with moisture either permanently or seasonally. Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes, and bogs, among others. The water found in wetlands can be saltwater, freshwater, or brackish.

Abreviations Site Investigation Manual – 2013

Page xviii Ethiopian Roads Authority

ABBREVIATIONS AND ACRONYMS AASHTO American Association of State Highway and Transportation

Officials AAV Aggregate Abrasion Value

AC Asphaltic Concrete

ACV Aggregate Crushing value

AFCAP Africa Community Access Programme

AIV Aggregate Impact value

ASTM American Society for Testing and Materials

CBR California Bearing Ratio

CR Core recovery

DCP Dynamic Cone Penetrometer

DEM Digital Evaluation Model

DFID Department for International Development, UK

DMT Dilatometer test

DOSI Depth of Significant Influence

E Young’s Modulus

EMA Ethiopian Mapping Agency

ERA Ethiopian Roads Authority

10% FACT 10% Fines Aggregate Crushing Test

FWD Falling Weight Deflectometer

GI Group Index

GSE Geological Survey of Ethiopia

HCl Hydrochloric Acid

HS&E Health, Safety and the Environment

If Flakiness Index

Ie Elongation Index

ITCZ Inter-tropical Convergence Zone

MDD Maximum Dry Density

OMC Optimum Moisture Content

LAA Los Angeles Abrasion

LI Liquidity Index

LL Liquid Limit

Site Investigation Manual – 2013 Abbreviations and Acronyms

Ethiopian Roads Authority Page xix

MCV Moisture Condition Value

MDD Maximum Dry Density

MPa Mega Pascal

OMC Optimum Moisture Content

PI Plasticity Index

PL Plastic Limit

PMS Pavement Management System

PMT Pressure Meter Test

PSD Particle Size Distribution

PSV Polished Stone Value

RQD Rock Quality Designation

SAICE South African Institute of Civil Engineers

SASW Spectral Analysis of Surface Waves

SPT Standard Penetration Test

TCDE Transport Construction Design Enterprise

TRL Transport Research Laboratory

USCS Unified Soil Classification System

VST Vane Shear Test

WPI Weighted Plasticity Index

3-D Three Dimensional

Chapter 1 Site Investigation Manual-2013 Introduction

Ethiopian Roads Authority Page 1-1

1 INTRODUCTION 1.1 Background and Context

All roads, whether they are built above or below the ground surface, use naturally occurring soils and rocks as the basic foundation and construction materials. Unlike man-made materials, the properties of these soils and rocks are highly variable and a function of the complex natural processes that occurred in the geologic past. As a consequence, road construction engineers are faced with the challenge of using soils and rocks available near the project site, whose properties are often unknown and of variable quality.

Hence, the investigation of potential sites and alignments is a vital and integral part of the location, design and construction of a road and its associated structures. It provides essential information on the following:

Characteristics of the soils along the possible alignments; Availability of construction materials; Topography; Land use; Environmental issues; and Socio-political considerations.

Typical uses of the information are;

Selection of the route/alignment of the road; Location of water crossings and drainage structures; Provision of design information for the road pavements, bridges and other

structures; Identification of areas of possible geotechnical problems requiring specialist

investigation; Identification of areas of possible problem soils requiring additional investigation

and treatment; Location and assessment of suitable, locally available, borrow and construction

material.

From the above, it is evident that the main component of site investigations is focussed on what is generally described as ‘engineering’ or, more precisely, ‘geotechnical engineering’ and it is these investigations that will be the focus of this manual.

It is recognised that various other types of surveys are required for the design of a road. Hydrological surveys are required to determine the water flows that determine the drainage design of the road, including bridges; traffic surveys are required to estimate the numbers of vehicles that will use the road, both motorised and non-motorised; surveys are required to evaluate environmental impacts and how to control them; surveys are required in which the local communities are consulted about the road project; and so on. Guidance on conducting these surveys is provided in the respective ERA Manuals. It is important to note that the Route Selection manual and Geotechnical Design manual produced as part of the 2013 series of ERA manuals provide more comprehensive guidance on route selection and geotechnical design than was previously provided in the 2002 Site Investigation Manual and other manuals in the 2002 series.

Chapter 1 Introduction Site Investigation Manual-2013

Page 1-2 Ethiopian Roads Authority

Not all projects will require the same detailed surveys. Road projects fall into one of the following categories:

A new road that follows the general alignment of an existing track or trail; Upgrading a lower class of road to a higher class; A completely new road where nothing currently exists.

Some realignment, and therefore site investigation, will almost certainly be necessary when upgrading an existing road and considerably more will be required when converting a track into an all-weather route. Major site investigations are usually only needed when designing and building a completely new road. In all cases the extent and quality of any investigation has a strong influence on the selection of the most cost-effective route and road design.

Roads of all standards require sufficient investigation to provide sufficient data and information to enable the engineer to optimise the design. In this respect, it is the job of the design engineer to ensure that a well-designed and organised site investigation is undertaken. The design engineer must therefore specify a site investigation programme for the site investigation teams (survey, materials, geotechnical, socio-environmental) that will provide adequate information and data to examine the feasibility of all the routes and designs under consideration.

Site investigation techniques encompass a large range of methods and the amount and type of exploration that is needed for a specific road will depend on the nature of the proposed project and the environment in which it is to be built. Information is provided in the manual on the various site investigation techniques that could be used depending on the prevailing circumstances.

Each site investigation technique has its own purpose and when two or more are taken together in the right combination they can provide a valuable insight into the subsurface conditions. Deciding which technique to use when, where and how, is normally made by geotechnical engineers with a good geological knowhow or engineering geologist with a background of road design. The practice can also be performed by a team comprising pavement engineers and geologists.

It is also important to emphasise that without exception, it is always more cost-effective to undertake an appropriate site investigation (depending on the category and importance of the road) from the start of the project rather than trying to rescue an inadequate investigation during construction or even worse, after the construction has been completed.

The preparation of this document represents the interests of the Ethiopian Road Authority (ERA) to revise and upgrade the 2002 site investigation manual. It is the responsibility of the user to ensure that the use of this document conforms to the policies and engineering practices of ERA.

1.2 Objectives

The main objective of this manual is to provide sufficient guidance on site investigation for road design so that the necessary input data can be developed and proper engineering principles applied to the design of new roads, or upgrading and rehabilitation of existing roads. This manual is prepared to provide project engineers with tools to assist in the



rational development of site investigation programmes, the execution of suitable in-situ and laboratory tests and the interpretation of data obtained from these programmes and test results.

1.3 Scope The ERA (2002) site investigation manual provided the foundation for this manual. However, this edition of the manual contains a number of major additions and modifications in the contents and the structure. For example, unlike the previous manual, information is provided on the general distribution of soils and rocks in the country. In many cases in Ethiopia the soils which form the foundation or sub-grade of the roads in any specific climatic, traffic and terrain conditions become the most critical component in the design and construction of the road. For this reason, the document attempts to address the entire range of soil materials potentially encountered in different regions of Ethiopia so as to assist engineers in selecting appropriate investigation techniques for each landform, geological makeup and climatic region.

It should however be stressed that this manual is not all encompassing in terms of explaining every technique and procedure that can be used to investigate roads during the design phase. Practitioners will need to use other references to broaden their knowledge and use diverse approaches for different conditions. In addition, this manual must be used in conjunction with other documents such as the ERA Pavement Design Manual, Geotechnical Design Manual, Bridge Design Manual, Drainage Design Manual and the Route Selection Manual. Moreover, although laboratory tests that are useful for pavement design are listed in appropriate sections and appendices, detailed procedures are not covered in this manual.

The procedures given in this manual should be adhered to, unless otherwise directed by ERA. However, it should also be understood that careful consideration to geotechnical engineering practice should be observed in the use of the manual, and under no circumstances shall the manual be used as an excuse to disregard professional and expert judgment.

In addition to the guidance given on site investigation requirements, the document also recommends a phased investigation approach that will be essential to all projects irrespective of size. As a good indication, international practice suggests that expenditure of 2% of the project costs on adequate site investigation has the potential of saving the client between 10% and 100% on over-expenditure of project foundation and structural costs.

1.4 Stages of Site Investigation Some form of site investigation is required at all stages in the development of a road project. In general there are four stages leading up to and including Final Engineering Design. These are;

1 Identification and general planning; 2 Pre-feasibility study; 3 Feasibility Study or Preliminary Engineering Design;



4 Final Engineering Design.

More detail is presented in Chapter 5 related on the site investigations associated with each stage. It should also be noted that not all stages are required for all projects.

1.5 Approach

Figure 1.1 highlights the phased approach for site investigations and moves away from the concept of a single-phased ground investigation. It is only as the investigation proceeds that one can assess the need (or otherwise) for further often more sophisticated investigation. However, this approach must not be seen as an “open cheque book” for additional costs for site investigation. Any cost variation should be dealt with up front with the Client and the additional investigation techniques agreed.

Without a phased approach as shown in Figure 1.1 investigations may be left incomplete and the engineer unable to draw the correct conclusions. The engineer responsible for the design of the road should not be left guessing on the required design parameters. Hence, sufficient investigation must be carried out to determine the parameters with a reasonable degree of confidence.

1.6 Manual Structure

The first five chapters of the manual, including the introduction, provide general information on the physiography, climate, geology and soil distribution specific to Ethiopia; soil and rock classification; commonly used site investigation techniques; and the application of these to the various stages of site investigation highlighted in Section 1.4 and Figure 1.1. These chapters provide valuable information mainly at prefeasibility and feasibility stages as part of the desk study to understand the prevailing terrain and climate; and to plan for more detailed investigations, using the most appropriate techniques, to support the design stage.

Information specific to design requirements and special investigations are covered in Chapters 6 and 7 with construction material surveys and a review of construction covered in Chapters 8 and 9.

Reports and checklists are presented in Chapter 10. These are essential components of a successful site investigation programme. The reports allow geotechnical engineers to present the information obtained during the investigation of the sub-grade, fills and embankments, foundation characteristics and the behaviour of construction materials in the design phase. The checklists are important to assess the work done and for completion of the site investigations.

References are given at the end of the document and provide a source of more detailed information to supplement what is presented in this manual.



Project Start-up

Pre-feasibility

Feasibility

Design

Construction

Post Construction

Figure 1-1: Phased Approach to Site Investigation (Geotechnical aspects)

Appoint Geotechnical Specialist

Identify initial site risks, geotechnical constraints & estimate scope of works

Client brief on type/class of road, structures, geometry etc

Desk study and walkover survey making maximum use of existing data & local

experience

Update scope of site investigation based on the possible alignments

Identify alternative, feasible alignments (up to three) with alternative solutions for each

alignment

Comprehensive desk study with limited fieldwork and lab testing

Evaluate risks and benefits of each site related to an appropriate cost for the

respective investigations (value engineering) in line with the client brief

Feasibility Report Project Feasible?

Yes

No Abandon Project

Monitor and value engineer as site conditions are exposed. Verify design assumptions

Detailed site investigation along chosen alignment focussing on soil characterisation, terrain/slope

characterisation and founding conditions of structures

Use for pavement design

Undertake Geotechnical Design

Interpretive Report

Factual Report

Long-term monitoring of pavement performance through visuals, deflections and

riding quality

Additional testing required?

Geotechnical design required?

Yes

Yes

No

No



1.7 Types of road projects The first important issue before conducting any site investigation in an area is to know whether the project involves new construction, rehabilitation or reconstruction.

New construction is the construction of a pavement system on a new alignment that has not been previously constructed.

Rehabilitation is defined as the repair and upgrading of an existing pavement. Typically, this involves the repair or removal and construction of additional bound pavement layers, and could include partial-depth or full-depth recycling or reclamation.

Reconstruction (including upgrading) is defined as the complete removal of an existing pavement system, typically down to and including the upper portions of the foundation soil, and the replacement with a new pavement structure.

This manual is primarily based on the site investigation requirements for the design of new roads. However, references are made to rehabilitation and reconstruction projects at different sections whenever it is assumed that the contents are applicable to them. A short note on techniques useful to investigate the condition of existing pavements is given in Chapter 9. 1 .7.1 New Construction

There are two or more investigation phases that new construction may require. The first phase is to identify the best of several possible routes, or to evaluate sub-grade and foundation alternatives. This phase of investigation may not require a detailed exploration. Rather, it is limited to a geologic reconnaissance and some sampling, and identification of surface and subsurface conditions to perform a generalized site characterization. (See the ERA Route Selection Manual for more details).

Once a route is selected, the site investigation programme for the design of new roads requires a complete evaluation of the vertical and horizontal variability of the sub-grade, and the characteristics of construction materials. This may use all or many of the site investigation phases mentioned in Chapter 5. Little will be known in advance of the soil profiles along the new alignment. Therefore, a comprehensive surface and subsurface exploration programme and material characterization is required at this stage, although access to the site is often limited due to adverse terrain condition. The locations and dimensions of cuts and fills, and structural elements such as bridges and culverts should also be identified as accurately as possible.

In addition, for new road construction projects, samples from the sub-grade immediately beneath the pavement and from soils that will be used as fill material will be required to obtain the design-input parameters. For designs based on sub-grade strength, lab CBR or DCP CBR values can be used to determine the support characteristics of the sub-grade (see Chapter 6). Location of the groundwater table is also an important aspect of the subsurface exploration programme for new construction to evaluate water control issues such as sub-grade drainage requirements with respect to both design and construction. Other design issues include the presence of problem soils and the identification of soft or otherwise unsuitable materials to be removed from the sub-grade.

New construction has been, and still is, the main focus of most pavement design procedures in various regions of Ethiopia. However, this focus may shift to rehabilitation



and reconstruction projects in the coming years, when the Ethiopian Road Authority (ERA) and regional road agencies switch from a strategy of road access expansion to a more extensive road maintenance programme. 1.7.2 Rehabilitation

In specific circumstances, site investigations could involve the restoration and repair of road failures (including landslides), assessment of embankment stability, slope stabilization, sub-grade and pavement settlement, and replacement of old foundation systems. The restoration of a road section, or the addition of structural capacity to an existing pavement, is known as rehabilitation.

The details and extent of the site investigation for rehabilitation projects depend on many factors such as the condition of the road and the nature of any distress; whether the road segment will be returned to its original, as-built condition, or whether it will be upgraded. If the road is distressed, the type of distress should be investigated (e.g. shallow basecourse failure; deep seated failures; settlement of a structure; landslides; drainage and water flow problems; evidence of imminent collapse). The proposed geometry, location, changes of structures (for instance culvert to bridge), and the required design life of the road, are also required.

In general, rehabilitation projects need some pits and trenches to be dug. As access will not be a problem, pits and trenches can be excavated easily and quickly using backhoes and dozers. This can even be done as the rehabilitation progresses. In some cases, such as deep-seated landslides, some borings may be required prior to rehabilitation to evaluate the properties of the slope and sub-grade materials. For minor problems, the pit and trenches in the pavement can be used to investigate the in-situ and disturbed properties of the sub-grade. In the field, DCP tests can be carried out to examine structural properties as well as layer thicknesses (Appendix A). Non-destructive evaluation using the Falling Weight Deflectometer (FWD) can also be used to determine in-place material properties for rehabilitation design (see Chapter 3). 1.7.3 Reconstruction (including upgrading)

The practice of upgrading and reconstruction includes activities like roadway replacement, full depth reclamation, or road widening. In Ethiopia, upgrading often involves the replacement of a gravel road by a new asphalt concrete (AC) or surface treated road on the same or slightly different alignment. Except for the demolition of the existing pavement during construction, upgrading and reconstruction are very similar to a new pavement in terms of design.

Before upgrading and reconstruction, a preliminary investigation of the type, severity, and amount of visible distress on the surface of the existing pavement and the condition of the road (whether gravel or surfaced) can indicate issues that need a more extensive investigation. Original design documents and construction records are often available for reconstruction projects. However, even with the presence of these records, additional subsurface investigation is usually needed to confirm and validate the new pavement design parameters. The engineering parameters measured during the original construction often change with time. In addition, previous data such as traffic counts may no longer be valid. Hence, it is advisable that design values are obtained for the prevailing materials and



sub-grade condition using current data, especially when the existing pavement is beyond its original design life.

However, depending on the purpose and scale of the project, foundation soil properties for reconstruction of some roads can be determined from original design records and in situ testing, similar to rehabilitation design. For road widening projects, the use of in-situ tests on the shoulder section of the old road will be helpful. The characterization of the new or recycled unbound sub-base and base materials should be determined through laboratory tests, similar to the design of a new road. Subsurface investigations for upgrading and reconstruction projects can also be undertaken using non-destructive methods or geophysical tests performed over the old pavement.

1.8 The Site Investigation Team

The following attributes are needed by the team or from individuals involved in any site investigation activity for pavement design:

Concepts of soil and rock mechanics; Knowledge of the geology of the area; and Experiences on direct and indirect exploratory methods.

It is also important that all engineers and geologist on the team have sufficient general knowledge in all areas of pavement design and construction to communicate effectively with one another, and to guide site inspectors and drilling technicians. In addition, team members should have the enthusiasm and drive for work and the ability to take responsibility for decisions in the field when necessary.

1.9 Other Factors 1 .9.1 Health, Safety and the Environment

Any type of site investigation undertaken in Ethiopia must comply with the prevailing Occupational Safety and Health Act of the country (Labour Proclamation No. 377/2003 - Part 7) and all environmental legislation whether national, regional or local. In addition, it is the responsibility of the Client to obtain any site-specific health, safety and environmental (HS&E) requirements and to make these available as part of any procurement process for site investigation works. It is also advisable that an HS&E plan is prepared by the appointed contractor and approved by the Client prior to commencement of the fieldwork (also see section 1.9.6). 1 .9.2 Site Access

It is the Client’s responsibility to obtain the necessary permission to access the land. This may include permission from land owners, and permission to undertake work within previously developed areas. 1 .9.3 Presence of Existing Services

It is the Client’s responsibility to obtain and indicate the presence of any existing services on the site prior to commencement of fieldwork. This is particularly crucial with respect to



fuel lines, telecommunication lines, major electrical installations and water reticulation lines. It is also the responsibility of the Client to apply for and obtain the necessary excavation permits. 1.9.4 Security

Allowance should be made to provide security for equipment and personnel in certain areas as required. For example, the employment of security guards to protect valuable drilling equipment left overnight on unprotected sites. 1.9.5 Socio-political considerations

A site may be located which has socio-political sensitivities. In these circumstances, it is essential that the local community is informed by the Client of the need to undertake a site investigation, along with the extent and nature of the investigation. 1.9.6 Proximity to Existing Roads and Waterways

If fieldwork needs to be conducted adjacent to an existing road or waterway then safety aspects must be taken into account. This would affect the personnel working on the site and the general public in proximity to the site. Potential hazards could include: dust pollution; spillages and pollution of potable water sources; traffic accommodation; and strongly flowing river crossings. Appropriate safety and precautionary measures will need to be implemented.

Chapter 2 Site Investigation Manual – 2013 Physiography, Climate, Geology and Soil Distribution


2 PHYSIOGRAPHY, CLIMATE, GEOLOGY AND SOIL DISTRIBUTIONS 2.1 Introduction

Before commencing the design of any type of road it is the responsibility of the engineers and geologists involved in the project to become familiar with all aspects of the environment. Environmental information on the climate, topography and geological aspects of the project site is an essential requirement to the understanding of the engineering characteristics of the area and is a prerequisite for the planning, design and construction of roads. The information is useful to identify the broad-scale climatic and terrain conditions of the area within which a route corridor is to be placed and thus provide a basis for:

Evaluating alternative locations during planning; Defining the geotechnical situation of the selected route during design; and Analysing external influences during construction.

Ethiopia is a country of great geographic diversity with high, rugged mountains, flat topped plateaus, incised river valleys, broad lowlands, and rift valley basins. Over the geological past, volcanic eruptions, tectonic movements, landslides and erosion have occurred throughout the country to create long escarpments and deep gorges. The location of the country within the tropics, in association with the physical conditions and variations in altitude, has also resulted in a great diversity of climate, soil, and vegetation. In the following sections, basic environmental information on the physiography, geology, climate, soil distribution and vegetation of the country are given to provide the user of this manual with a basic knowledge of these parameters during site investigation for the design of roads.

2.2 Physiography and landform

As shown in Figure 2.1, Ethiopia is divided into three physiographic regions: the north-western and the south-eastern highlands separated asymmetrically by the rift valley. The highlands are formed from lava flows that have created extensive plateaus, dissected by very deep, river-worn gorges, and marked by isolated summits rising to more than 4,000 m. In the west, the north-western highlands give way to a large low lying flat plain. The south-eastern highlands on the other hand grade into a semi-arid lowland further east and south towards Somalia and Kenya. The tropical lowlands on the periphery of the plateaus contrast markedly with the upland terrain. The north-western highlands are considerably more extensive and rugged and are divided into north and south sections by the Blue Nile gorge.



Figure 2. 1: Physiographic regions of Ethiopia

The rift valley is a large open basin, bounded from the east and west by long and high scarps of volcanic rocks. It is very wide in the north and narrow in the central and southern portions. Its undulating but relatively flat floor is occupied mainly by lake basins and volcano-tectonic depressions. Some of these lakes hold fresh water recharged by small streams. Others contain salts and soluble minerals. Existing literature indicates that the Ethiopian rift valley is affected by NE–SW trending normal faults. There are also suggestions of the presence of extensions in the E–W direction which can be related to rifting. Hence, the rift valley is thought to be geologically active and earthquakes are common in the region.

Many places in the Ethiopian highlands have a general elevation in the range of 1,500 to 3,000 metres above sea level. Interspersed on these landscapes are high mountain ranges and level-topped peaks known as Ambas. The highest mountain in the country at an elevation of 4,620 metres is Mt. Ras Dashen located in the northern part of the country. In contrast, lowlands with an elevation of less than 1,500 m are common in the rift valley. Lowlands are also present in the Somali region and in areas bordering Sudan, Kenya and Somalia. The lowest place in the country at about 115 m below sea level is the Dalol (Denakil) depression, a large, triangle-shaped basin located in the northern part of the Afar region. Active volcanoes occur in the Denakil area, and hot springs and steaming fissures are found in surrounding areas. The existence of small volcanoes, hot springs, and many deep gorges indicates that large segments of the landmass in the country are still geologically active.



In general, the highlands, lowlands and associated basins in Ethiopia can be divided into ten distinct landforms on the basis of broad terrain characteristics useful for engineering purposes. These are:

The northern, western, central and eastern highlands; The western, southern and Ogaden lowlands; and The northern, central and southern rift valley basins.

All of Ethiopia's rivers originate in the highlands and flow outward through gorges and basins. The general westward inclinations of the highland areas dictate that rivers such as the Blue Nile, Tekeze, and Baro belong to the drainage system of the Nile. The Awash river flows in the northern half of the rift valley and goes to the saline lakes found near the border with Djibouti. The southeast part of the country is drained by the Genale and Shebele rivers and their tributaries, while the Omo river flows in the southwest towards Lake Turkana.

2.3 Climate 2.3.1 Climatic Zones The major factors influencing rainfall in Ethiopia are the Inter Tropical Convergence Zone (ITCZ) and winds blowing from the Atlantic and Indian Oceans. The variation in altitude throughout the country also influences climatic conditions. In addition, the micro-climatic changes over small distances are often created by differences in micro-relief. The traditional classification of climatic zones in Ethiopia is based on altitude and temperature. It divides the country into five climatic zones are shown in Figure 2.2 and summarised in Table 2.1.

Table 2.1: Ethiopian Climatic Zones

Climatic Zone Elevation (m) Average Temperature (°C)

Average Annual Rainfall (mm)

Wurch (cold) > 3 200 < 10 < 800 Dega (cool-cold) 2 300 – 3 200 10 - 16 1 000 - 2000 Weina Dega (warm-cool) 1 500 – 2 300 16 - 20 1 200

Kolla (hot-warm) 500 – 1 500 20 - 28 600 (1 000 in places)

Berha (hot) < 500 28 - 34 < 400



Figure 2.2: Traditional climatic zones in Ethiopia.

Generally, central, eastern and northern areas of Ethiopia experience a bimodal rainfall pattern, receiving the major rains from June to September and small spring showers between February and May. Western and south-western parts of the country are characterized by a unimodal rainfall pattern brought about by wind systems to give continuous rains from March or April to October or November. Southern and south-eastern parts get two periods of rain from September to November and from March to May.

North-eastern parts of the country comprise part of the western escarpment of the rift valley and the adjacent Afar depression. These lowlands have one very limited rainy season anytime between November and February. In all regions of the country, the amount of rainfall and length of the rainy season decreases when one goes from south to north and from west to east (Figure 2.3). The mean monthly temperature varies slightly throughout the year, although the difference between the minimum and maximum temperatures is high only in the dry season. According to the National Metrological Agency of Ethiopia, the highest mean maximum temperatures in the country, in the range of 40oC to 45oC, are recorded in the Afar depression. The other hot areas are the north-western lowlands close to the border with Sudan, which experience a mean maximum temperature of 40oC in June, and the western and south-eastern lowlands with mean maximum temperatures of 35oC during April. Most of the Somali, Dire Dawa and Afar regions are also hot for several months in a year. The lowest mean temperatures in the range of 5oC to 15oC or even lower are recorded in the morning or at night between October and January in the highland areas, with an elevation of over 2,000 m above sea level. In these areas, the midday warmth diminishes quickly by late afternoon and nights are usually cold.



Figure 2.3: Rainfall distribution of Ethiopia.

2.3.2 Climatic Indices

While the most common measure of climate is rainfall, both Thornthwaite (1948) and Weinert (1980) developed indices for the evapotranspiration of an area. For the purpose of site investigation in Ethiopia it is recommended that the Thornthwaite Moisture Index (Im) is used.

According to Emery (1985) the index derived by Thornthwaite provides a rational classification of climate based the potential evapotranspiration which is defined as:

The amount of moisture that would be transferred from vegetation covered soil to the atmosphere by evaporation and transpiration if it were constantly available in optimum quantity.

A comparison of the potential evapotranspiration with precipitation gives the variation in soil moisture and hence, a measure of water surplus or water deficiency on an annual basis. Thornthwaite assumed that the stored water was equivalent to 100 mm of rainfall and a water deficiency occurred when the rainfall was 100 mm less than the potential evapotranspiration. By combining the potential evapotranspiration with the water surplus and water deficiency, a moisture index can be obtained.

Potential evapotranspiration (Ep) can be derived from the following (Nata Tadesse et al, 2010):

퐸 = 1.610푇퐽

Ep = Potential transpiration in cms/month. Tn = Mean monthly air temperature (°C). n = 1, 2, 3 ….12 is the number of considered months.



J = Annual heat Index given by the equation:

퐽 = 푗

j = the monthly heat index expressed as:

푗 =푇5

.

A = 0.49 + 0.0179J – 0.0000771J2 + 0.000000675J3

The overall availability of moisture during the year can be assessed by the moisture index (Im):

퐼 =(100 퐷 − 60 푑)

퐸

D = Drainage D = soil moisture deficit (approximately d = Ep - R – 4) R = Rainfall

Table 2.2 shows the moisture index in terms of the moisture regions defined by Thornthwaite.

Table 2.2: Thornthwaite Moisture Regions Moisture Region Moisture Index (Im)

Per-humid A >100

Humid

B4 80 to 100 B3 60 to 80 B2 40 to 60 B1 20 to 40

Sub-humid C2 0 to20 C1 -20 to 0

Semi-arid D -40 to -20 Arid E -60 to -40

2.4 Geology In pavement design and construction, it is necessary to understand the geological history of the project area. In particular it is essential to:

Determine the major geological processes that led to the formation of rocks and soils in the area;

Know the regional and local stratigraphy; Draw attention to important features like major faults and landslides Assess whether any construction activity, especially earthworks, will cause major

changes to the existing environment; Obtain an appreciation of the regional groundwater conditions; and Form a logical basis for the location of proven sources of construction materials.



The geology of Ethiopia provides a variety of rocks and soils (Figure 2.4). The oldest rocks in the country are metamorphosed igneous and sedimentary rocks of Precambrian age. They are exposed in parts of Harar, Dire Dawa, Sidamo, lllubabor, Welega, Gojam, and Tigray. The metamorphic rocks in the south and west of the country, where granitic rocks and gneisses predominate, are more strongly metamorphosed than the Precambrian sequences in the north.

After a time of intense erosion in the Paleozoic, a shallow sea spread over much of the south-eastern part of the country in Mesozoic times and then extended farther north and northwest as the land continued to subside. This process first formed an accumulation of sandstone followed by depositions of mudstone and limestone as the depth of water increased. Much of the Blue Nile basin, Tigray, and places in Dire Dawa and Harar, are covered by Mesozoic sedimentary rocks. The Blue Nile in particular provides long cliffs of sandstone, limestone, and gypsum intercalated with relatively soft units of mudstone, shale, and marl.

Extensive fracturing occurred in the Cenozoic, followed by major displacements along the rift system. Faulting in late Tertiary was accompanied by widespread volcanic activity. This resulted in the outpouring of vast quantities of basaltic lava known as the Trap Series over much of the country, accompanied by the eruption of large amounts of ash and tuff. Most of the highlands in the northwest, west, central and south-eastern part of the country are now covered by these rocks. More recent volcanic activity is associated with the development of the rift valley, being concentrated within the rift and along the edge of the adjoining plateau.

Figure 2.4: Generalized geological map of Ethiopia.

At present, the rift valley is covered by Cenozoic volcanics and recent sediments. The volcanics are dominantly basaltic lava flows, rhyolites and ignimbrites intercalated with volcano-clastic deposits derived from tuff and volcanic ash. Volcanism has persisted into the present time in the Afar region within small eruptive centres.



Many areas in the rift valley are covered by alluvial and lacustrine deposits. The youngest sediments in the country are of Quaternary age. These include conglomerates, sands and clays which are accumulated in the Afar depression and the northern end of the rift valley. Sediments are also present in dried lakes of the southern part of Afar, in the central and southern part of the rift valley, and in the lower part of the Omo River. Undifferentiated Quaternary sediments and superficial deposits occur intermittently along the border with Sudan and Kenya.

2.5 Soil type and distribution

During road design and construction, soil engineering maps are very essential. These maps show the distribution of soils, and describe their origin, physical characteristics and engineering properties. However, national or regional based soil engineering maps do not exist in Ethiopia. Consequently, maps are often only available in association with specific road construction projects. In the absence of engineering soil maps, it is common practice to use agricultural soil classification systems of the type given in Figure 2.5.

Figure 2.5: Agricultural soil map of Ethiopia.

In road design, it is necessary to use maps and material categories that are useful for engineering purposes. Such maps and categories need to be comprehensive (covering all materials), meaningful in an engineering context (so that engineers will be able to understand and interpret them), and relatively descriptive. Engineering maps and categories should normally be prepared to facilitate an easy transition from field observations and descriptions made during site investigation to general classification of soil and rock properties used for design.

The distribution of soils in Ethiopia is a function of climate, regional landform, local topography and the underlying parent materials. Drainage is also an important factor in the formation of some soils. Table 2.3 summarizes the engineering characteristics and distribution of the most common soil types in Ethiopia.



Generally, many places in Ethiopia are covered by thick autochthonous (residual) soils. These soils, which are generally red and black in colour, are classified as oxisols and vertisols, respectively, in agricultural or pedological soil maps. Laterites are also present and the general trend of soil cover in the country is that black soils are replaced by red soils which in turn grade to lateritic soils when one goes from central areas towards the west. Transported soils in the form of lacustrine, alluvial and aeolian deposits are present in the rift valley, along major river basins, and in depressions and lowlands.

The red soils normally occur on sloping ground close to local high points where there is good drainage (Dumbleton, 1967), a vegetative cover with little organic matter and high temperature and rainfall. Water removes the more soluble bases and silica, leaving the soil rich in iron (in the form of iron oxide) and aluminum (as clay minerals of the kaolin group). Deposits of these soils are present in the western part of Ethiopia (western and north-western highlands), southern lowlands and southern rift, most part of the central highlands, and in pockets of well drained lands throughout the north-east and eastern highlands.

Red soils can be formed from many kinds of rocks if the weathering conditions, climate and drainage are suitable. In Ethiopia, they have developed mainly on volcanic (basalts, ryholites, etc) and pyroclastics rocks. In the western part of the country they have also been seen on granitic terrains. The iron oxide in these soils, which accounts for their dark red colour, occurs in a hydrated (goethite) and an unhydrated form (hematite). Goethite and hydrated halloysite predominate under wetter conditions. The clay mineral is usually kaolin of the halloysite type, which occurs as hydrated and meta-halloysite. Hydrated halloysite is readily converted to meta-halloysite on drying. Kaolin in the form of halloysite has a disordered structure, which gives rise to a soil of higher potential plasticity than well-ordered kaolinites. Red clays in the wetter regions of Ethiopia often show this nature of possessing high plasticity and should be subjected to plasticity tests before they are used for road construction purposes.

The black soils are formed when volcanic rocks and some sediment are weathered under humid, alkaline conditions. Because of poor drainage, these soils are rich in soluble bases and silica. Black clay soils, also called “black cotton soils”, contain montmorillonite and other smectite group clay minerals. The presence of montmorillonite allows them to absorb much water and expand upon wetting. The poor drainage pre-condition means these soils can also contain some calcite grains. The black colour is largely due to organic matter.

Black soils are widely distributed in Ethiopia, especially in the highlands. Known as vertisols, they are present in the central, north-western and eastern highlands and western lowlands. They are fertile and used intensively for agriculture. It is estimated that 7.6 million hectares of vertisol area are located in the highlands with a height of greater than 1,500 m above sea level (Jutzi and Abebe, 1986). The remaining area (over five million hectares) is located at elevations below 1,500 m. The general slope range of the landscape on which vertisols occur is 0 – 8% (Debele, 1985). They are more frequent in 0 - 2% slope range and are usually found in landscapes of restricted drainage such as seasonally inundated depressional basins, alluvial and colluvial plains, undulated plateaus, valleys and undulating side slopes.

Laterites and lateritic soils are present in the western lowlands near the border with Sudan and in some lowlands of the southern region. They are reddish highly weathered soils that contain oxides of iron and aluminium and may have also some amount of quartz and



kaolinite. Laterites may have hardened either partially or extensively into gravel like or rock like masses, they may have cemented other materials into rock like aggregates, or they may be relatively soft but with the property of self-hardening after exposure.

Laterites are thought to be formed by the influence of a fluctuating water table to allow solution and transfer of soluble silica, iron, and aluminium ions, resulting in iron and aluminium oxides accumulating in the upper part of the profile. In Ethiopia, laterites have been developed on common igneous and metamorphic rocks. Most of the lateritic soils are clays, or sandy or gravelly clays, which behave as soils of medium plasticity. Sinkholes similar to those in limestone are known to occur in some laterite profiles. These sinkholes develop when voids are formed by the removal of silica and silicate minerals.

In-situ laterite profiles are often permeable. Many of the structural features which cause the high permeability are near-vertical. Lateritic soils usually make excellent earth-fill construction materials. When used as fill, lateritic soils are characterized by a high effective friction angle and medium to low density and permeability. In most cases, they are readily compacted despite often having high and poorly defined water content. Sometimes, lateritic clays are readily compacted at water contents between 40% and 50%. However, some particularly silty laterites with high halloysite contents can be difficult to compact. The ferricrete gravels and weak rocks in the near-surface crust zone are used as base or sub-base material in pavements for roads and airstrips. Lateritic soils are often non-dispersive.

Lacustrine deposits are common in the rift valley, in various lowlands, and along river basins. The group includes soils which have been deposited in lakes and depressions and in surrounding flood-plain. Lacustrine deposits consist of a heterogeneous sequence of clay, thin lenses of sand and silt, pumice fragments and other volcanic sediments. In some parts of the rift valley, these deposits are reported to be 40–50 m thick on average. Several small, loosely interconnected erosion pipes are observed in these deposits. The relative density of such deposits is variable, but the upper few metres are likely to be loose to medium sand and silt, hence, will be relatively compressible. Deeper deposits are more likely to be dense, less compressible and could have a high effective friction angle.

The alluvial soils form a skeletal layer above the lacustrine deposits. They are largely soft, irregular in grading and rounding, and have light-grey to yellow-coloured weathering sheath surrounding the fresh interiors. The alluvial sands and silts had origins from the pumice layers and supplied to the rift during periods of flooding. Alluvial soils deposited elsewhere outside the rift can vary from clays of high plasticity to coarse sands and gravels. These soils are characterized by great variability in engineering property, both vertically and laterally. Typical geotechnical properties of these soils include high void ratios which are related to porosities of above 60%, low bulk densities and low moisture content. The tensile strength of these soils is found to be low because of their clay mineralogy.



Table 2.3: A summary of the characteristics and distribution of soils in Ethiopia Type of soil Engineering characteristics Availability in Ethiopia

Red clay soils

The red colour is the result of iron oxide; they contain kaolin in the form of halloysite, which has a disordered structure and may produce a soil with a potential of high plasticity than soils with well-ordered kaolinites.

Present in large quantities in the west, south-west, north-west and western parts of Ethiopia, and in well drained areas of other parts of the country

Black clay soils

Black clay soils (black cotton soils) contain a clay mineral called montmorillonite which promotes expansion on wetting. These soils often have high dry strength, and are highly plastic with a liquid limit of above 40%. A measure of the activity (the ratio of the plasticity index to the clay fraction) is a good indication of swelling potential.

Available in central, north-west and eastern highlands and western lowlands. They are fertile and used for agriculture. They are more frequent in 0 - 2% slope range and are usually found in landscapes of restricted drainage such as seasonally inundated depressional basins, alluvial and colluvial plains, undulated plateaus, valleys and undulating side slopes.

Lateritic soils

In-situ laterite profiles are often permeable. Lateritic soils usually make excellent earth-fill construction materials. When used as fill, lateritic soils are characterized by high effective friction angle and medium to low density and permeability

Laterites and lateritic soils are present in the western lowlands near the border to Sudan and in some lowlands of the southern region.

Lacustrine deposits

Material deposited within lakes by waves, currents, and organo-chemical processes. Deposits consist of unstratified organic clay or clay in central portions of the lake and typically grade to stratified silts and sands in the peripheries. Usually very uniform in horizontal direction, fine-grained soils generally compressible.

Lacustrine deposits are common in the rift valley, in various lowlands, and along river basins. The group includes soils which have been deposited in lakes and depressions and in surrounding flood-plain.

Alluvial soils

They are largely soft, irregular in grading and rounding; possess high void ratio which is related to porosities of above 60%, low bulk densities and low moisture content. Tensile strength of these soils is found to be low because of their clay mineralogy

The alluvial soils form a skeletal layer above the lacustrine deposits in the rift valley of Ethiopia.

Aeolian (wind blown) soils

These soils consist mostly of silt with minor amounts of sand and clay. Loess is the most common type of windblown soils. Due to the method of deposition, loess has an open (honeycomb) structure with very high void ratios. The clay component of loess plays a pivotal role because it acts as a binder (along with calcium carbonate) holding the structure together. However, upon wetting, the calcium carbonate bonds dissolve or the negative pore pressures within the clay reduce and the soil undergoes shear failures and/or settlements.

Aeolian (windblown) sands in the form of dunes are common in the Afar region. Moreover, windblown loess deposits are reported from the Lower Omo Basin in southern Ethiopia. Aeolian soils are also believed to be present in Somali region and in north-eastern part of the country. In Ethiopia, wind-blown sediments are characteristics of relatively dry periods in the past.

Collapsible soils

These soils appear to be strong and stable in their natural (dry) state, but rapidly consolidate under wetting, generating large and often unexpected settlements. The basic characteristics of collapsible soils are categorized as high porosity (more than 40%), low saturation (less than 60%), high silt content (more than 30%), and rapid softening in the water

Most of the rift valley alluvial soils including the aeolian deposits of the Lower Omo basin are compressible and collapsible.

Dispersive soils

These soils deflocculate in the presence of relatively

pure water to form colloidal suspensions and are, therefore, highly susceptible to erosion and piping. They contain a high amount of sodium in their pore water. Dispersive clays cannot be identified by standard engineering index tests

Dispersive soils have not been definitively associated with any specific geologic origin but most have been found as alluvial clays in the form of slope wash, lake bed sediments, loess deposits, and flood plain silts and clays. Dispersive soils are common in the rift valley, in lowland areas, and in some places of the highland where the annual rainfall is less than 1,000 mm.



Aeolian (windblown) sands in the form of dunes are common in the Afar region. Moreover, windblown loess deposits are reported from the Lower Omo Basin in southern Ethiopia. Aeolian soils are also believed to be present in the Somali region and in the north-eastern part of the country. In Ethiopia, wind-blown sediments are characteristic of relatively dry periods in the past. In the Lower Omo Basin, the loess deposits contain a predominance of silt accompanied by some clay and fine sand. The size proportion and the alignment of soil grains indicate the general pattern of the wind direction in the past. Aeolian silts have porosities in excess of 50%.

Most of the rift valley alluvial soils including the aeolian deposits of the Lower Omo basin are compressible and collapsible. This originates from the fact that the soil grains are loosely arranged and prone to considerable settlement due to minor changes in water content. The high void ratio values are characteristic of soils that could collapse upon wetting. Usually the soil collapse is caused by minor changes in water content or the weakening of soil cement. The magnitude of soil collapsibility depends on initial porosity. The basic characteristics of collapsible soils are categorized as high porosity (more than 40%), low saturation (less than 60%), high silt content (more than 30%), and rapid softening in the water.

Dispersive soils are common in the rift valley, in lowland areas, and in some places of the highlands. These soils deflocculate in the presence of relatively pure water to form colloidal suspensions and are, therefore, highly susceptible to erosion and piping. They contain a high amount of sodium in their pore water. However, there are no significant differences in the clay contents of dispersive and non-dispersive soils. Dispersive soils can derive from any rock, although in Ethiopia they are associated with volcanic rocks. They are commonly found in regions where the annual rainfall is less than 1,000 mm. Suspicion of their presence is indicated by the occurrence of erosion gullies and piping at unprotected road shoulders and cuts, drainage ditches, and other surfaces from which vegetation has been removed.

2.6 Land cover and land use

Ethiopia was once heavily wooded. Now most parts of the country are sparsely vegetated with natural forests existing in some areas of the western and southern parts of the country (Figure 2.6). The highlands are characterized by extensive cultivation. The central and southern part of the rift valley is also a zone of agricultural activity.

As shown in Figure 2.6, a significant proportion of the country is classified as “Bareland”. This is especially common in the eastern and north-eastern semi-arid and arid lowlands of Afar and Somali regions. Grasslands are distributed throughout the country. In relatively dry areas, the proportion of grass reduces and is replaced by patches of shrubs and bushes.

The intermediate zones between humid and semi-arid parts of the country are areas of bushes and shrubs. The woodland areas are characterized by a more discontinuous canopy and smaller trees than the high forest region. The escarpments along the rift valley are areas where wooded grasslands are common. Portions of villages, towns and cities consisting of planted eucalyptus trees around settlements are included in the woodland regions. The high forest region is found mostly in the southwest and west. It consists of coniferous forest in parts of the central and western highlands and of mixed tree species in the southwest.



Figure 2.6: Land cover and land use map of Ethiopia.

Most of the central and northern highlands are intensively cultivated for rain-fed agriculture as well as livestock grazing. As shown in Figure 2.6, pockets of lands in these locations are either barren or covered by grass. In these regions it is estimated that about 70% of the land is under annual crops during the rainy season. The grazing land is intensively used, and in many cases very few trees are visible. In contrast, the land used for agricultural in the central, southern and western part of the country contain patches of natural forests or woodlands.

Wetlands that include large lakes and swampy areas also cover a significant percentage of the country. In addition, the extent of urban and built-up areas is continually increasing although the proportion of land used for this purpose is still relatively small.

Chapter 3 Site Investigation Manual-2013 Investigation Methods and Techniques


3 INVESTIGATION METHODS AND TECHNIQUES 3.1 Introduction This chapter contains information on site investigation methods and techniques including the use of aerial photographs and remote sensing, geophysical methods, test pits, borings, and in-situ tests. Usually, if a site investigation is to be effective, it must be carried out in a systematic way using techniques that are relevant, reliable and cost-effective. The choice of methods for site investigation is determined by the type of road project, the practical problems arising from site conditions, and the terrain and climate. For any road project, it is advisable to start investigations using standard methods with sophisticated and expensive procedures being employed only when the nature of the geotechnical problem has been determined. A wide variety of methods have been used for site investigation, and Table 3.1 shows those used most frequently.

Table 3.1: Common site investigation techniques.

Applicability Investigation technique Description

Desk study

Topographic and thematic maps

Collation of available maps, references, reports, records. Background information on geological, geomorphological hydrological, vegetation and climatic can be collected at this stage. May be used to define project organization with respect to sites, materials, and project objectives.

Remote sensing May involve techniques ranging from aerial photography to satellite imagery interpretation. Can be used for terrain evaluation and the preliminary organization of projects into convenient sites or soil masses.

Field study

Geophysics

Seismic refraction is the most generally used procedure. Best utilized to interpolate or extrapolate in situ conditions in conjunction with boreholes. Caution required in cases where stronger material overlies weak layers. Cross-hole seismic data can be correlated with geotechnical parameters. The logging of boreholes by means of a suite of geophysical procedures is now a well-established ground investigation procedure in many projects. Other geophysical procedures utilized are resistivity, gravity and magnetic.

Tests pits

May be either hand or machine dug. Particularly cost effective in the examination and logging of material fabric and the delineation of mass structure. Caution should be exercised in geotechnical interpretation of areas where weak materials underlie strong layers. Very useful for obtaining bulk undisturbed samples in sensitive materials.

Auger boring This ranges from hand augers to machine driven hollow stem augers with undisturbed sampling and in situ testing.

Boreholes

May be sunk by a number of percussion or rotary methods. The techniques employed should be chosen to take into account the type and condition of material involved. Special precautions and care should be taken in attempting to recover undisturbed samples in sensitive soils or those whose fabric is of geotechnical significance. In some locations, options may be restricted by economic or access constraints.

In situ testing

Includes the currently utilized in situ ground investigation techniques such as standard penetration testing (SPT), dynamic cone penetration (DCP), pressure meter test (PMT), plate load pest, and vane shear test (VST).

Chapter 3 Site Investigation Manual – 2013 Investigation Methods and Techniques


3.2 Topographic and thematic maps

The initial phase of site investigations consists of a review of landforms and geological conditions at the site and in its environs. This includes a desk study of topographic maps at different scales. In Ethiopia, topographic maps prepared by the Ethiopian Mapping Agency (EMA) are often published at scales of 1:250,000 and 1:50,000. Larger scale maps can be prepared by the EMA and other private companies upon request. The information obtained from topographic maps is useful as a guide for planning subsequent explorations. The denoting of important features such as bench marks on topographic maps at an early stage is often a requirement for road design. Later during the investigation all other features should be marked on these maps, preferably aligned to a regional system of coordinates.

Thematic maps such as rainfall, geology, and geomorphological maps are also necessary during site investigation. The presence of these maps and the information that can be extracted from them can reduce the scope and financial requirement of the investigation. Geologic maps should be examined prior to sub-surface investigation to provide a reasonable idea of what may be encountered during construction. The Geological Survey of Ethiopia provides a variety of maps and reports on the many aspects of the country’s geology. Most of the geological maps are regional and encompass large areas, but could be useful for the general assessment of the geology associated with the project site. Other types of geologic and geomorphologic mapping also exist and are useful to use in the first steps of a project. Rainfall data at daily, monthly and annual basis can be obtained from National Metrological Agency of Ethiopia.

3.3 Remote Sensing Remote sensing is the collection of data about an object with a device not in contact with it. More commonly, the term refers to the imagery and image information derived from airborne platforms and satellites carrying sensory equipment. Remote sensing data from aerial photographs and satellite images can be used directly during road design to identify terrain conditions, buried streams, site accessibility, right-of-way surveys and general soil and rock formations. The aerial photographs are also helpful to prepare a digital elevation model (DEM) of an area, with which three dimensional topographic features can easily be studied.

Aerial photographs and satellite images can effectively be used to identify terrain conditions, geologic formations, escarpments, site access conditions and the location of construction materials. For example, soil formations can be interpreted from aerial photos using drainage patterns. U-shaped gullies are found in stratified sandy or silty soils. A broad gully is indicative of a clay or silty clay soil and V-shaped gullies are found in semi-granular soils. A radial pattern indicates a hill or a volcanic cone. A parallel drainage pattern is characteristics of a regionally sloping terrain or a system of faults and rock joints. Whenever a drainage pattern appears rectangular, it is almost certain that rock stratum is near the ground surface.

Aerial photography is also used for the following to provide important information at the preliminary phase of road design:

Identification of ground phenomena that suggest instability which should be avoided if possible when locating a new road.



Areas susceptible to landslides are associated with steep scarps, hummocky surfaces, depressions, disturbed drainage and vegetation conditions; and

Provision of up-to-dated information regarding current land use.

Aerial photographs are produced in black and white (panchromatic) or in colour. Many of the aerial photographs in Ethiopia are available in black and white. Aerial photographs are usually taken as flight strips with 60% or more overlap between pictures along the flight line, and from 20 to 30% side overlap between parallel flight lines. On aerial photos, every ground feature has a distinctive ‘tonal signature’. Thus, a road covered by cinder gravels appears dark grey on panchromatic aerial photos. In contrast, when the gravels are from a crushed limestone, the road appears light grey. The contrast is normally greater when colour photos are used as they contain hue and chroma as well as tonal and textural information.

Practical scale recommendations for route surveys are 1:30,000 to 1:16,000 for desk study, from 1:16,000 to 1:8,000 for preliminary survey, and 1:10,000 to 1:500 for detailed investigation, with the exact scale determined by the type of information being sought. Photographs taken in early times are obtained at scales in the range of 1:50,000 to 1:20,000. The choice of the scale depends upon the intended usage, the presence of cloud and the extent it affects flying height, and the problems associated with scale distortion and its acceptable limits. Aerial photos in Ethiopia can be purchased from the Ethiopian Mapping Agency.

Interpretation of aerial photographs and other remote sensed data require considerable experience and skill, and the results obtained depend on the proficiency of the interpreter. Spot checking in the field is an essential element in any photo-interpretation and accuracy is often limited where dense vegetation obscures ground features.

Although locally available aerial photographs are sufficient for a standard ground survey at any stage of a road project, satellite images can also be used at the planning phase or at early stage of the design. For roads, regional investigations using LANDSAT images are normally good enough. Other images such as SPOT and IKONOS are relatively expensive. Standard LANDSAT image comes approximately at a scale of 1:100,000, although enlargements are also available. These images provide a broad view of the terrain, and are useful to understand the regional geology, landforms, drainage, vegetation cover, and land use at the time of route selection or when realignment is considered during construction.

A digital elevation model (DEM) is a digital representation of the ground surface. Recent developments in technology allow the use of radar technology to generate DEMs. Alternatively, DEMs can be produced from stereoscopic pairs of aerial photographs. Older methods of developing DEMs often involve interpolating digital contour maps obtained by direct surveys of the land surface. This method is still used in mountainous areas, where interferometry is less satisfactory due to terrain roughness.

The use of DEMs range from extracting terrain parameters to modelling landslides, creation of relief maps, 3D visualizations, and rectification of aerial photography or satellite imagery. Some simulation techniques such as elevation-flattening applied to a selected sloppy area on a DEM produces terraces similar to those created by actual earth-moving equipment. Moreover, moderate Gaussian blur applied to road selections removes excess height from elevated protrusions and adds data to bisected valleys, creating virtual road cuts and fills.



3.4 Geophysical methods Geophysical methods are used as part of the preliminary site investigation phase of a road project or to supplement the information collected by other exploration programmes. They can be used for establishing stratification of subsurface materials, the depth to the bedrock, level of groundwater, limits of soil deposits, the presence of voids and buried pipes, and depths of existing foundations. They can also be used to assess the depth of excavation and boring.

Geophysical methods offer some advantages and disadvantages. The advantages are they are non-invasive (can be carried out from the surface or exiting boreholes) and non-destructive (do not alter soil conditions). They can also provide information between data points (e.g. boreholes). The surveys can usually be performed quickly and cover a relatively large areas at a reasonable cost. The disadvantages are that the methods require expensive equipment and skilled operators.

Geophysics also assumes sub-horizontal layering or boundaries, and provides no samples. It is also difficult to develop good stratigraphic profiles where hard material overlies weak rocks. For all these reasons, the interpretation of a geophysical survey must always be confirmed by boreholes. Geophysical data should also be interpreted by a professional in the field with a background in road engineering.

There are different kinds of geophysical methods. Table 3.2 provides a summary of these methods and their significance in engineering. The selection of these methods depends on regional and local site conditions, the purpose of investigation, time and cost. For road design, the most commonly used methods are seismic refraction and electrical resistivity.



Table 3.2: Comparison of Geophysical Methods

Method Basic field procedures Applications Limitations

Seismic refraction

Impact load is applied to the ground surface. Seismic energy refracts off the soil or rock layer interfaces and the time of arrival is recorded on the ground surface using several dozen geophones positioned along a line or performing repeated events using a single geophone.

Depth to bedrock Depth to water table Thickness and

relative stiffness of soil or rock layers.

Does not work if stiffness decreases with depth or if soft layer underlies stiff layer.

Spectral-analysis of surface waves (SASW)

Impact load is applied to the ground surface. Surface waves propagate along the ground surface and are recorded on the ground surface with two geophones positioned along a line.

Depth to bedrock Measurement of

shear wave velocity Thickness and

stiffness of surface pavement layer

Qualitative indicator of cracking in pavement

Resolution decreases significantly with increasing depth

Accurate interpretation may require a significant amount of expertise

Interpretation is difficult if a stiff layer overlies a soft layer and soft layer properties are desired

Electrical resistivity

DC current is applied to the ground by electrodes. Voltages are measured at different points on the ground surface with other electrodes positioned along a line.

Depth to water table Groundwater

contamination and salinity

Soil layer thickness Delineation of

certain features (e.g., sinkholes, waste trenches)

Slow; must install electrodes directly in the ground

Resolution decreases significantly with increasing depth

Resolution is difficult in highly heterogeneous deposits

Ground penetrating radar (GPR)

Electromagnetic energy is pulsed into the ground. This energy reflects off boundaries between different soil layers and is measured at the ground surface.

Depth to water table Identification of

buried objects Thickness of

pavement layers Void detection

Not effective below the water table or in clay

Depth of penetration is limited to about 10 m

Gravity The earth’s gravitational field is measured at the ground surface.

Identification of large subsurface voids

Identification of large objects possessing unusually high or low densities

Results are non-unique (i.e. more than one subsurface condition can give the same result)

Primarily, large-scale surveying tool

Applications in road engineering are limited

Magnetics The earth’s magnetic field is measured at the ground surface.

Identification of ferrous materials

Identification of soil and rock containing large amounts of magnetic minerals

Results are non-unique Primarily a large-scale

reconnaissance tool Applications in road

engineering are limited



3.5 Seismic refraction

This method is illustrated in Figure 3.1 and utilizes the fact that seismic waves travel at different velocities in different materials. In rock and soil masses, the velocity increases with an increase in substance strength and compactness or consolidation. Seismic refraction using shear waves is the method most commonly used for delineation of boundaries between soil and weathered rock, and within weathered rocks. The method is becoming increasingly popular for road site investigation and general geotechnical engineering practice as it has the potential to provide quantitative data regarding the shear wave velocity of the subsurface materials.

Figure 3.1: Illustration of the Geophysical Seismic Refraction Method.

The shear wave velocity is directly related to small-strain material stiffness, which in turn, is often correlated to material strength, and soil and rock types. However, seismic refraction can only be used when the velocity of wave propagation increases in the successively deeper strata. Hence, it doesn’t detect velocity inversions. Therefore, if a hard rock overlies a weak material, or if the underlying material is a thin layer of less consolidated soil, it is very difficult to identify the boundary between the two using this method. Complications also sometimes arise in loose deposits where the velocity of transmission increases gradually with depth.



3.6 Electrical resistivity The electrical resistivity method (Figure 3.2) measures the resistance of the ground to induced electrical current. Interpretation assumes a horizontally layered model. The method has been used to locate fault zones, zones of deep weathering and cavities. It can also be used in the exploration of alluvial deposits where permeable gravel and sand beds can be distinguished from low permeability clays or rocks. This capability has been applied in searches for construction materials beneath alluvial terraces. The resistivity of sound igneous rocks is far higher than that of loose saturated soils. However, some dry sedimentary deposits can have fairly high resistivity. Resistivity depends mainly on the quantity and ionization of the water contained in the subsoil, and to a lesser degree on the mineralogical composition.

Figure 3.2: The Basic Installation of Electrical Resistivity Apparatus

In valleys, the results of resistivity surveys are affected by the irregular terrain and by changes in the electrical properties of dry materials on the valley sides and the wet material beneath the valley floor. Resistivity methods have been used in route investigations and can give useful results when the ground conditions are favourable. However, in many cases the results are disappointing either because the strata boundaries offer insufficient contrast, or because there are natural anomalies such as caverns and solution cavities present.

3.7 Pits and Trenches

Test pits and trenches are used to examine and sample soils in situ for the determination of the thickness of the top part of the subsurface and depth to groundwater. Exploration pits permit detailed examination of the soil and rock conditions at shallow depths and relatively low cost. They are an important part of site investigation where significant variations in soil conditions occur, soil materials with boulders and debris exist that cannot be sampled with conventional methods, or buried features must be identified. Table 3.3 compares the



different types of pits related to their use and limitations. They range from manual to machine excavated holes. In addition to being a low cost investigation option, the soil profiles are visible and can be logged and photographed, undisturbed samples can be collected, in-situ tests can be carried out, and the strength of the sub-grade can be determined from the resistance of the ground.

It must however be stressed that all reasonable safety precautions must be adhered to when excavating and logging pits and trenches. Suitable safety equipment and adequate shoring of deeper pits and trenches should be provided. Under no circumstances should personnel be allowed to enter pits or trenches for observation, logging and sampling purposes if unstable ground is suspected. In such circumstances the risks must be evaluated (risk assessment) and mitigated (signed off) by a qualified and experienced professional. In the absence of an Ethiopian guideline, documentation such as the SAICE (South African Institute of Civil Engineers) code of practice: "The safety of persons working in small diameter shafts and test pits for geotechnical engineering purposes" should be followed.

Table 3.3: Comparison of Different Types of Test Pit and Trenching Methods

Exploration Method General Use Capabilities Limitations

Hand-excavated test pits and shafts

Bulk sampling, in-situ testing, visual inspection.

Provides data in inaccessible areas, less mechanical disturbance of surrounding ground.

Time-consuming, limited to depths above groundwater level.

Backhoe excavated test pits and trenches

Bulk sampling in-situ testing, visual inspection, excavation rates, depth of bedrock and groundwater.

Fast, economical, generally less than 3 m deep, can be up to 6 m deep.

Equipment access, generally limited to depths above groundwater level, limited undisturbed sampling.

Drilled shafts

Pre-excavation for piles and shafts, landslide investigations, and drainage wells.

Fast, more economical than hand excavated, minimum 750 mm diameter maximum 2 m diameter.

Equipment access, difficult to obtain undisturbed samples casing may obscure visual inspection, and costly mobilization.

Dozer cuts

Bedrock characteristics, depth of bedrock and groundwater level, rip-ability, increase depth capability of backhoes, level area for other exploration equipment.

Relatively low cost, exposures for geologic mapping.

Exploration limited to depth above groundwater level.

The depth of the exploration pit is determined by the purpose of the investigation, but is typically about 2 – 3 metres. In areas with high groundwater level, the depth of the pit may be limited by the water table. Exploration pit excavations are generally unsafe and uneconomical at depths greater than 5 metres. Pits should be backfilled and compacted after investigation. It may be possible to leave pits open for an inspection, but in this case fencing is required.

During excavation, the sides of the pit should be cleaned by chipping continuously in vertical bands, or by other appropriate methods so as to expose a clean face of soil or weathered rock. Survey control at exploration pits should be done to accurately determine



the ground surface elevation and plan locations of the exploration pit. Measurements should be taken and records should include the orientation, plan dimensions and depth of the pit, and the depths and the thicknesses of each stratum exposed in the pit. In logging the exploration pit, a vertical profile should be made parallel with one pit wall. After the pit is logged, the pit may be photographed or video logged. Photographs should be located with reference to project stationing and baseline elevation. A visual scale should be included in each photo or video.

A logical extension of the use of test pits is the excavation of trenches, which provide continuous exposures where there is little natural outcrop. Trenches are especially useful to investigate cut slopes, valley sides and bridge abutments where lateral variations in material conditions are expected. Trench exposures are logged in a similar manner to test pits.

3.8 Boring The main objective of boring is to extend the knowledge obtained from surface mapping, test pits and trenches below the depth limitations of these methods and to provide control for the interpretation of any geophysical investigations. Boring is also useful to provide samples from these greater depths and access for test equipment. Boring has little effect on the environment. Holes can be easily covered, backfilled or neatly preserved. Surface disruption is commonly restricted to the preparation of a boring pad on sloping ground. Boring has the disadvantage that information obtained is almost always indirect, either from the observation of resistance to rig penetration, by the measurement of in situ properties with equipment lowered down the hole, or by the logging of samples recovered. Direct observation of the ground is restricted to the use of down-the-hole-camera, television or other techniques.

In the design and construction of roads boring is necessary only in certain circumstances such as when there is a need to investigate major bridge foundations and drainage structures, in areas where landslides are common and realignment is difficult, and when unforeseen problems are encountered in the sub-grade or road cuts. In such cases, boring up to the depth of sound rock is often necessary to obtain adequate information.

There are different methods to perform borings in soils. Some of these methods are summarized in Table 3.4. The method used should be compatible with the soil and groundwater conditions to ensure that soil samples of suitable quality are obtained. Below the groundwater level, drilling fluids are often needed to stabilize sidewalls and the bottom of the boring in soft clays or cohesionless soils. Without stabilization, the bottom of the hole may heave or the sidewalls may contract, either disturbing the soil prior to sampling or preventing the sampler from advancing down. In most investigations, borings are performed with solid and hollow-stem augers, or rotary wash boring methods.



Table 3.4: Soils and Soft Rock Boring Methods

Method Procedure Applications Limitations/Remarks

Auger boring

Dry hole drilled with hand or power auger; samples recovered from auger flights.

In soil and soft rock; to identify geologic units and water content above water table.

Soil and rock stratification destroyed; sample mixed with water below the water table.

Hollow-stem auger boring

Hole advanced by hollow-stem auger; soil sampled below auger as in auger boring above.

Typically used in soils that would require casing to maintain an open hole for sampling.

Sample limited by larger gravel; maintaining hydrostatic balance in hole below water table is difficult.

Wash-type boring

Involves light chopping and strong jetting of soil; cuttings removed by circulating fluid and discharged into settling tub.

Soft to stiff cohesive materials and fine to coarse granular soils.

Coarse material tends to settle to bottom of hole; should not be used in boreholes above water table where undisturbed samples are desired.

Bucket auger boring

A 0.6 to 1.2 m diameter drilling bucket with cutting teeth is rotated and advanced. When each advance is completed, the bucket is retrieved from the boring and soil is emptied on the ground.

Most soils above water table; can dig harder soils than above types and can penetrate soils with cobbles and boulders if equipped with a rock bucket.

Not applicable in running sands; used for obtaining large volumes of disturbed samples and where it is necessary to enter a boring to make observations.

Rotary wash boring

Power rotation of drilling bit as circulating fluid removes cutting from the hole. Changes indicated by rate of progress, action of drilling tools, and examination of cutting in drilling fluid. Casing usually not required except near the surface

Applicable to all soils except those containing much large gravel, cobbles, and boulders. Applications are increasing since it is usually the most rapid method of advancing a borehole.

Difficult to determine changes accurately in some soil strata. Not practical in inaccessible locations because of heavy truck mounted equipment. Soil samples and rock cores are usually limited to 150 mm.

Percussion drilling (Churn drilling)

Power chopping with limited amount of water at the bottom of the hole. Water becomes slurry and should be periodically removed with bailer or sand pump. Changes known by rate of progress, action of drilling tools, and composition of the slurry. Casing required except in stable rock

Used in combination with auger or wash borings for penetration of coarse gravel, boulders and rock formations, useful to probe cavities and weakness in rock by changes in drill rate

Not preferred for ordinary exploration or when undisturbed samples are required because of difficulty in determining strata changes, disturbance caused below chopping bit, difficulty of access, and usually higher cost.



3.8.1 Auger boring An auger is an apparatus with a helical shaft that can be manually or mechanically advanced to bore a hole in soil. The practice of advancing a borehole with a mechanical auger consists of rotating the auger while applying a downward pressure to penetrate soil and weak rocks. The auger may be continuous, where the helix extends along the entire length of the shaft, or discontinuous when the auger helix is at the bottom of the drill stem.

The most common type of boring in cohesive soils uses a spiral flight auger to penetrate and remove the material below the surface. The simplest form is a hand auger which is usually restricted to about 3m by the physical effort involved. Hand driven augers are often used to obtain shallow subsurface information from sites with difficult access or terrain where vehicle accessibility is not possible. Several types of hand augers are available with the standard post hole type barrel auger as the most common. In stable cohesive soils, hand augers can be advanced up to 5m or a little more. Maintaining an open hole in granular soils is often difficult. Boulders and cobbles, if present, will create significant problems.

Most augers are power driven. The common power driven auger rig equipped with either 100mm or 150mm diameter solid or hollow flight augers can reach up to 30m in relatively hard soils. A steel blade “V” bit will penetrate most fine-grained soils and very weak rocks. A tungsten-carbide bit will grind slowly through weak and medium strong rock.

Auger boring allows the logging of disturbed material collected from the flights during drilling. By removal of the augers, it is practical to regularly recover tube samples and carryout in situ testing of the material properties. Auger boring is suited to the investigation of areas with thick soil deposits which extend beyond the practical limit of pits and trenches. In many cases, it is used as a rapid method of establishing the depth and general properties of the material overlying rock. A major difficulty in auger boring in cohesion-less soils or soft clays is the stability of the sides of the drill hole particularly below groundwater.

Solid stem augers are generally limited to stiff cohesive soils where the boring walls are stable for the entire depth of boring. The auger must be removed from the borehole to allow access to the hole for sampling or testing devices. Because the auger must be periodically removed from the borehole, a solid stem auger is not appropriate in sands and soft soils or in soil deposits where groundwater is close to the surface. A drill bit is attached to the leading section of flight to cut the soil. The flights act as a screw conveyor, bringing cuttings to the top of the hole. Additional augers are added as the auger drills into the earth.

Hollow stem augers are very similar to solid stem augers except, as the name suggests, they have a large hollow centre. Both augers have the auger flights continuous along the entire length of the auger. For both of these types of auger the drill cuttings are returned to the ground surface via the auger flights. Hollow stem augers are commonly used in clay soils or in granular soils above the groundwater level where the boring walls may be unstable. The auger has a circular hollow core that allows for sampling through the centre of the auger. The augers form a temporary casing to allow sampling below the bit. The cuttings produced from this boring method are mixed as they move up the auger flights and are, therefore, of limited use for visual observation purposes. This should be noted during soil description.



3 .8.2 Wash type boring

Wash-type borings use circulating drilling fluid (e.g. water or mud) to remove cuttings from the borehole. Cuttings are created by the chopping, twisting, and jetting action of the drill bit that breaks the soil or rock into small fragments. Tri-cone bits are often used in dense soil or soft rock. If bentonite cannot be used, casings are often necessary to prevent cave-in. The use of a casing requires a significant amount of additional time but results in a protected borehole. When drilling-mud is used during subsurface boring, it is difficult to classify the soil from the auger cuttings because of contamination with the mud. The properties of the drilling fluid and the quantity of water pumped through the drill bit determine the size of particles that can be removed from the boring with the circulating fluid. In formations containing gravels, cobbles, or larger particles, coarse material may be left at the bottom of the boring. In these instances, cleaning the bottom of the boring with a larger diameter sampler may be needed to obtain a representative sample of the formation.

3 .8.3 Rotary wash boring

The rotary wash boring method is generally the most appropriate method for use in soil formations below the groundwater level. In rotary wash boring, the sides of the borehole are supported either with the use of a drilling fluid or casing. When a drill casing is used, the boring is advanced sequentially by first driving the casing to the desired sample depth, and then cleaning out the hole to the bottom of the casing, inserting the sampling device, and finally obtaining the sample from below the bottom of the casing. The casing is usually selected based on the outside diameter of the sampling and stiffness considerations.

Where drilling fluids are used to stabilize the borehole walls, the casing should still be employed at the top of the hole to protect against ground sloughing. In addition to stabilizing the borehole walls, the drilling fluid (commonly water and bentonite) also removes the drill cuttings from the boring. In granular soils and soft cohesive soils, bentonite is typically used to increase the weight of the fluid and minimize soil stress reduction at the bottom of boring. For borings advanced with the use of fluids, it is important to maintain the level of the fluid at or above the ground surface to maintain a positive pressure for the full depth of boring.

The properties of the drilling fluid and the quantity of water pumped through the bit determine the size of particles that can be removed from the boring. Examination of the materials suspended in the wash fluid and the drill cuttings provides an opportunity to identify changes in soil conditions between sample locations. In some instances (especially with uncased holes) the drilling fluid return is reduced or lost. This is indicative of open joints, cavities, gravel layers, high permeable zones, and other rock conditions that may cause a sudden loss in pore fluid and must be noted on the logs and described accordingly.

3 .8.4 Drilling in rock

Rock core drilling procedures are used when formations are encountered that are too hard to be sampled by soil sampling methods mentioned above. This could for instance be the case in bridge foundation investigation for drilled shafts and piles. Defining the top of a rock stratum from soil boring operations can be difficult when large boulders exist deep



below river beds. A penetration of 25mm or less by a 51mm diameter split-barrel sampler following 50 blows using standard penetration energy or other criteria established by a geotechnical engineer should indicate that soil sampling methods are not applicable and rock drilling or coring is required. In many instances, geophysical methods, such as seismic refraction, can be used to assist in evaluating the top of rock elevations in an expedient and economical manner.

Drilling into rocks can be performed by either a rotary or percussion system. Rotary coring is most commonly used when an intact core of the rock is desired. The drilling bits are specifically designed to core rock, and the inner and outer tubes or casings are used to capture the intact core. Percussion drilling is often used to penetrate hard rock. The drill bit works much like a jackhammer, rising and falling to break up and crush the rock material. Air is commonly used to clean the hole and transport the cuttings to the ground surface.

Rotary rock coring can be accomplished with either conventional or wire-line equipment. With conventional drilling equipment, the entire string of rods and core barrel are brought to the surface after each core run to retrieve the rock core. Wire-line drilling equipment allows the inner tube to be uncoupled from the outer tube and lifted rapidly to the surface by means of a wire line hoist. The main advantage of wire-line drilling over conventional drilling is the increased drilling production resulting from the rapid removal of the core from the hole.

Wire-line drilling also provides improved quality of recovered core, particularly in soft rock, since it avoids rough handling of the core barrel during retrieval of the barrel from the borehole and when the core barrel is opened. Wire-line drilling can be used on any rock coring job. Typically, it is used on projects where boreholes are greater than 25m deep and rapid removal of the core from the hole has a greater implication on cost. Wire-line drilling is also an effective method for both rock and soil exploration though cobbles and boulders.

3.9 Pit, Trench and Boring Logs

Field logs of pits, trenches and borings are basic records that contain the descriptions of exploration procedures and subsurface conditions encountered during excavation. For this reason, logs should be prepared in a standard format that is simple and comprehensive. The formats to be used for a given type of logging depend on local practices.

The information that needs to be recorded in pit and trench logs is: The name of the road project; Client and Consultant name (including Contractor’s name for construction); Absolute (geographic) locations of the pit or trench (with coordinates if available); The total depth of the pit or the length of trench; Road chainage; Relative position and offset of the pit from the proposed centreline; The descriptions of soils (consistency, moisture, density, geologic origin); and Soil sampling interval and depth.



Likewise, in addition to the above information, boring logs should contain: Records on borehole and casing diameter; Boring orientation and total depth; The types and depths of soil and rock strata; Engineering description of rocks; Comparative resistance to boring; Type of drilling operation used to advance and stabilize the hole; Sampler type; Total and solid core recovery; Loss of drilling fluid; The depth to steady level of groundwater; The name of the individual who has provided the description; The person that approved the accuracy of the total input, and Any other remarks and observations.

Photographs should also be taken of the test pit and boring locations, as well of the soil horizons in the test pit. This will help revising the interpretation during design. Selected, representative photographs should also be included in the site investigation report. Samples of pit and boring log templates are given in Tables 3.5 and 3.6. These templates are used in this document simply to provide the reader with an idea of the basic information that should be included in a boring log. Specific projects are likely to require more detailed log formats.

Table 3.5: Example of a Pit Log

Project name Client Ethiopian Road Authority (ERA)

Project No. Consultant

Chainage Easting Total pit depth (m) Location Northing Position from centreline Right/Left

Date Elevation (m) Offset from centreline (m) ---

Logged by Inspected by

Depth (m) Description

x Clay, with some sands and gravels, very dark (black) at the top but becomes reddish downward, slightly weathered, stiff, plastic,

y Colluvium, dark brown, slightly weathered and discoloured, dense

z Tuff, , light yellow to grey, slightly weathered and discoloured, dense



Table 3.6: Example of a Standard Boring Log

Borehole log description Sheet 1 of X Project Name Project (Contract) No. Client Ethiopian Road Authority Consultant Contractor Borehole No Borehole orientation Logged by: Easting Borehole length (m) Northing Starting BH diameter (mm) Elevation Finishing BH diameter (mm) Location Casing diameter (mm) Road Chainage Drilling method Inspected by: Purpose Core tube Starting date Core barrel size (rocks) Ending date Core recovery ratio (%)

Depth (m)

Casing depth (m)

Water level (m)

SPT Description Graphic

log CR (m)

RQD (%)

Sample details Blows

(N) Drive (mm)

From-to (m) No

0.3 No Dry

Loose sand with rounded gravels of basalt and rubbles of limestone

0.1 0

1.0 No Dry

Dark brown, slightly plastic clayey sand with sub-rounded cobbles of basalt

0.5 0

1.4 1.0 Dry

Loose sand with cobbles of basalt with a 5cm basalt core at the end

0.3 0

1.9 Dry Brown clayey sand with cobbles of basalt and limestone

0.4 0

5.2 3.0 Dry

Sand mixed with 40-50%, large (2cm in diameter), sub-rounded limestone cobbles, very loose and unconsolidated

2.1 0 3-3.4 1

6 Dry Limestone cobbles with 10-20% gravel, highly rounded and loose

0.5 0

3.10 Sampling Soil samples obtained from surface and subsurface investigations for engineering tests are either disturbed or undisturbed. Disturbed samples are those obtained using equipment that destroy the structure of the soil but do not alter its composition. Sources can be pits, trenches, auger flights or rotary boring techniques. Disturbed samples are usually collected using split-barrel samplers or continuous helical flight augers. Specimens from these samples can be used for identification of soil components, general classification purposes,



and determination of some engineering characteristics such as grain size, consistency limits and compaction.

Undisturbed samples consist of material which is extracted from the site and transported to the laboratory with a minimum disturbance. Undisturbed soil samples are required for performing laboratory strength and consolidation tests on cohesive soils having consistencies ranging from soft to stiff. High-quality samples for such tests are particularly important for approach embankments and for structural foundations and wall systems that may stress compressible strata. The goal of high-quality undisturbed sampling is to minimize the potential for alteration of the soil structure and changes in moisture content or void ratio.

The ideal undisturbed sample is a cube of soil specimen, hand cut from a test pit or trench, carefully packed and sealed on site and transported to the laboratory without delay. However, due to economic factors and the limitations of depth of test pits, it is more usual to use different size thin-walled steel tubes (“Shelby” tubes) to obtain samples of soft to stiff cohesive soils from boreholes. The tubes are pushed into the soil using an adaptor connected to the drill rods. Common thin walled tube sizes are 50mm, 63mm, and 75mm.

Samples should be identified and sealed against moisture loss using either a sample tube sealing device or layers of molten wax as soon as the sample is recovered from the hole. Undisturbed samples should preferably be tested within two weeks of sampling as they rust into the tube and dry out. On extrusion in the laboratory, a proportion of undisturbed samples often prove to have been partly disturbed by the sampling process and it is prudent to take enough samples to allow for this. If possible, all specimens should be kept and photographed for an inspection by the pavement design engineer for tendering purposes and use during construction.

In rock coring, the dimensions and types of core barrel, type of coring bit, and drilling fluid are important variables. The minimum depth of rock coring should be determined based on the local geology of the site and the type of structure to be constructed. There are single, double and triple barrels to take a core from a rock. Since the double core barrel isolates the rock from the drilling fluid stream to yield better recovery, it is the minimum standard of core barrel that should be used in practice when an intact core is required for testing. Rock is sampled with core barrels having either tungsten-carbide or diamond core bits.

Rock coring can be performed in different core sizes. The standard size is the NX, which has a diameter of 54 mm. Generally, large core sizes will lead to less mechanical breakage and yield greater recovery, but the associated cost for drilling will be much higher. Since the size of the core affects the percentage recovery, the core barrel size should be clearly recorded on the log. Additionally, the core barrel length can increase recovery in fractured and weathered rocks. In these rocks, a core barrel length of 1.5m is recommended. Core barrel lengths should not be greater than 3m because of the potential for damage.

In many instances, clear water is used as a drilling fluid in rock coring. Sometimes, drilling mud is required instead of clean water to stabilize collapsing holes or to seal zones where there is loss of drill water. However, drilling mud clogs open joints and fractures and can adversely affect permeability measurements and piezometer installations.

The suitability of cores for structural foundations depends on the quality of cores measured as core recovery. The core recovery (CR) is the length of rock recovered from a core run.



The recovery ratio is the ratio of the length of core recovered to the total length of the core drilled on a given run, expressed as a percentage. It is an indication of soundness and degree of weathering of rocks and an important parameter for foundation design.

The rock quality designation (RQD) is a quantitative measure that represents a modified core recovery percentage. By definition, the RQD is the sum of the lengths of all pieces of sound core over 100mm long divided by the length of the core run. The RQD is an index of rock quality. Problematic rocks (highly weathered, soft, fractured, sheared, and jointed) typically yield lower RQD values than more intact rocks. Thus, RQD is simply a measurement of the percentage of sound rock recovered from an interval in a borehole. It should be noted that the original definition of RQD was based on measurements made on NX-size cores.

All borings should be properly closed at the completion of the field exploration. This is typically required for safety considerations and to prevent groundwater contamination.

3.11 In-situ tests

In many cases it is preferable to describe or measure the properties of soils and rocks at the investigation site. This can be done with the help of in-situ tests. In pavement design, in-situ tests can be used to rapidly evaluate the variability of sub-grade conditions, identify uniform sections, locate regions that require sampling and testing, and provide estimates of design values.

For sub-surface investigation, the most utilized in-situ methods are the standard penetration test (SPT), the dynamic cone penetrometer (DCP), and the cone penetrometer test (CPT). DCP and CPT especially offer more efficient and rapid way of sub-grade characterization and have a greater reliability than SPT. Other in-situ tests, such as pressure meter (PM) and dilatometer test (DMT), are also useful to obtain in-situ design properties, but are time intensive and require special skilled personnel. The vane shear test (VST) measures the undrained shear strength of soft to firm clays and is useful in stability analyses of cuts and fills.

The SPT is useful to estimate the relative density, effective friction angle, deformation modulus of cohesionless soils, and to assess the liquefaction potential of saturated sands.

The DCP and CPT provide information on subsurface soils, without sampling disturbance effects, with data collected continuously on a real-time basis. In situ strength characteristics at the prevailing density and moisture content are obtained as the DCP or CPT progresses. Since all measurements are taken in the field and there are no samples to be tested, it is possible to save considerable cost. However, it should be noted that where pavement design methods and catalogues are based on soaked CBR strength, the predicted CBR from the in situ DCP rate of penetration should be used with care.

DCP is more qualitative than CPT. It is performed with low cost, lightweight equipment, and a few personnel, and is suitable for general assessment as described in Table 3.7. the DCP is an excellent tool to perform initial exploration of pavement surfaces in rehabilitation projects (Appendix A). Results of DCP tests from the main pavement can also be compared to those in the shoulders for road widening and upgrading projects. The DCP can also be an effective tool to evaluate the suitability of the sub-grade after cut, fill,



or stabilization, although it is only useful for identifying variations in the upper part of the ground.

CPT provides more quantitative results, can be correlated directly to design properties and types of sub-grade, and can be used to greater depths in fine grained and sand type soils than the DCP. CPT is useful to obtain estimates of the relative density, effective friction angle, drained Young’s Modulus (E) of cohesionless soils, and the undrained shear strength of soft cohesive soils. The CPT is particularly useful in alluvial foundations where sandy soils are inter-layered with clays as the instrument is able to detect the layering better than most boring techniques. The CPT has been used successfully to locate softened zones, and it is best suited to define the subsurface stratigraphy. The test is also relatively consistent and repeatable.

Each investigation method listed above has advantages and limitations that should be considered when planning a subsurface investigation. The results from SPT are for instance highly variable and uncertain, and the method cannot be used in soft clays and silts. DCP is an index test where no samples are obtained. It is highly variable in gravelly soils and limited in depth to about 1m, although this depth is adequate for most rehabilitation projects and good for rapid surface characterization. In some instances, extraction of the cone can also be difficult after the test. The drawback of CPT is the lack of samples and limitations with pushing past obstructions. It requires a skilled operator to run, is affected by electronic drift and noise, and is unsuitable for gravel or boulder deposits unless a special rig is attached.

The relevance of each in-situ test also depends on the type of the road, the problem at hand, the stage in design, and, as shown in Table 3.7, the material types encountered during site investigation.

Table 3.7: Common In-Situ Tests for Foundation Investigation

Type of test Best suitable for

May not be applicable for

Properties that can be determined for road design

and construction Remarks

Standard Penetration Test (SPT)

Sand and Silt

Gravel, questionable results in saturated silt.

Crude estimate of modulus in sand. Disturbed samples for identification and classification. Evaluation of density for classification.

Test best suited for sands. Estimated clay shear strengths are crude and should not be used for design.

Dynamic Cone Penetrometer (DCP)

Gravel Sand, Silt and Clay

Clay with varying gravel content. Chemically stabilised materials .

Qualitative correlation to CBR. Identify spatial variation in sub-grade soil and stratification.

Good to evaluate the sub-grade, cut and fill.

Cone Penetrometer (static cone) Test (CPT)

Sand, Silt and Clay

Undrained shear strength and correlation to CBR in clays, density and strength of sand and gravel. Evaluation of sub-grade soil type, vertical strata limits, and groundwater level.

Use piezocone for pore pressure data. Tests in clay are reliable only when used in conjunction with other calibration tests.

Chapter 4 Site Investigation Manual – 2013 Soil and Rock Description and Classification


4 SOIL AND ROCK DESCRIPTION AND CLASSIFICATION

4.1 Introduction Description is the process of visual observation and estimation of the relative percentage of each component of a soil or rock to accurately explain and identify the material being investigated and sampled. It is mainly carried out in the field but may be refined in the laboratory. Classification is a laboratory-based process of grouping soils and rocks with similar engineering characteristics. During pitting, trenching or boring, the engineer describes the soils encountered. Samples are later returned to the laboratory where classification may be carried out based on the measured properties.

4.2 Soil description

Soil description is the systematic identification and complete naming of individual soils. In site investigation, a thorough and accurate description of soils is important in establishing general engineering properties for the design of the road and anticipated behaviour during construction. Soils are described in accordance with AASHTO M 145 or ASTM D 2488.

The description of the soil should include the following: Apparent consistency (e.g. soft, firm, etc for fine-grained soils) or density

adjectives (e.g. loose, dense, etc. for coarse-grained soils); Water content condition adjective (e.g. dry, moist or wet); Colour description (e.g. brown, grey etc.); Main soil type name (e.g. sand, clay, silt or combinations); Descriptive adjective for main soil type (e.g. for coarse-grained soils: fine, medium,

coarse, well-rounded, angular, etc., for fine-grained soils: organic; inorganic, compressible, laminated, etc.);

Particle-size distribution adjective for gravel and sand (e.g. uniform or well-graded);

Plasticity adjective (e.g. high or low) and soil texture (e.g. rough, smooth, slick, waxy, etc.) for inorganic and organic silts or clays;

Descriptive term for minor type(s) of soil (with, some, trace, etc.); Minor soil type name if the fine-grained minor component is less than 30 % but

greater than 12 %; or the coarse-grained minor component is 30 % or more (e.g. silty for fine grained, sandy for coarse-grained minor soil type);

Descriptive adjective “with” if the fine-grained minor soil type is 5 to 12 % (e.g. with clay) or if the coarse-grained minor soil type is less than 30 % but 15 % or more (e.g. with gravel);

Inclusions (e.g. concretions or cementation).

Additional information that may be included in the soil description form includes apparent consistency (for fine-grained soils) or a density adjective (for coarse-grained soils), geologic origin, the presence of roots and any sign of organic matter, and the existence of mica, gypsum, salt, etc. The following is an example of a complete soil description.

Clayey gravel with sand and cobbles; approximately 50 % coarse, sub-rounded to sub-angular gravel; about 30 % fine to coarse, sub-rounded, less strong sand; roughly 20 % fines with medium plasticity, high dry strength, no dilatancy, medium toughness; dark

Chapter 4 Soil and Rock Description and Classification Site Investigation Manual – 2013


brown, relatively dry, weak reaction with HCl; original field sample had about 5 % (by volume) sub-rounded cobbles with maximum size of 150 mm.

Sometimes, short descriptions can be used in local practice where much of the above detail is difficult to obtain. When this is the case, a simple form of describing a fine grained soil could be: soft, wet, grey, high plasticity clay with fine alluvial sand. A coarse grained soil is described in the same manner as dense, moist, brown, silty sand with an appreciable amount of gravel.

4.3 Coarse grained soils

Coarse-grained soils consist of a matrix of either gravel or sand in which more than 50% by weight of the soil is retained on the 75 micron (µm) sieve. Coarse-grained soils may contain fine-grained soil (i.e. soils passing the 75 µm sieve) but the percentage by weight of the fine-grained portion is less than 50%. The gravel and sand components are defined on the basis of particle size as indicated in Table 4.1. The particle size distribution is identified as well-graded or poorly-graded. Well-graded coarse-grained soil contains a good representation of all particle sizes from largest to smallest, with 12 % fines. Poorly graded coarse-grained soil is uniformly graded, i.e. most of the coarse grained particles are about the same size with 12 % fines. Gap graded coarse grained soil can be either a well graded or poorly graded soil lacking one or more intermediate sizes within the range of the gradation.

4.4 Fine grained soils Fine-grained soils are those having 50% or more by weight of material that pass the 75 µm sieve. The fines are either inorganic or organic silts and/or clays. To describe fine grained soils either in the field of laboratory, plasticity and soil-type adjectives should be used.

Table 4.1: Particle Size Definition for Gravels and Sands Component Grain Size Determination

Boulders* 300 mm + Measurable Cobbles* 300 mm to 75 mm Measurable

Gravel Coarse 19 mm to 75 mm Measurable

Fine 19 mm to 4.75 mm Measurable

Sand

Coarse 4.75 mm – 2.00 mm (#4 to #10 sieve)** Measurable and visible to the eye

Medium 2.00 mm – 0.425 mm (#10 to #40 sieve)** Measurable and visible to the eye

Fine 0.425 mm- 75 micron (#40 to #200 sieve)** Measurable but barely discernible to the eye

*Boulders and cobbles are not considered part of the soil's classification, but described as “with cobbles at about 5% (volume)”.

** Previous ASTM nomenclature.

Fine-grained soils where the organic content appears to be less than 50% of the volume should be described as soils with organic material or organic clays and silts. If the soil



appears to have an organic content greater than 50% by volume it should be described as peat.

Some subjective indicator tests may be useful in determining the plasticity characteristics of fine grained soils in the field. These tests include the dilatancy or reaction to shaking test, the dry strength test, the toughness test and plasticity tests. The description of these tests is given in Table 4.2. For each test, the fraction which passes the 0.425 mm sieve is used and corresponds to the fraction which is required for determination of Atterberg limits. For the purpose of the visual tests, screening is not important and the removal of coarse particles is adequate. Table 4.3 presents a method to describe plasticity as a function of dry strength.

Table 4.2: Field Identification Procedures for Fine Grained Soils

Test Test Procedures and Interpretation of Results

Dilatancy

Prepare a pat of moist soil with a volume equivalent to a 25-mm cube. Add water, if necessary, to make the soil soft but not sticky. Place the pat of soil in the open palm of one hand and shake horizontally; strike vigorously against the other hand several times. If the reaction is positive, water appears on the surface of the pat; the consistency of the pat then becomes livery; and the surface of the pat becomes glossy. Next, squeeze the sample between the fingers. The water and gloss should disappear from the surface of the pat; the soil will stiffen and crack or crumble. The rapidity of the appearance of water on the surface of the soil during shaking and its disappearance during squeezing help to identify the character of the fines in the soil. Very fine clean sands give the quickest and most distinct reaction, inorganic silts give a moderately quick reaction, and plastic clays have no reaction.

Dry Strength

Mould a pat of soil to the consistency of putty. If the soil is too dry, add water; if it is too sticky, the specimen should be allowed to dry by evaporation. After the consistency of the pat is correct, allow the pat to dry (by oven, sun, or air). Test its strength by breaking and crumbling between the fingers. The dry strength increases with increasing plasticity. High dry strength is characteristic of high plasticity clays. Silty sand and silts have only slight dry strengths, but can be distinguished by feel when powdered; fine sands feel gritty whereas silts feel smooth like flour. It should also be noted that shrinkage cracks may occur in high plasticity clays. Therefore, precautions should be taken to distinguish between a shrinkage crack as opposed to a fresh break which is the true dry strength of the soil.

Toughness and Plasticity

A specimen of soil which is about the size of a 25-mm cube should be moulded to the consistency of putty; add water or allow drying as necessary. At the proper moisture content, roll the soil by hand on a smooth surface or between the palms into a thread about 3-mm in diameter. Fold the thread of soil and repeat the procedure a number of times. During this procedure, the water content of the soil is gradually reduced. As drying occurs, the soil begins to stiffen and finally loses its plasticity and crumbles at the plastic limit. After the thread has crumbled, the pieces should be lumped together and a kneading action should be applied until the lump crumbles. For higher clay contents, threads are stiffer and lumps are tougher at the plastic limit than for lower plasticity clays.



Table 4.3: A Field Method to Describe Plasticity in terms of Dry Strength

Plasticity Adjective Dry strength

0 Non-plastic None; crumbles into powder with mere pressure.

1 - 10 Low plasticity Low; crumbles into powder with some finger pressure.

>10 - 20 Medium plasticity Medium; breaks into pieces or crumbles with considerable finger pressure.

>20 - 40 High plasticity High; cannot be broken with finger pressure; spec. will break into pieces between thumb and a hard surface.

>40 Very plastic Very high; can’t be broken between thumb and a hard surface.

Other tests which may help in distinguishing sands and fine grained soils in the field are summarized in Table 4.4. The dispersion (settlement in water) test and the bite test can be used to determine the presence of and relative amounts of sand, silt, and clay fractions. The odour and the peat tests are useful for determining the presence of organic matter, the acid test for identifying the presence of a calcium carbonate cementing agent, and the slaking test for determining whether the rocklike material is shale. Like the other three tests mentioned in Table 4.2, these tests are also performed on particles that pass the 0.425 mm sieve which is the division between medium and fine sand. For field classification purposes, screening is not necessary. Instead, the removal of the coarse particles that interfere with tests is sufficient.

When both fine and coarse grained soils are present in a soil mass in appreciable quantity, the description is in such a way that the name of the second soil type is used as an adjective. For example when the percentage of the fine-grained soil type is less than 30% but greater than 12% of the total sample, the soil is described as “silty” or “clayey”, depending on which particle size is dominating. Similarly, the description becomes “gravelly” or “sandy” if the coarse-grained component is 30% or more of the total sample. When the percentage of the fine-grained soil is 5 to 12%, the description contains “with silt” or “with clay”. The coarse grained equivalent is used when the sand and gravel is less than 30% but higher than 15% of the total sample.



Table 4.4: Additional Tests to Identify Fine Grained Soils in the Field

Test Test Procedures and Interpretation of Results

Disperssion test

Place a few hundred grams of soil in a jar containing water. Shake the jar and allow the soil to settle. The rate of settling can be used to judge the (settlement predominate soil type(s) whereas the thicknesses of the various soils can be used to judge the gradation of the soil. Sands settle in 30 to 60 seconds, silts settle in 30 to 60 minutes, and clays may in water) remain in suspension overnight. The interface between fine sands and silts occurs where individual grains cannot be discerned with the unaided eye. The cloudiness of the water indicates the relative clay content.

Bite test Place a pinch of soil between the teeth and grind lightly. Fine sands grate harshly between the teeth; silts have a gritty feeling but do not stick to the teeth; clays tend to stick to the teeth, but do not have a gritty feeling

Odour test Organic soils have a musty odour which diminishes upon exposure to air. The odour can be revived by heating a moist sample or by exposing a fresh sample.

Peat Peat has a fibrous texture and is characterized by partially decayed sticks, leaves, grass, and other vegetation. A distinct organic odour is characteristic of peat. Its colour generally ranges from dull brown to black.

Shine Moist highly plastic clay will shine when rubbed with a fingernail or pocketknife blade; lean clay will have a dull surface.

Acid test

The presence of calcium carbonate in a soil can be determined by adding a few drops of dilute (3:1 ratio of water to acid) hydrochloric acid to the soil. The relative amount of calcium carbonate in the soil can be determined by the effervescence (fizzing reaction) which occurs. Degrees of reaction range from none to strong. For some very dry non-calcareous soils, the illusion of effervescence as the acid is absorbed by the soil can be eliminated by moistening the soil before the acid is applied.

Slaking test

Certain shales and other soft rock-like materials disintegrate upon drying or soaking. The test is performed by placing the soil in the sun or oven to dry completely. After the sample has been dried, it should then be soaked in water. The degree of slaking should be reported.

4.5 Soil classification The most widely used soil classification is the Unified Soil Classification System (ASTM D 2487). The USCS outlines field procedures for determining plasticity, dilatancy, dry strength, particle size, and other engineering parameters. The AASHTO classification system (M 145), which is also commonly used for highway projects, groups soils into categories having similar load carrying capacity and service characteristics for pavement sub-grade design. The USCS is provided here for information but in most cases of site investigation in Ethiopia, the AASHTO classification system is recommended.

The USCS is based on identifying soils according to their textural and plastic characteristics, and on their grouping with respect to behaviour. Soils seldom exist in nature separately as sand, gravel, or any other single component. They are usually found as mixtures with varying proportions of particle sizes. Each component part contributes its characteristics to the soil mixture. The USCS is based on those characteristics that control how the soil behaves as an engineering material.



As illustrated in Figure 4.1, the following properties have been found most useful for this purpose and form the basis of soil identification. They can be determined by simple tests and, with experience, can be estimated with some accuracy.

Percentages of gravel, sand, and fines (fraction passing the 75 micron sieve). Shape of the grain-size-distribution curve. Plasticity and compressibility characteristics. In the USCS, the soil is given a

descriptive name and a letter symbol indicating its principal characteristics. Soils are primarily identified as coarse grained, fine grained, and organic. On a textural basis, coarse-grained soils have 50% or more by weight of the overall soil sample retained on the 75 µm sieve (No. 200 sieve in Figure 4.1) and fine-grained soils are those that have more than 50% by weight passing the 75 µm sieve (No. 200 sieve in Figure 4.1). Highly-organic soils are, in general, readily identified by visual examination. The coarse-grained soils are subdivided into gravel and gravelly soils (G) and sands and sandy soils (S). Fine-grained soils are subdivided on the basis of their liquid limit (LL) and plasticity properties. The symbol L is used for soils with LL of 50 and less and the symbol H for soils with LLs in excess of 50. Peat and other highly organic soils are designated by the symbol Pt. The Unified Soil Classification System is shown in figure 4.1.



Figure 4.1: The Unified Soil Classification System (USCS).

The AASHTO soil classification system is shown in Table 4.5. The AASHTO classification system is useful to determine the relative quality of the soil material for use in earthwork structures, particularly embankments, sub-grades, sub-bases and bases.



Table 4.5: The AASHTO soil classification system

General classification Granular materials (35% or less of total sample passing 75 micron (No. 200 sieve)

Silty-clay materials (More than 35% of total sample passing 75

micron (No. 200 sieve)

Group classification A-1 A-3 A-2

A-4 A-5 A-6 A-7

A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7 A-7-5, A-7-6

Siev

e an

alys

is,

perc

ent p

assi

ng 2 mm

(No 10) 50 max

0.425 mm (No. 40) 30 max 50 max 51 min

75 micron (No. 200) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min

Cha

ract

eris

tics

of fr

actio

n pa

ssin

g N

o 40

(0

.425

mm

) LL 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min

PI 6 max NP 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min*

Usual significant constituent materials

Stone fragments, gravel and sand Fine sand Silty or clayey gravel and sand Silty soils Clayey soils

Group Index** 0 0 0 4 max 8 max 12 max 16 max 20 max

Classification procedure: With required test data available, proceed from left to right on chart; correct group will be found by process of elimination. The first group from left into which the test data will fit is the correct classification. *Plasticity Index of A-7-5 subgroup is equal to or less than LL minus 30. Plasticity Index of A-7-6 subgroup is greater than LL minus 30. **See group index formula below. Group index should be shown in parentheses after group symbol as: A-2-6(3), A-4(5), A-7-5(17), etc. GI = (F-35)[0.2+0.005(LL-40)] + 0.01(F-15) (PI-10)



According to the AASHTO system, soil is classified into seven major groups, A-1 through A-7. Soils classified under groups A-1, A-2 and A-3 are granular materials where 35% or less of the particles pass through the 75 µm (No. 200 sieve). Soils where more than 35% pass the 75 µm (No. 200 sieve) are classified under groups A-4, A-5, A-6 and A-7. Soils where more than 35% pass the 75 µm (No. 200 sieve) are mostly silt and clay-size materials.

To evaluate the quality of a soil as a sub-grade material, the group index (GI) is also used along with the groups and subgroups of the soil. The group index is written in parenthesis after the group or subgroup designation. The GI is shown in Table 4.5 where F is the percentage passing the 75 µm (No. 200 sieve), LL is the liquid limit, and PI is the plasticity index. Rules related to interpreting the GI are:

If the equation yields a negative value, GI is taken as zero. The group index calculated from the equation is rounded off to the nearest whole

number (e.g. GI = 3.4 is rounded off to 3; GI=3.5 is rounded off to 4). There is no upper limit for the group index. The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3

will always be zero. When the group index for soils belonging to groups A-2-6 and A-2-7 is calculated,

the partial group index for PI should be used, or GI = 0.01(F-15) (PI-10).

4.6 Engineering characteristics of soils

In the previous sections, soils are divided into coarse and fine grained using either the USCS or AASHTO classification systems. The engineering characteristic of a soil mass depends on the proportion of these two groups of soils, and is governed by the one which dominates. 4.6.1 Coarse grained soils

Grain size distribution is the main important factor that controls the engineering behaviour of granular soils. Much can be learned about a soil’s behaviour from the shape and location of the grain size distribution curve (poorly-graded, well-graded, and gap-graded). For instance, densification of a well-graded soil causes the smaller particles to move into the voids between the larger particles. As the voids in the soil are reduced, the density and strength of the soil increases. Specifications for select fill should contain required ranges of different particle sizes so that a dense, non-compressible backfill can be achieved with reasonable compactive effort.

A poorly graded or uniform soil is composed of a narrow range of particle sizes. When compaction is attempted, inadequate distribution of particle sizes prevents reduction of the volume of voids by infilling with smaller particles. Uniform soils are not good for selected fill. However, uniform soils do have an important use as drainage materials. The relatively large and permanent void spaces act as conduits to transmit water.

In general, the following are the main engineering characteristics of coarse grained soils: Generally very good foundation material for supporting structures and roads; Generally very good embankment material; Generally the best backfill material for retaining walls;



May settle under vibratory loads or blasts; Dewatering may be difficult in open-graded gravels due to high permeability; Generally not expansive.

4 .6.2 Fine grained soils

The plasticity index (PI) is the main parameter that governs the engineering performance of fine-grained soils. In addition to the PI, the liquidity index (LI) and liquid limit (LL) are also useful indicators of the engineering performance of fine grained soils. The PI represents the range of water content over which the soil remains plastic and normally the higher the PI, the higher the percentage of clay particles in the soil. Also, the more plastic a soil, the more likely it is to be compressible. It will have a greater potential to shrink and swell and it will be less permeable.

In general, the engineering characteristics of inorganic clays (A-6 and A-7) are the following:

Generally possess low shear strength; Plastic and compressible; Can lose part of shear strength upon wetting; Can lose part of shear strength upon disturbance; Can shrink upon drying and expand upon wetting; Generally very poor material for backfill; Generally poor material for embankments; Can be practically impervious; Clay slopes are prone to landslides.

The engineering characteristics of inorganic silts (A4 and A-5) are: Relatively low shear strength; Relatively low permeability; Difficult to compact.

4.7 Rock 4 .7.1 Description An engineering description of a rock should include those features which are significant in influencing its engineering performance. Most rocks are cut by discontinuities which characteristically have little or no tensile strength. In many cases, the engineering performance (strength, compressibility, permeability and durability) of any rock mass is influenced by these fractures and their inclusion is clearly an important aspect of rock description.

Ideally, the best way to obtain a comprehensive description of a rock mass is by careful examination of large exposures. However, in many site investigations of rock it is not possible to gain access to large exposures and hence the rock mass description has to be derived mainly from borehole information. Boreholes provide a reasonable means for examining the rock material but do not permit a comprehensive description of the discontinuities.



In road design and construction, describing rocks is often necessary in situations such as rock cuttings, blasting and tunnelling. Rocks should also be described during an investigation of bridge foundations. In addition, deep seated landslide and groundwater investigations may require complete rock descriptions. When providing rock descriptions, there is a need to use technically correct geological terms. Local terms are acceptable if they describe distinctive characteristics. Rock cores should be logged when wet for consistency of colour descriptors and greater visibility of rock features. Methods of rock material and discontinuity descriptions are summarized in Appendix B and should include as a minimum the following items:

Rock (unit or formation) name; Lithology with lithological descriptors;

o Composition (mineralogy); o Grain/particle size; o Texture; o Colour;

Bedding/foliation/flow structure; Discontinuities (includes fracture indexes); Weathering; Strength/Hardness.

Generally, it is advisable to write the description in the order listed above for ease of understanding. For instance:

“Sandstone, red, very fine-grained, thinly-bedded and highly fractured, iron-oxide cemented, slightly weathered, relatively strong" 4.7.2 Rock name Like any soil mass, every rock should be properly identified by its name. Rock names or rock unit names are not only required for identification purposes but may also provide indicators of the depositional environment and geologic history, geotechnical characteristics, and correlations with other areas. Hence, during site investigation, a simple descriptive name should be provided to allow designers to better understand the possible engineering characteristics of the rock.

Unlike soils where naming is largely based on engineering properties, rock names or rock unit names are connected with geological parameters such as stratigraphy (lower and upper sandstone), lithology (basalt, rhyolite, etc.), age (e.g. Tertiary lava flows), genesis (igneous, sedimentary, etc.) or a combination of these. The resultant names seldom bear any relation to engineering performance. In many cases, a full petrographic analysis is required to classify a rock specimen for geological applications. Such classification systems are too elaborate for engineering purposes, and usually provide little or no information of engineering significance. For engineering use the classification systems should be simplified and the number of rock names kept to a minimum. This is especially true in road construction. As much as possible, rock names should be geologically correct but simple enough for general understanding. 4.7.3 Lithological descriptions

Rocks are classified into three major groups (igneous, sedimentary, and metamorphic), based on their genesis as shown in Table 4.6. Igneous rocks are formed from the



solidification of molten magma. Sedimentary rocks are results of the accumulation of fragmental rock and organic material or by chemical precipitation. Metamorphic rocks are products of alteration of existing rocks through the action of heat and pressure. These three classes are further subdivided into different rock groups according to mineral and chemical composition and texture (grain size).

Igneous rocks are divided into intrusive (plutonic) and extrusive (volcanic). Intrusive igneous rocks are formed by the crystallization of magma deep in the earth while extrusive igneous rocks are produced when lava solidifies on the ground surface. The grain sizes of igneous rocks can range from very coarse (equivalent to gravel in a soil) to very fine (equivalent to silt and clay) and is related to the rate of cooling of the parent magma. Coarse-grained igneous rocks are associated with slow cooling rates, and fine-grained igneous rocks with a rapid process of solidification. If the magma cools very rapidly, in such a way that there is no time for crystals to develop, then a rock with glassy texture is produced. Intrusive igneous rocks are very coarse- to medium-grained while extrusive rocks are usually fine-grained (aphanitic), glassy or porous.

The main rock forming minerals in igneous rocks include quartz, feldspar, mica (muscovite, and biotite) and ferro-magnesian (mafic) minerals. Classification is based on the relative proportions of quartz, feldspar and mafic minerals. High proportions of quartz and feldspar give the rock a light colour, whereas the presence of a significant amount of mafic minerals results in a dark appearance. Light coloured igneous rocks, such as rhyolite, are called acidic while those which appear dark (for example basalt) are known as basic. Most igneous rocks are placed in these two categories. It is often difficult to identify intermediate igneous rocks in hand specimen.

Some igneous rocks, such as basalt, exhibit large crystals embedded in a finer-grained matrix. Such rocks are termed porphyritic and the large crystals are termed phenocrysts. Extrusive igneous rocks often have numerous spherical or ellipsoidal voids (vesicles). These are produced by the inclusion of gas bubbles within the magma as it cools. When these vesicles are present, the rock is said to be vesicular. In some cases, these voids may be filled with minerals. Such mineral filled inclusions are termed amygdales and the rock is known as amygdaloidal.



Table 4.6: Rock groups and types Igneous

Intrusive (Coarse Grained)

Extrusive (Fine Grained) Pyroclastic

Granite Rhyolite Pyroclastic breccia Syenite Trachyte Agglomerate Diorite Andesite Tuff (Ash) Gabbro Basalt Scoria (Cinder)

Peridotite Pumice Pegmatite

Sedimentary Clastic rocks Chemically formed Organic remains

Shale Limestone Chalk Mudstone Dolomite Lignite Clay-stone Gypsum Coal Siltstone Halite

Sandstone Conglomerate

Limestone Metamorphic

Foliated Non-foliated Slate Quartzite

Phyllite Amphibolite Schist Marble Gneiss Hornfel

Pyroclastic rocks are formed by the accumulation of volcanic material generated by the explosive fragmentation of magma. Characteristically, there are more pyroclastic rocks associated with acidic than basic magmas. Acid magmas are more viscous and release only little gas which results in high explosions. This may occur when the rising magma comes into contact with ground water. The different types of pyroclastic rocks are the following:

Ash tuff: rock dominated by ash particles (pyroclasts whose average size is less than 2 mm). When welded it is simply referred to as tuff.

Lapilli tuff: rocks dominated by lapilli (pyroclasts with a size in the range of 2mm to 62 mm).

Pyroclastic breccias: rocks containing at least 75% bombs (pyroclasts whose average size exceeds 62 mm), and in which angular pyroclasts predominate.

Agglomerate: rocks containing at least 75% bombs, mainly rounded.

Most sedimentary rocks are cemented aggregates of transported fragments derived from pre-existing rocks. Typically these rocks comprise rock fragments resistant to weathering and minerals derived from the chemical decomposition of pre-existing rocks (clay minerals) bound together with chemical precipitates (or cementing agents) such as iron oxide and calcium carbonate. Other forms of sedimentary rock include accumulations of organic debris (typically shell fragments or plant remains), and minerals that have been chemically precipitated.

As shown in Table 4.6., sedimentary rocks are broadly classified into three sub-groups: Clastic sedimentary rocks are formed by the accumulation of rock or mineral

fragments.



Organic sedimentary rocks are derived from accumulations of dead plants and other biodegradable material.

Chemical sedimentary rocks are formed by the accumulations of minerals chemically precipitated from surface or groundwater. Rocks formed by chemical precipitation generally have a crystalline texture. Most chemically precipitated rocks are water soluble.

Metamorphic rocks are derived from pre-existing rocks of all types (igneous, sedimentary and metamorphic) in response to marked changes in temperature or stress or both. An increase in temperature or pressure can cause the formation of new minerals and the partial or complete recrystallization of the parent rock with the development of new textures.

Metamorphic rocks are classified into foliated and non-foliated as shown in Table 4.6. Foliated metamorphic rocks contain laminated structure resulting from the segregation of different minerals into layers during the process of metamorphism. Non-foliated metamorphic rocks are re-crystallized and are generally massive or contain no distinct structures. 4 .7.4 Rock colour

Rock colour is not in itself a specific engineering property, but may be an indicator of the influence of other significant geologic processes that may be occurring in the rock mass (e.g. the presence of water, the action of weathering, etc.). In basic and acidic igneous rocks, it is normally associated with mineral composition of the constituent particles. In clastic sedimentary rocks, it may even be linked with the type of cementing material.

Colour is often the most noticeable feature of a rock but is possibly the most difficult to describe accurately. The colour of rock should be assessed objectively applying similar precautions to those advised in assessing the colour of soils. Wherever possible, colour should be compared with a standard chart. 4 .7.5 Bedding Often, sedimentary rocks contain a series of beds that need to be estimated (measured) and included in the description. Similarly, foliation and igneous layering should also be properly described. If present, these structures can affect the degree of rock fracturing during excavation and blasting. Moreover, they control the mechanism and extent of slope failure. Table 4.7 provides terms to describe the thickness of beds. The inclination of bedding or foliation with respect to the road cut should also be investigated and measured from the horizontal.

Table 4.7: Terminology for Layer Thickness

Very thickly bedded > 2 m Thickly bedded 600 mm – 2 m

Thinly bedded 60 mm – 600 mm

Very thinly bedded 20 mm – 60 mm

Laminated 6 mm – 20 mm

Thinly laminated < 6 mm



4.7.6 Weathering Weathering is the process of alteration and breakdown of rock. It comprises physical disintegration (physical weathering) and chemical decomposition (chemical weathering), which normally act together. The two extremes of weathering are an unaltered fresh rock and a residual soil. If unaffected, a complete weathering profile may be present in which a residual soil grades downwards through weathered rock into unaltered ‘fresh’ rock. Intermediate states of weathering are difficult, if not impossible, to identify if the dominant weathering mechanism and the appearance of the end members are not known, particularly in weak rocks.

Weathering is strongly influenced by climate (rainfall and mean temperature). In arid areas, physical disintegration is the main process, whereas chemical decomposition dominates in humid tropical regions. However, it should be pointed out that the progress of chemical decomposition usually relies on fractures formed partly as a result of physical disintegration. Similarly, fractures may develop in response to changes in volume and weakening from chemical weathering.

The main processes of chemical weathering depend on the presence of water and may result in the alteration or dissolution of the component minerals grains. In the case of sedimentary rocks, the cement which binds the grains together is also prone to chemical attack. Typically, the chemical decomposition of the rock material starts at discontinuity walls and works inwards towards the centre of the intact blocks. This is often associated with discoloration penetrating the rock from the discontinuity walls. In cases were cement is removed by solution, the rock may be friable adjacent to discontinuities, and the zone of discoloration may be absent. The degree to which the discoloration has penetrated the rock will indicate the degree of weathering.

The effect of only slight or moderate chemical decomposition will be to influence the shear strength and compressibility of the discontinuities with little effect on the intact rock. When the volume of chemically decomposed rock exceeds that of the fresh rock in intact blocks, the rock properties will be affected. It is likely that when the rock material is highly weathered, the discontinuities will not have a significant effect on the performance of the rock mass as would be the case if the rock material were fresh. This is particularly true with respect to compressibility.

Physical weathering of rock will generally cause the formation of new fractures, together with the opening of existing discontinuities. The action of physical weathering may only be recognized from variations in discontinuity spacing and aperture measurements. These measurements form an essential feature of rock mass descriptions. The decrease in discontinuity spacing and the general loosening of intact blocks of rock associated with this weathering process will have a significant influence on the performance of the rock mass.

In some areas, the simple weathering profile of fresh rock overlain by residual soil may not occur due to variations in rock type and geological structures. It is then possible for weathered rock to pass laterally into unweathered rock and for altered layers to exist below fresh rock. Alteration refers to changes in the chemical or mineralogical composition of a rock produced by the action of hydrothermal fluids. Alteration effects may be significant at increasing depths.



Table 4.8 shows the terminology used to describe the weathering state of a rock mass. A detailed description of weathering for selection of road construction materials is also given in Chapter 8. Generally, weathering is well described using rock cuts or exposures in the field. Attempts to describe the degree of weathering using rock cores are often very difficult. This is because a borehole does not provide a sufficient volume of the rock mass to permit an accurate assessment of the state of weathering. Indeed, the weathered state of the rock obtained from boreholes is based almost entirely upon the condition of the rock material. In cases where the processes of physical weathering dominate, the rock material will appear relatively fresh even when the fracture state of the rock mass may indicate a high degree of weathering throughout the area.

Table 4.8: Terminology for Rock Mass Weathering

Term Description Grade

Unweathered No visible sign of rock material weathering, perhaps slight discoloration on major discontinuity surfaces. I

Slightly weathered

Discoloration indicates weathering of rock material and discontinuity surfaces. All the rock material may be discoloured by weathering and may be somewhat weaker externally than in its fresh condition.

II

Moderately weathered

Less than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discoloured rock is present either as a continuous framework or as core-stones.

III

Highly weathered

More than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discoloured rock is present either as a discontinuous framework or as core-stones.

IV

Completely weathered

All rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact. V

Residual soil All rock material is converted to soil. The mass structure and material fabric are destroyed. There is a large change in volume, but the soil has not been significantly transported.

VI

Fresh No visible sign of weathering of the rock material.

Discoloured The colour of the original fresh rock material is changed. The degree of change from the original colour should be indicated. If the colour change is confined to particular mineral constituents, this should be mentioned

Decomposed The rock is weathered to the condition of a soil in which the original material fabric is still intact, but some or all of the mineral grains are decomposed

Disintegrated The rock is weathered to the condition of a soil in which the original fabric is still intact. The rock is friable, but the mineral grains are not decomposed

4 .7.7 Rock strength Rock strength is controlled by many factors including the degree of consolidation, cementation, crystal bonding, degree of weathering and alteration. Determination of relative rock strength can be estimated in the field using the Schmidt hammer rebound test



or the Point Load test. The latter is especially more reliable. The results can be refined later in the laboratory. Terminologies for describing rock strength based on the unconfined compressive strength are given in Table 4.9.

Table 4.9: Description of rock strength in the field

Description Field identification Unconfined

Compressive Strength (MPa)

Extremely weak Indented by thumbnail. 0.3 – 1

Very weak Crumbles under firm blows with point of geological hammer, can be peeled by a pocket knife. 1 – 5

Weak Can be peeled by a pocket knife with difficulty, shallow indentations made by firm blow with point of geological hammer.

5 – 50

Strong Specimen requires more than one blow of geological hammer to fracture. 50 – 100

Very strong Specimen requires many blows of geological hammer to fracture. 100 – 250

Extremely strong Specimen can only be chipped with geological hammer. > 250

4.7.8 Rock discontinuity Structural breaks or discontinuities generally control the mechanical behaviour of rock masses by forming planes of weakness or surfaces of separation. These weak planes or separations include foliation and bedding planes, joints, fractures, and shear zones. Discontinuities usually control the strength, deformation, and permeability of rock masses. In many cases, engineering problems are related to these discontinuities rather than to intact rock strength. Discontinuities should, therefore, be adequately described. This description should include all observable characteristics such as spacing, orientation, continuity, openness, surface conditions, and fillings.

The spacing between discontinuities is defined as the perpendicular distance between adjacent discontinuities. It is described in the field using the guide given in Table 4.10. The orientation is the direction of continuity and inclination. It is an important parameter to assess deformation and stability on rock faces. Seepage on road cuts may also be affected by the orientation of discontinuities. Continuity measures the continuous nature of a discontinuity. A continuous joint or fracture is weaker and more deformable than a short discontinuous one bridged by intact bedrock. The aperture (openness) is a separation of a discontinuity. It is measured normal to the fracture surface. Aperture affects the strength, deformability, and seepage characteristics. Fillings are secondary materials that exist within the openings of discontinuities. Describing fillings should contain their composition, thickness, alteration, weathering, and strength.

Fracture density is based on the spacing between all natural fractures in an exposure or along cores from boreholes, excluding mechanical breaks. Fracture frequency, on the other hand, is the number of fractures occurring within a unit length or survey-line.



Table 4.10: Discontinuity Spacing

Description Spacing of discontinuity Very widely spaced > 3 m

Widely spaced 1 – 3 m Moderately spaced 300 mm – 1 m

Closely spaced 30 - 300 mm Very closely spaced < 30 mm

Chapter 5 Site Investigation Manual – 2013 Site Investigation Stages


5 SITE INVESTIGATION STAGES

5.1 Introduction As indicated in Figure 1.1, road design and construction involves a multi-phased approach to site investigation. This is especially true in projects that involve new construction, or in those that pass through mountainous areas or are located in remote regions. The lengthy duration, limited access, and limited coverage of field surveys in these places may demand that site investigations be carried out in several phases to obtain the information necessary at each stage of the project cost effectively. Throughout the development of the project, the final alignment and profile may deviate from those originally anticipated. Dividing the investigation into phases provides a rational approach to produce the most acceptable design options with the best possible solutions to various problems.

Table 5.1 provides further detail on the basic procedural steps needed in a typical site investigation during the design of a road. The various steps or phases or stages are often carried out in the form of desk study, reconnaissance survey, preliminary investigation, and final investigation. The final work of compiling field and laboratory data and writing a report is usually performed in the office.

5.2 Desk study The objective of a desk study is to:

Identify the design data needs for the project; Assess design requirements; Evaluate performance criteria; and Search for areas of concern on site and potential variability of local geology.

After defining different options of road alignments earlier in the planning phase (see the ERA Route Selection Manual), it is essential that a complete description of the site, its accessibility, work requirements, and other preliminary information is made available for the upcoming surface and subsurface investigations. This can be done through the desk study programme. The desk study is also important to decide the number of design phases needed and the exploration methods to be used. A desk study also involves the identification of sources of information and the review of existing documents. 5.2.1 Identifying sources of information

The first step in the desk study is to identify the source of existing data. There are a number of very helpful sources of information in Ethiopia that can and should be used in planning surface and subsurface investigations for roads. Much of this information can be obtained for free or is available at low cost from various institutes. Their review at this stage can minimize surprises in the field, assist in determining boring locations and depths, and provide very valuable geological information which may have to be included in the site investigation report.

Chapter 5 Site Investigation Stages Site Investigation Manual – 2013


Table 5.1: Basic Steps for a Typical Investigation to Design a Road Task Site investigation phase

Identify sources of information

Desk study Locate available preliminary information useful to the project, Obtain other pertinent preliminary project related information, Find engineering and location study reports, Search for any design related documents. Review available information

Desk study

Review any site investigation reports and information for projects in the vicinity with emphasis placed on those in the same region, Review published information. Place emphasis on documents obtained from ERA, Obtain survey information such as cross sections, drawings, and plans. Plan field investigation

Reconnaissance survey Review checklists for site investigations, Determine types of investigations and equipment requirements, Determine site restrictions. A site visit may be required, Develop a preliminary boring and testing plan. Conduct field investigation

Preliminary and final investigation

Mobilize site investigation equipment, Excavate pits, trenches and boreholes, Prepare borehole logs, Describe soil and rock masses. Plan sampling and testing


Determine sampling and testing requirements, Perform in-situ tests, Record field information. Summarize field data.


Summarize soil and rock survey information, Summarize subsurface profile information, Review roadway cut and embankment checklists, Identify unstable slopes and landslide areas, Locate potential sources of construction materials and estimate extents, Identify water problem areas. Laboratory tests


Engineering property tests on the characteristics of the sub-grade, Classification and index tests of construction materials, Tests to confirm design values. Write a report

Office work Compile field and laboratory information, Draft report (to be revised by appropriate experts), Final report.

The task of looking for existing data generally involves researching for past information about the site. In road design, existing information may be in the form of unprocessed and processed data. Unprocessed data include topographic maps, aerial photos, and satellite images. Initial base maps for road projects are usually generated from existing topographic maps. Existing aerial photographs can be used as temporary base maps if topographic maps are not available. Prior photo-analysis is critical if the available time and funding is



limited. This is especially true in regional and rural roads where the provision of appropriate access is a priority. Processed data comes in the form of:

Geological maps; Existing borings; Previous bridge plans with plotted borings; Previous geotechnical reports; Previous subsurface investigations at or near the project site; Previous construction records of structures (i.e. pile length, drivability problems,

unpredicted settlement, etc.); Well records; Property ownership information; Locations of rivers, bridges, culverts, etc.; Flood zone maps or flood level records; Engineering (agricultural) soil maps; Site plans showing locations of ditches, driveways, culverts, utilities and pipelines: Pre-design plans, profiles and cross sections.

Table 5.2 provides names of federal and local agencies in Ethiopia where data relevant to site investigation exists. ERA’s library and documentation from Transport Construction Design Enterprise (TCDE) are also good sources of information. 5.2.2 Reviewing available information

A review of available information will help in early recognition of the characteristics of the site and potential geo-hazards. It facilitates appropriate scoping of the later stages of the subsurface investigation programmes, and can be used to assess the economics of filed investigation. In addition, it enables the formulation of a preliminary geotechnical ground model, and assists in the creation of efficient designs for roadway structures (e.g. foundations, retaining walls, tunnels, slope stabilization works, etc.).

For a road construction project, basic sources of geotechnical information should be reviewed to determine landform boundaries. A necessary part of reviewing available data is to identify the major geologic processes that occurred at the project site in the past. This permits the geotechnical specialist to develop an understanding of how the local soils and rocks will behave during and after the construction of the new structure or roadway.



Table 5.2: Ethiopian Data Sources for Site Investigation Item Functional Use Source Benefit

Utility Maps Identifies buried utility locations; Identifies access restrictions Prevents damage to utilities

Federal, Regional and Local agencies/ authorities and Utility Companies.

Telecommunication and water supply line identifications prior to an investigation prevents expensive repairs.

Aerial Photographs

Identifies man-made structures, potential borrow source areas;

Provides geologic and hydrological information which can be used as a basis for site reconnaissance;

Track site changes over time

Ethiopian Mapping Agency Evaluating a series of aerial photographs may save time during construction material survey.

Topographic Maps

Provides good index map; Allows estimation of site topography;

Identifies physical features; Can be used to assess access restrictions

Ethiopian Mapping Agency Engineer identifies access areas and restrictions, identifies areas of potential slope instability; and can estimate cut and fill before visiting the site.

Satellite images and digital elevation models (DEM)

Provides topographical and hydrological information which can be used as a basis for site investigation and design purposes

International organizations such as SPOT, Landsat, and SRTM.

Satellite images and DEMs are useful for hydrological analysis of watersheds around river crossings.

Existing Subsurface Investigation Report

May provide information on nearby soil and rock type, strength parameters, hydro-geological issues, foundation types previously used

Geological Survey of Ethiopia (GSE), Transport Construction Design Enterprise (TCDE), other regional and local agencies.

A report for a nearby roadway widening project provides geologic, hydrogeological, and geotechnical information for the area, reducing the scope of the investigation.

Geologic Reports and Maps

Provides information on nearby soil and rock type and characteristics,

Hydro-geological issues, Environmental concerns

Geological Survey of Ethiopia (GSE)

A report on regional geology identifies rock types, fracture and orientation and groundwater flow patterns.

Soil maps Identifies site soil types Permeability of site soils Climatic and geologic information

Ministry of Agriculture, local soil conservation and research institutes.

The local soil survey provides information on near-surface soils to facilitate preliminary borrow source evaluation.

Water Well Logs Groundwater levels Provide stratigraphy of the site and/or regional area Water Wells Drilling Agency Wells indicate the presence of water in the surroundings of

the site.

Climatic data (rainfall and temperature)

Rainfall distribution Maximum and minimum temperatures

National Metrological Agency of Ethiopia

Climate controls the degree and type of weathering and may indicate the type of materials present in the site.

Land use / land cover

Soil maps Road maps Water bodies Forest map

Ministry of Agriculture, local agencies, universities and research institutes

Land use or land cover maps assist to identify the physical and biological cover over the land, including water, vegetation, bare soil, and artificial structures.



5.3 Reconnaissance survey

It is essential that the geotechnical engineer, and if possible the pavement design engineer and other team members, conduct a short reconnaissance visit to the project site to develop an appreciation of the topographic and geologic features of the site, and become aware of access and working conditions as part of the desk study phase of site investigation.

The reconnaissance survey creates an opportunity to learn about the design and construction plans, general site conditions, access restrictions for equipment and personnel, traffic control requirements during field investigations (for rehabilitation and upgrading projects), location of utilities, type and condition of existing facilities (i.e. pavements, bridges, etc), adjacent land use (schools, churches, etc.), right-of-way constraints, environmental issues, problem soils, erosion features and surface settlements, flood levels, availability of water, and the presence of benchmarks and other reference points to aid in the location of boreholes.

Right-of-way is needed to investigate any non-reclamation land and should be obtained during the reconnaissance survey to prevent work delays. Although "walk on" permission can be obtained easily and may not be necessary in many parts of Ethiopia, permission for trenching and boring requires a formal request and discussion with local administrative bodies.

Moreover, the reconnaissance survey is important to get an understanding of the processes which have developed the present geological situation at and in a broad region around the site; to explain the geomorphology of the project area in terms of local and regional landforms; to draw attention to important features such as major landslides occurring at or close to the site; to get an appreciation of the regional groundwater conditions; and to form a logical basis for the location of suitable sources of construction materials.

5.4 Preliminary site investigation

The next step after reviewing existing documents and the reconnaissance survey is the implementation of an appropriate ground exploration programme for the purpose of collecting all the information important for the design of the road. This exploration programme is usually performed in the form of preliminary and final phase investigations. When properly planned, this type of two-phase investigation provides sufficient surface and subsurface information for each stage of design while limiting the risk of unforeseen problems. Prior to initiating both the preliminary and final site investigation programmes, the geotechnical engineer needs to know the type of road, traffic load, performance criteria, location, and geometry.

The initial or preliminary design stage investigation is typically performed early in the design process prior to defining the proposed structural elements or the specific locations of bridges, embankments or other structures. Accordingly, the preliminary design investigation typically includes techniques sufficient to define the general geology, soil and rock characteristics, sub-grade conditions, and other features important to road design. In addition, a substantial portion of the site investigation time should go into the preliminary design phase to refine road alignment and profile. Some boreholes could be undertaken partly as an experiment to determine the best method for boring, sampling and in-situ testing in the final stage of investigation. At the end of this stage, there should be sufficient



knowledge of the site to allow a preliminary design of the roadway and associated structures.

5.5 Final site investigation

A final site investigation programme builds on the results of the desk study, reconnaissance and preliminary site investigations, and is conducted to confirm the extent and implication of key geotechnical issues. During this stage of site investigation, all the elements of the design are refined, checked and quantified. The scope of a final site investigation must be sufficient to allow design and construction to proceed with a low level of risk. Generally, the results of a detailed geotechnical site investigation are compiled into a site investigation and materials report.

It is possible that some of the final investigation work may be difficult and expensive to undertake because of access to the project site and availability of equipment. Often these problems are easier to overcome during the construction of the project when site access has been obtained and construction equipment is readily available. Hence, there is often a great temptation to postpone necessary investigation until construction begins. However, this practice is not recommended as it is possible that postponed items from the main site investigation programme could have revealed ground condition problems that would invalidate the project design.

There are times from a cost perspective that it will be impractical to take site investigation equipment into the field more than once. This is especially true where access is limited for new alignments and also for the lower volume roads. In cases of this nature, both the preliminary and final site investigation stages can be combined into one exploration phase, and the data collected will form the basis for the determination of the centre line of the road. This single phase investigation should also permit preliminary estimates of material quantities and construction costs. An additional investigation may only be needed during construction and directed towards approving design aspects, fine-tuning some vital decisions, and identify specific problem areas.

In cases where the traffic is expected to grow quickly and the road design demands a two stage site investigation with more or less equal emphasis and detail, the trenching and boring scheme, the sampling frequency, and testing for each stage should complement each other to avoid repetition while maintaining sufficient coverage.

Chapter 6 Site Investigation Manual – 2013 Design Data Surveys


6 DESIGN DATA SURVEYS

6.1 Introduction The site investigation for preliminary and final design focuses on the following aspects:

1. Soils and rock conditions and the situation of the sub-grade to support the pavement and earthworks operations. An assessment of the sub-grade strength is required during site investigation for pavement design, together with the conditions of compaction and reuse of excavated materials. Also, the interface between soil and fresh rock should be identified as far as possible in order to understand the extent of the earthwork and associated problems.

2. Road cut sections in particular, should be estimated as a function of height, with consideration given to previous local experiences, and the geological and soil formations. The influence of potential erosion and disturbance following road cuts should also be investigated. The side slopes to be adopted for the road cuts should also be established.

3. Embankment stability, as well as a potential for excessive settlements, may occasionally become problematic if soft and compressible deposits are encountered. It is at this stage that the magnitude and rate of anticipated settlements should be estimated together with the potential for embankment failure. This will allow for the selection of remedial solutions and the determination of the possible need for further investigations and studies. Significant problems related to expansive and collapsible soils should also be identified and solutions proposed.

4. Soil and foundation conditions at streams and river crossings (e.g. bridges and culverts) also need attention during site investigation. The goal is to define the soil characteristics in the vicinity and optimize the location of the structures, prior to the final foundation design.

5. It is important to verify the existence of material sources at desirable intervals and their quality and suitability, although a precise estimate of the volumes may be difficult during site investigation. If problems become apparent with the availability of certain materials, then a more precise determination of the quantities should be made at this stage. Alternate solutions should be proposed if a scarcity of materials becomes apparent.

6. In certain areas, water may be scarce for construction purposes and in particular for providing proper moisture content during compaction of the soils and pavement layers. Since this problem is serious in some regions of Ethiopia, it is important to search for water sources, estimate their yields and record the distances from the construction site. In regions where water is scarce, a separate and dedicated hydro-geological study may be needed.

In this chapter, attention is given to investigation needs that are common to any aspect of road design such as the conditions of sub-grades, earthworks and foundations. The investigation for construction materials is discussed separately in Chapter 7.

Geotechnical problems that are not commonly observed in all road construction sites are discussed under “Special Investigations” in Chapter 8. In an equally divided, two phases of exploration (preliminary and final) for design, special investigations can be carried out when the inputs are needed, and the necessary equipment and personnel are available. A summary of geotechnical needs and testing considerations in pavement design is given in Appendix C.

Chapter 6 Design Data Surveys Site Investigation Manual – 2013


The design data survey assumes that the vertical and horizontal alignments of the road have been to a large extent fixed. It is impossible to know the depth of sampling unless there is knowledge about the approximate vertical alignment of the road. If the vertical alignment is changed, the investigation data may translate into prohibitively large pit depths in a cut or the design depth may be entirely within an embankment, where qualities of available fill materials decide the required pavement design rather than in-situ soils. Any change in horizontal alignment may also need additional site investigation along the new centreline.

6.2 Sub-grade characterization

The performance of a road is significantly affected by the characteristics of the sub-grade. Desirable properties of the sub-grade include strength, stiffness, drainage, ease of compaction and low compressibility. These properties can have a significant influence on road performance and long-term maintenance. The sub-grade must be strong enough to resist shear failure and have adequate stiffness to minimize vertical deflection. It should also form a suitable platform to achieve the required compaction of the pavement layers above sub-grade level. Stronger and stiffer materials provide a more effective foundation for the riding surface and will be more resistant to stresses from repeated loadings and environmental conditions.

A critical component of site investigation is, therefore, the characterization of the sub-grade which can comprise naturally unprocessed or treated in-situ materials, a capping or drainage layer, or a combination of these. As shown in Figure 6.1, there are three locations a sub-grade can assume in pavement design: at the existing ground surface, at the top of an embankment, or at the bottom of a cut section. The exploration required for road cuts and embankments will be discussed in the followings sections. In this section, emphasis is given to sub-grades at existing natural grounds whose performance is controlled by the in-situ conditions.

In cases where the in-situ conditions of the sub-grade materials are unsuitable, cost-effective methods of improving the existing situations must be identified. An important part of this process is the balance between initial construction costs and long-term maintenance costs. These trade-offs are best resolved during site investigation. 6 .2.1 Location and spacing of test pits and borings

Common investigations for sub-grade characterization include test pits and trenches, hand auger probes, and occasional borings. The location, spacing and depth of pits and borings for characterizing the sub-grade depend on the type of road; the soil and rock formations; the known variability in stratification; and the anticipated loads from traffic to which the sub-grade will be exposed. A prior review of all possible documents as well as a preliminary visual inspection of the entire road alignment (reconnaissance survey) assists in developing a plan for the location of pits.



Figure 6.1: Different Options of Sub-grade Locations

In many circumstances, the exact location of pits is determined on the basis of subsurface conditions. Test pits should be located along the road alignment as well as within the lateral extent of anticipated excavations to ensure material representation. In the case of a new road, excavation normally starts at the existing ground level, not far from the anticipated centreline. During upgrading and reconstruction, test pits should be dug through the pavement layers. Depending on the type and extent of distresses, pits could also be necessary on both sides of the road (or near the shoulders) for rehabilitation projects.

Often, the location and spacing of pits are decided based on the phase of investigation. For route selection studies, very wide pit spacing (up to 5km) may be acceptable particularly in areas of uniform or simple subsurface conditions. During preliminary investigation, a closer spacing is necessary but the number would be limited to an amount needed for basic design purposes. This spacing is refined during final investigation with more pits excavated to supplement those dug earlier in areas where there are special problems to solve.

Table 6.1 provides guidelines for selecting minimum boring depths and spacing for sub-grade characterization. This table should be used only as a first step in estimating the minimum number of borings for a particular design, as actual boring spacing will be dependent upon the project type and geologic environment. In all cases, it is recommended that the depth of the exploration should be such that the depth of significant influence (DOSI) is explored. In areas underlain by heterogeneous soil deposits and rock formations, it will probably be necessary to exceed the minimum depth of investigations to capture variations in soil and rock type.



Table 6.1: The Frequency and Depth of Investigation for Sub-grade Characterization

Application

Minimum number and location of exploration points Minimum depth of

exploration Preliminary site investigation

Final site investigation

Sub-grade characterization

A minimum of one exploration point at each km of the anticipated alignment.

The spacing can be wider (up to 5km) in relatively uniform conditions.

Investigate to a depth 1.5m below the proposed sub-grade level. In the case of a new alignment, the depth from the natural ground surface should not be less than 2m unless a rock stratum is encountered.

Additional exploration points are needed at significant changes in soil types.

The spacing could be as low as 500m when information is required on specific problems.

In some places, the depth should increase to fully penetrate soft, highly compressible soils.

Representative large amounts of samples for CBR testing should be taken from pits or borings located not more than 10km apart.

Representative large amount of samples for CBR testing should be taken from pits or borings when necessary.

The presence of groundwater less than 3 m beneath the sub-grade, irregular bedrocks, or big boulders may all need a limited amount of shallow borings (up to 15m).

Generally, pits could be excavated at each kilometre of the anticipated alignment for preliminary design, with more pits needed where there are significant changes in soil types. Significant changes in subsurface conditions are those which affect the engineering properties of soils, as well as their bearing strength (CBR). The spacing of pits during final site investigation should be planned with an aim to fill the gaps that are identified after the preliminary design. Generally, this spacing could be as low as 500m in areas with geotechnical, hydrological or environmental problems and can go up to 5km when the subsurface condition is relatively uniform or appropriately determined with pits (trenches) dug for preliminary design.

The average frequency of one pit per kilometre during preliminary site investigation and associated sampling procedures apply to soil identification. For soils classification, this spacing could increase to two kilometres. Representative large amount of samples for CBR testing should be taken from pits or borings located not more than 10km apart.

It is sometimes not possible to dig trial pits to the full depth of soil layers or in highly weathered rocks. In these cases, the use of a hand or power augers is recommended. In some circumstance, a number of borings may be necessary to investigate the materials that lie below pavements. This is especially true in areas where thick problem soils and soft deposits exist; and when the road alignment passes through landslide zones, solution cavities, and unconsolidated soils. In these cases, the location and spacing of boreholes depend on the location of these specific problems, with one or more borings needed in specific areas.



6.2.2 Depth of test pits and boreholes

Similar to the location and spacing, the depth of pits and borings is determined by the nature of the subsurface. In pavement design, the depth of influence is usually assumed to relate to the magnitude and distribution of traffic loads. Current AASHTO (1993) and many other standards limit this depth at 1.5m below the proposed sub-grade level. This depth increases in special situations such as when deep deposits of very soft soils exist.

For the purpose of sampling and description, pits should be dug to at least 50cm below the expected sub-grade level. In the case of a new alignment, the depth from the natural ground surface should be not less than 2m unless a rock stratum is encountered. Almost all investigation in upgrading, reconstruction and rehabilitation projects can be done with the help of an excavator. Assuming permission of access has been obtained, an excavator should be used at all identified locations along the alignment for digging a pit (or trench) of up to 5 m deep. All necessary safety requirements should be observed during the excavations.

Special problems requiring deeper exploration may include thick highly compressible deposits (e.g. peat or marsh areas) or expansive or frost-susceptible soils. Moreover, the presence of groundwater less than 3 m beneath the sub-grade, irregular bedrocks, or big boulders may all need a limited amount of shallow borings (up to 15m).

Where borings are drilled to a rock stratum, it is generally recommended that a minimum of 3m length of rock core be obtained to verify that the boring has indeed reached bedrock and not encountered the surface of a boulder. Buried basaltic boulders are common in the Ethiopian highlands where Trap Series lavas exist. It is also possible to find limestone and sandstone columns moved to lowlands as rock falls, where sedimentary rocks are present.

In general, subsurface investigation programmes, regardless of how well they may be planned, must be flexible to adjust to variations in subsurface conditions encountered during investigation. The geotechnical engineer should at all times be available to confer with the field inspector. On critical projects, the geotechnical engineer should be present during the field investigation. There should also be a communication with the design engineer to discuss unusual field observations and changes to be made in the investigation plans.

The position of each test pit and borehole must be accurately recorded. In every test pit, all layers, including topsoil, shall be properly described and their thicknesses measured. All layers of more than 50cm shall be sampled. This will promote a proper assessment of the bulk of materials excavated in cuts and to be used in embankments. The samples shall be taken over the full depth of the layer by taking a vertical slice of material. 6.2.3 Laboratory testing

During sub-grade characterization, samples are necessary for further visual description and laboratory tests. The number of test specimens depends on the number of soil layers identified from pits as well as their engineering behaviour. There are no general guidelines on the number of tests, but the availability of in-situ tests often reduces the total amount and type of laboratory tests. Most of the sub-grade test specimens should be taken from as close to the top of the sub-grade as possible, extending down to a depth of 50cm below the



planned sub-grade elevation. However, some tests should be performed on soils encountered at a greater depth, especially if those deeper soils are soft. Likewise, the extent of the laboratory programme depends on the criticality of the design and on the complexity of soil conditions.

Soil laboratory tests commonly used in pavement design and their AASHTO or ASTM designations are summarized in Appendix D. The primary test to assess the strength of the sub-grade is the California Bearing Ratio (CBR) test. Table 6.2 summarizes the purpose, procedures and limitations of this test. Where possible, CBR tests should be performed on undisturbed specimens that represent the natural conditions of the sub-grade.

Table 6.2: The California Bearing Ratio (CBR) Test

AASHTO ASTM

T 193 D 4429 (for field); D 1883 (for laboratory)

Purpose To determine the bearing capacity of a compacted soil under controlled moisture and density conditions.

Procedure

The test results are expressed in terms of a bearing ratio which is commonly known as the California Bearing Ratio (CBR). The CBR is obtained as the ratio of the unit load required to cause a certain depth of penetration of a piston into a compacted specimen of soil at a measured moisture content and density, to the standard unit load required to obtain the same depth of penetration on a standard sample of crushed stone. Typically soaked conditions should be used to simulate anticipated long-term conditions in the field.

The CBR test is run on three identically compacted samples. Each series of tests is run for a given relative density and moisture content. The engineer must specify the conditions (dry, at optimum moisture, after soaking, 95% relative density, etc.) under which each test should be performed.

Remarks

CBR is a practical bearing capacity test, yet provides only discrete point test data for evaluation. Most CBR testing is laboratory-based, thus the results will be highly dependent on the representativeness of the samples tested. The test results are used for pavement designs using locally specified limits.

In addition to the CBR, sub-grade samples shall be tested for in situ moisture content (AASHTO T 265), Atterberg limits (AASHTO T 89 and T 90), percent passing the 0.425 mm ASTM sieve, weighted plasticity index (WPI), standard and modified compaction tests (AASHTO T99 and T180), and swelling at natural density and moisture conditions.

CBR testing of in situ untreated sub-grade and fill material to determine the design CBR and swell shall comprise single point or at times three point tests as described in Table 6.2. Testing shall target at most 95.0% maximum dry density (MDD) and 100% optimum moisture content (OMC) using modified compactive effort. The target MDD for testing can be increased to 97.0% for fill materials depending on the standard of the road. The moisture contents after soaking shall be measured on the whole CBR specimen after testing.



Soaking periods for determining both the CBR values and swell shall be: Unsoaked for low pavement moisture-content environments and dry areas such as

many parts of the Afar and Somali regions, and the northern and southern lowlands; Four-day soaking is necessary for locations with circumstances other than those

mentioned above. Testing in many places of the central, western and eastern highlands and the central and southern part of the rift valley can be done under this condition.

It is always advisable to consider local climatic and topographic condition when deciding the soaking period. In deep gorges for example, a mixed approach of unsoaked in the valley floor where day-time temperatures are high, soaking for four-days in the middle and even an extended soaking at the top could be considered to better simulate natural conditions.

In many cases, an assumed design CBR could be assigned on the basis of previous test data and performance of soils in similar environments. Some regional road authorities and federal institutes may have considerable experience and performance data on specific soil types in local climate and topographic conditions. Use of this information reduces the cost of sub-grade evaluation and also helps ensure a consistent approach to the determination of sub-grade CBR within each region. The approach involves the assessment of sub-grades on the basis of local geology, topography and drainage, with regular routine soil tests.

In addition, many highly- to extremely-weathered rocks in the highlands of Ethiopia tend to breakdown during construction and release moisture-sensitive clay fines. For such sub-grade materials, the effects of construction should be simulated by repeated cycles of compacting. Adequate compaction of the sub-grade and subsequent pavement layers is an essential ingredient for obtaining high-quality road pavements and cannot be overemphasised. Good compaction reduces settlement, increases strength and density, and decreases the sensitivity of the sub-grade soil to changes in moisture content.

In the case of upgrading gavel roads, the following approach shall be considered when the geometric standards are acceptable to maintain the existing alignment.

Where more than 10cm of existing gravel wearing course is in place, samples of the sub-grade shall be submitted to grading by 0.075mm sieve, Atterberg limits, and standard compaction test. In addition, the field moisture content and field dry density shall be measured to decide whether to leave the sub-grade undisturbed or subject it to re-compaction. In-situ dry density and moisture content shall be determined by nuclear methods. Table 6.3 summarizes other alternatives with their advantages and limitations.

If the degree of compaction is found to be at least 95% of the maximum dry density, then CBR values shall be measured in the laboratory at field dry density from samples of the sub-grade materials. These values may be supplemented by direct measurements of the sub-grade strength using DCP. If the degree of compaction is not satisfactory, the sub-grade will need re-compaction and the CBR values shall then be measured at a compaction of 95% MDD.



Table 6.3: Options for Measuring Dry Density

Procedure ASTM Comment on use

Sand replacement density D1556

Sand replacement (cylinder) is the commonest method employed. Time consuming, potential difficulties with unstable holes in granular materials.

Core cutter density D2937 Can only be used in cohesive soils free from coarse-grained materials.

Water replacement density D2167 Used effectively only in fine materials.

Nuclear density D2922(b) Gamma radiation a function of material wet density. Source is inserted in small hole in ground. Requires careful calibration for each chemically variable soil type.

During the upgrading of a gravel road on the same alignment, the existing gravel wearing course may provide extra material either for sub-base, or for improved sub-grade. Measurements of the thickness and width of gravel wearing course shall then be recorded every 500m. One sample per kilometre of existing gravel wearing course shall be taken, where the gravel layer is at least 10cm thick. Each sample shall be subjected to tests related to grain size analysis, Atterberg limits, and compaction and CBR tests.

In rehabilitation projects, the number and type of tests will depend on the condition of the existing road. The initial survey should be used to show where problems that require material property information exist. In general, however, representative samples would be tested for the purpose of material identification and engineering description. 6 .2.4 Subsurface profile

On the basis of all subsurface information (i.e. from the literature review, geophysical evaluation, in-situ testing, soil borings, and laboratory test data), a subsurface profile is developed to evaluate the regional behaviour of the sub-grade. Longitudinal profiles are typically developed along the roadway alignment, and a limited number of transverse profiles may be included for key locations, such as at major bridge foundations, cut slopes, or high embankments. The subsurface information should also be presented in plan view, providing a map of general trends and changes in subsurface conditions.

Vertical and plan view profiles provide a means of summarizing pertinent subsurface information and illustrating the relationship of the various investigation points. By comparing the vertical profiles with the plan view, the subsurface conditions can be related to the topography of the site, providing a sense of lateral distribution over a large horizontal extent.

The preparation of subsurface profiles requires geotechnical judgment and a good understanding of the geologic setting for accurate representation of the ground conditions. In developing a two-dimensional subsurface profile, the profile line (typically the roadway centerline) needs to be defined on the base plan, and the relevant pits and borings should be projected to this line. Due to the subjective nature of the interpretation required, subsurface profiles and plan views should not be included in the construction bid documents. The subsurface profile should be presented at a scale appropriate to the depth of the borings, frequency of the borings and soundings, and overall length of the cross-section.



6.3 Road Cuts and Embankments Many roads require the design and construction of road cuts and embankments. Road cuts involve excavation of a cut slope or construction adjacent to a natural slope. Embankments are, on the other hand, fills constructed on natural soil. The embankment fill is either imported or relocated from another portion of the project and placed on the existing ground. In most cases, cuts and fills are needed to meet grade and curvature requirements where the topography is changing. Fills are also needed when the road passes through low lying areas susceptible to flooding and inundation because of runoff or an increasing groundwater table. Fills can also be used to form temporary access routes.

The different topographic and geologic conditions for road cuts and embankments result in different geotechnical requirements during field explorations and engineering design. Table 6.4 summarizes the information needed to assess cuts and embankment fills.

The primary geotechnical concern for road cuts is the stability of the slope. The stability assessment requires characterization of the different geological layers and groundwater

Table 6.4: Information Needs During the Design of Road Cuts and Embankments

Purpose of investigation Engineering evaluations Required information for analyses

Excavations and road cuts

Slope stability Bottom heave Liquefaction Dewatering Lateral pressure Soil softening/progressive

failure Pore pressures

Subsurface profile (soil, ground water, rock)

Shrink/swell properties Unit weights Hydraulic conductivity Time-rate consolidation parameters Shear strength of soil and rock

(including discontinuities) Geologic mapping including

orientation and characteristics of rock discontinuities

Embankments and embankment foundations

Settlement (magnitude & rate) Bearing capacity Slope stability Lateral pressure Internal stability Borrow source evaluation

(available quantity and quality of borrow soil)

Required reinforcement

Subsurface profile (soil, ground water, rock)

Compressibility parameters Shear strength parameters Unit weights Time-rate consolidation parameters Horizontal earth pressure

coefficients Interface friction parameters Pullout resistance Geologic mapping including

orientation and characteristics of rock discontinuities

Shrink/swell/degradation of soil and rock fill



conditions of the existing material. Engineering design activities focus on the evaluation of short- and long-term stability for different groundwater and material strength conditions.

The primary geotechnical design issues for embankments, on the other hand, include bearing capacity, slope stability and long-term settlement. These design issues are often controlled by not only the engineering characteristics of the fill, but also the geological property of the material below it. Consequently, site investigation for the embankment focuses on material exploration while evaluating the response of the foundation to the load from the new fill. 6 .3.1 Road cuts This section addresses existing slopes adjacent to roadways or slopes resulting from roadway excavations. Existing slopes are referred to as natural slopes, while those excavated are called road cuts or cut slopes. As discussed in Chapter 2, many regions in Ethiopia are mountainous. The construction of roads in these regions often demands the excavation of deep cuts to meet geometric standards. In theory, the stability of cut slopes is determined on the basis of the factor of safety obtained from analytical methods. However, the application of stability analyses to the design of road-cuts is not always successful because of the heterogeneity and spatial variability of rocks and soil masses.

Hence, natural slopes with a history of instability often need surface and subsurface investigations. These investigations should as a minimum consider the types of materials in the cut; slope stability and the different types of movements that may occur in the region; sub-grade materials and their strength; moisture regime; and the level and movement of groundwater. It is also advisable to look for scarps, anomalous bulges, odd outcrops, broken contours, ridge top trenches, fissures, terraced slopes, abrupt changes in slopes or in stream directions, and springs or seepages. These may indicate the presence of past movements. It is necessary to examine not only the sides of the road but also the entire region.

The first indication of possible instability problems can be obtained from a study of the topography. Topographic maps, aerial photographs and site reconnaissance all provide useful data on whether instability is likely to occur or has occurred in the past.

Moreover, an understanding of the local geology is necessary. Initially, this involves the use of all available geological, agricultural soil and engineering soil maps and reports. Because of the high degree of heterogeneity and anisotropy in material property, failure along road cuts in the highlands of Ethiopia doesn’t usually follow the classical "slip circle", but will be associated with pre-existing planes. Hence, when rock slopes are encountered, a complete survey of the orientation and characteristics of joints is essential. In addition, the degree of weathering along these discontinuities should be inspected. When inspection and visual survey is not enough, it is often useful to excavate a pit or trench. In deep cuts especially where an interference with existing stability and groundwater conditions is expected, a long trench across the face of the slope would allow the defining of the surface and subsurface geology of the area in which the cut will be constructed. Trenches are more preferable than pits to inspect cuts because of their dimension. Depending on the geology and degree of weathering, up to three trenches are normally enough to investigate a 120m long slope cut. This is indicated in Table 6.5. The



trenches should be located at places where material changes are expected and located by a geotechnical expert.

Table 6.5: Suggested spacing and depth of trenches and boreholes for road cuts

Application Minimum spacing

Minimum depth Preliminary investigation Final investigation

Cut slopes

1) A minimum of three exploration points for every 120 m (uniform conditions) of the slope length.

1) In deep cuts, a minimum of one boring should be performed for each proposed cut slope.

1) Exploration depth should be, at a minimum, 4.5 m below the minimum elevation of the cut unless a hard stratum is encountered.

2) At critical locations (e.g. maximum cut depths or soft strata) a minimum of three exploration points in the transverse direction of the slope are needed.

2) For longer cuts, horizontal spacing for borings parallel to the cut should generally be between 50 to 200 m, based on site geology.

2) Exploration depth should be great enough to fully penetrate through soft strata into competent material (e.g. stiff to hard cohesive soil, compact to dense cohesionless soil, or bedrock).

3) For cut slopes in rock, perform geologic mapping along the length of the cut slope.

3) In rock exposures, the use of inclined boreholes to intersect steeply dipping discontinuities should be considered.

3) Where the base of cut is below ground-water level, increase depth of exploration to determine the depth of underlying pervious strata.

In areas where it is expected that the stability of the slope will be affected by the road cut, further investigations should be carried out using boreholes. Prior to this, however, a seismic refraction survey can be used to delineate the soil/rock or soft/hard rock boundaries. The number, spacing and depth of boreholes depend on the subsurface geology, the configuration and dimensions of possible unstable areas, and the expected mechanism of failure. Normally boreholes are needed in the final phase of site investigation and during construction after it is known that the information from trenches is insufficient. Where general instability exists, borings should be placed perpendicular to the centreline on the uphill side of the cut.

In deep cuts, a minimum of one boring should be performed for each proposed cut slope. For longer cuts, horizontal spacing for borings parallel to the cut should generally be between 50m to 200m, based on site geology. Wider spacing may be considered if, based on existing data and site geology, conditions are likely to be uniform and of low impact to construction and long-term cut slope performance. Borings should extend a minimum of 5 m below the anticipated depth of the cut at the ditch line to allow for possible downward grade revision and to provide adequate information for slope stability analysis. Boring depths should be increased at locations where base stability is a concern due to



groundwater and soft or weak soil zones. Borings should extend through any weak zones into sound material.

Hand augers, test pits, or trenches may be used for investigating sliver cuts (additional cut in an existing natural or cut slope) or shallow cuts, if the soil conditions are known to be fairly uniform. Moreover, detail investigation may be required in areas of cut to fill transition as the material at these points is expected to have variable moisture regimes.

In rock exposures, the use of inclined boreholes to intersect steeply dipping discontinuities may be considered. Besides, care should be taken to determine the groundwater conditions whenever deep cuts of appreciable height are proposed. In such areas, more borings are needed and shall be taken along the centreline as well as the edge of the roadway. It is also useful to monitor standing water levels in boreholes for as long as possible, and the nature of groundwater flow (unconfined water table, perched, and artesian) in the region.

For soil cuts, it is important to obtain soil samples in order to perform laboratory index tests such as particle size analysis, natural moisture content and Atterberg limits. Sampling should also be performed for the purpose of cut slope stability assessment (using strength and density parameters) and the evaluation of cut material as borrow sources. For rock cuts, discontinuity characteristics and weathering should be determined for design purposes.

In situ testing can be used to augment the exploration programme. In boreholes, SPT data taken at changes in strata or at intervals of 1.5m are generally sufficient for granular soils. On the other hand, a combination of SPT and undisturbed Shelby tube samples are necessary in cohesive soils. The vane shear test (VST) may also be performed in very soft to soft cohesive soils. It involves inserting a 4-bladed vane into the soil and rotating the device about a vertical axis, based on ASTM D 2573 guidelines. This test can yield meaningful results in rare cases where materials consist of normally consolidated clays without gravel, cobble-sized particles or interconnected planes of weakness. In general, however, it should be used in conjunction with triaxial testing unless there is a previous experience with the VST at the site.

Because it is generally desirable to obtain samples for laboratory testing, the DCP and CPT tests are not often used for routine exploration of cut slopes. However, these tests can provide information on the stratigraphic profile and can be used to evaluate in situ strengths.

Knowledge of groundwater elevations is critical for the design of cut slopes. In granular soil with medium to high permeability, reliable groundwater levels can sometimes be obtained during drilling. For boreholes located in medium to high permeability soils, groundwater levels should be recorded at the completion of drilling after the water level has stabilized and 12 hours after drilling is completed. In low permeability soils, however, false water levels can be registered, as it takes days for water levels to reach equilibrium. Generally, piezometers are needed to obtain accurate water level information and this is discussed in Chapter 7.

In landslide susceptible zones cuts can be designed for higher factor of safety (1.5 for example). This is because of the uncertainties involved in the determination of the material properties and the exact plane of failure, and because of the heterogeneous nature of geological materials. It is important to know that calculated factor of safeties are in reality



the minimum for situations at the site of the road cut. In many cases, when probable non-circular slip planes are subjectively assessed, it is possible that, the failure plane will, in all probability, not be that which is suggested by theoretical calculations.

Hence, the site investigation should involve the inspection of soil and rock exposures along existing road cuts to evaluate the performance of slopes. This can help determining the depths and gradients of road cuts qualitatively for the geological environment and climatic conditions in hand. Commonly used cut slope ratios that allow maintaining maximum stand up time on different soil and rock masses are given in the ERA Geotechnical Design Manual. In slopes with heterogeneous materials, the appropriate cut-slope angle can be determined on the basis of the types of soil and rock layers and the way they are deposited or intercalated.

6.3.2 Embankments

Many of the methods and procedures described previously for road cuts are also applicable to the evaluation of embankments slopes. However, in addition to the issue of slope stability, the potential for settlement must also be considered as shown in Figure 6.2.

Figure 6.2: Illustrations of Instability and Settlements Concerns in Embankments

Hence, the embankment foundation investigation should as a minimum consider the range of materials in the foundations and where appropriate the pavement sub-grade, settlement potential, side-slope stability, groundwater, moisture regime, drainage requirements, erosion resistance, haul distance, and environmental impact as shown in Table 6.6.



The first thing to determine during embankment site investigation is whether the foundation is made up of transported or residual soils, or rock. Many of the problems don't exist if rock is encountered at a shallow depth. If the underlying foundation is covered by transported soils, then problems are likely since the material may vary from soft alluvial clays to collapsing silts (sands) or expansive clays. It is, therefore, important to comprehend the particular transportation history and mechanism and the result this has on the nature of the soil and its distribution. Concerns also exist if unconsolidated residual soils are present in an area.

The type of field investigation will then depend on the types of soils encountered. If soils are predominately cohesive, then the primary design issues will be bearing capacity, side slope stability, and long-term settlement. These design issues will usually require collecting undisturbed soil samples for laboratory strength and consolidation testing. It may also be desirable to collect in-situ vane shear strength data and conduct CPT or DCP soundings. The vane shear test can provide valuable in-situ strength data, particularly in soft clays.

Cohesionless soils are less of a geotechnical design concern for static loading, as they exhibit good bearing capacity and low compressibility. Very few embankments founded on sands have failed in many countries. Settlements will generally be small and occur rapidly during placement of the fill. If cohesionless soils are located in seismically active areas and below the groundwater, then liquefaction will be a concern. But, this is rare in Ethiopia.

Where embankments cross alluvial deposits there will probably be a stream requiring a structure. Hence, embankments may need structures through them, and investigations should assess the interaction between these structures, the embankment and the in-situ material.

Table 6.6: Investigation Needs for Embankments Engineering

factor Material requirement Investigation requirements

Site controlling factors

Stability

The material, when placed and suitably compacted, must be capable of standing at the appropriate designed slope both in the short and long term.

Shear strength, unconsolidated undrained and consolidated drained, at the relevant moisture-density relationship.

Placement and compaction control, foundation conditions.

Erosion resistance

When placed in the embankment, particularly at or close to slope faces, the material must be capable of resisting erosion by rainfall, and surface run-off. Internal erosion decreases when non-dispersive soil is used. The long term effects of alternate wetting and drying must also be considered.

Grading and plasticity; fabric and mineralogy assessment. Possible use of soil dispersion and erosion tests.

Compaction control at the embankment edges, drainage and surface protection of earthworks both during and after construction.



Most embankment problems at streams are a direct result of poor drainage and consequent high pore-water pressures. This may be the result of inadequate design and construction, but is more probably because drainage paths were not recognized at the time of investigation or may have changed as a result of construction. Therefore, one very important issue during site investigation is to look for all signs of water and moisture along the alignment.

The size, complexity and extent of site investigation for embankments will depend primarily on the type, height and size of the embankment as well as the expected soil conditions. Generally, as summarized in Table 6.7, embankments with a height of 3m or less, constructed over average to good soil conditions (non-liquefiable, medium to very dense sand, silt or gravel, with no signs of previous instability) require only basic site investigation. A site reconnaissance combined with widely spaced test pits (500m apart), auger holes, or a few shallow borings to verify the anticipated site geology may be sufficient, especially if the geology of the area is well known, or if there is some prior experience in the region.

For larger embankments, or for any embankment to be placed over soft or potentially unstable ground, geotechnical explorations should in general be spaced no more than 120m apart of the embankment length for uniform conditions. In non-uniform situations, spacing should be decreased to 60m intervals with at least one pit in each major landform or geologic unit.

Table 6.7: Spacing and Depth of Exploration Points for Embankment Investigations

Application Minimum spacing

Minimum depth Preliminary investigation Final investigation

Embankment foundations

1) A minimum of one exploration point (pit) every 60 m (erratic conditions) to 120 m (uniform conditions) of embankment length along the centerline of the embankment.

1) When boring is considered, this should be in areas where thick soft deposits and potentially unstable ground are present.

1) Exploration depth should be, at a minimum, equal to twice the embankment height unless a hard stratum is encountered above this depth.

2) At critical locations, (for high embankments, thick underlying soft strata, embankments in hilly areas) a minimum of three exploration points in the transverse direction to define the existing subsurface conditions for stability analyses are necessary.

2) For bridge approach embankments, at least one borehole at abutment locations is needed. Shallow boreholes may also be needed for embankments in hilly areas.

2) If soft strata are encountered below the depth greater than twice the embankment height, the exploration depth should be increased to fully penetrate the soft strata into competent material (e.g., stiff to hard cohesive soil, dense cohesionless soil, or bedrock). For bridge approach embankments, the depth of boreholes should be deeper than the river floor.



When boreholes are considered, the depth of the borings will typically extend to twice the height of the embankment. However, depending on the foundation conditions, the required boring depth could be deeper or shallower than this. It is also important to determine the level of groundwater table. When structures are involved, it is economical to use the same borings to provide information for both the embankment and structural design.

The relevant soil characteristics related to stability and settlement, are strength and compressibility. Like cuts, the shear strength of the subsoil is required in embankments to estimate the stability of side slopes. This can be assessed by the description of the material or from in situ tests (CPT, DCP and VST). The VST is often used to evaluate the in-situ, undrained shear strength of soft to stiff clays and silts. Since few embankments require laboratory shear strength tests, the emphasis should be on obtaining good quality, reliable field results and, in particular, a measure of the distribution of shear strength within the strata.

Compressibility is considered as comprising two components representing the short-term and long-term behaviour. Short-term behaviour is modelled by elastic moduli and long-term by coefficients of volume change and consolidation. The elastic moduli can often be determined by in-situ tests, whereas assessment of long-term compressibility requires sampling and laboratory testing. In-situ tests are often useful to assess rates of settlement since these depend on subsoil permeability, which may be best measured in-situ. However, settlement in itself is not the main issue. Many embankments settle in the time after construction. It is the rate of settlement, in conjunction with the amount that is significant.

As with road cuts, it is often difficult to define acceptable criteria for factors of safety for sides of slopes of embankments, and in deciding the confidence level required for the calculated values. The same is true for bearing capacity and amount and rate of settlement. Often, it is not feasible to express these variables in the form of simple criteria, such as an allowable settlement equation. Hence, judgment and experience is requires to express predicted settlements in terms of the extent, and time. For this and any other related design issues, the reader can refer to the ERA Pavement and Geotechnical Design Manuals.

6.4 River crossings

The satisfactory performance of a river crossing such as a bridge or culvert normally depends on the proper selection, investigation and design of the foundations. While it is more appropriate to think about the location of river crossings at the time of route selection, the investigation during design assists in delivering appropriate foundation data for design and reduces the risk of facing unanticipated ground conditions during construction.

At each site where a drainage structure is to be constructed, the items that should be evaluated during site investigation include foundation conditions, drainage area, land use, allowable headwater, effects of adjacent structures, existing streams and discharge points, stream slope and alignment, stream capacity, soil erodibility, and environmental constraints. Discharge depends on the watershed drainage area, runoff characteristics, design rainfall intensity, and return period (frequency) of the design storm. All these shall be noted during site survey.



The type of drainage structure constructed in an area depends mainly on the storm event. This is, for example, based on a return interval of 20 to 100 years of flood, depending on the type and value of the structure and local practices. Normally, the structures considered for design are bridges, culverts, and low water crossings. The discussions in the following sections also focus on the foundations of these structures. Bridges are large but culverts have a limited flow capacity. On the other hand, low water crossings are less sensitive to flow estimates. 6.4.1 Bridges

Bridges are relatively expensive but are often the most desirable stream crossing structure because they can be constructed outside of the stream channel and thus minimize channel changes, excavation, or placement of fill in the natural channel. They reduce the disturbance of the natural stream bottom but they require detailed foundation investigations.

The function of the bridge foundation is to support loads from the bridge superstructure by spreading concentrated loads over a sufficient area, provide adequate bearing capacity, limit settlement under the imposed load, and to transfer loads through unsuitable foundation strata to sound formation. Hence, the choice of the most appropriate foundation type and size for bridges requires knowledge of the loading conditions, environmental and climatic effects over the life of the structure, plus an understanding of the subsurface conditions, locations and quality of rocks, groundwater conditions, local construction practices, and effects of scour.

Foundations for bridges include spread footings, driven piles or drilled shafts. Each one of these has its own advantages and disadvantages. Their selection depends on the road design needed (refer to the ERA Bridge Design Manual for further discussion).

Spread footings are normally used where the bearing capacity is high and settlements are small. Competent material such as rock must be present near the ground surface (roughly less than 3m below the ground surface) to avoid large excavations during construction.

Driven piles are used where the underlying soils cannot provide adequate bearing capacity or predicted settlements are excessive for a spread footing. The function of these piles when used in these areas is to transfer loads to deeper suitable strata through friction and end bearing. Driven piles are also important where the anticipated depth of scour is excessive.

Drilled shaft foundations are constructed by excavating a hole with drilling equipment and placing concrete with reinforcing steel in the excavation. Casing, slurry, or both may be necessary to keep the excavation stable. The size of the drilled shaft foundation typically ranges from 90cm to 3m in diameter. The length of drilled shafts can be up to 60m. Drilled shafts can be selected when significant scour is expected, the thickness of rock layers is small, bridge spans are long, driven piles are not economically viable due to high loads or obstructions to driving, there are limits on in-stream work, or earthquake loads are high. 6.4.2 Subsurface investigation The procedure to investigate the subsurface condition of bridges is the same as any other section of the road such as the sub-grade, cuts and embankments. The only difference may



be the need to have more relatively deep boreholes because of the requirement to drill down to sound rock or strong foundations. This is especially true in the case of drilled shafts.

Table 6.8 summarizes the information that is needed during the investigation of the different types of foundations of bridges. Investigation starts with the assessment of existing documents assisted by a reconnaissance survey to look at the performance of other drainage structures in the surroundings of the road. This will be followed by a preliminary investigation which may involve pits and geophysical tests at abutment areas and floor of the stream or river. During the final site investigation phase, boreholes are needed, although their number can be limited and more excavated as required at the time of construction.

When borings are finished the logs should be prepared based on the format given in Chapter 3 (Table 3.6). To the extent practical, the variation in properties within each soil layer should be documented. After the soil layer boundaries and descriptions are established, determine the extent and details of any necessary additional laboratory testing (e.g. consolidation and shear strength). A subsurface profile of the type shown in Figure 6.3 should be prepared from the information to provide a visual representation of the material that lies below the ground surface.

The final soil profile should include the average physical properties of stream-bed materials including size and gradation, consistency, shear strength and compressibility, as well as a soil group classification (refer to Chapter 4) and visual description of each deposit. Also, information is needed on the nature (perched, artesian, and unconfined) and level of groundwater. Boulders, voids and old channel deposits, if present, should also be noted.

The soil profile should be characterized at each bridge pier location. Usually, this will require a borehole at each centre pier and each end pier as shown in Figure 6.3. If drilled shafts are used, the common practice is to conduct an exploration at each shaft location. Sometimes, however, the borehole can move a little to avoid subsurface disturbances during operation. Where a drilled shaft foundation is anticipated, it is desirable to leave exploratory borings open for as long as practical to establish whether the hole will stay open or collapse. This information is useful in helping to determine if temporary casing will be required during construction.



Table 6.8: Information Needs for the Design of Different Types of Bridge Foundations

Purpose of investigation Engineering evaluations Required information for analyses

Shallow foundations

Bearing capacity Settlement (magnitude & rate) Shrink/swell of foundation soils (natural soils or embankment fill) Chemical compatibility of soil and concrete Heave and swell Scour (for water crossings) Extreme loading

Subsurface profile (soil, groundwater, rock) Shear strength parameters Compressibility parameters (including consolidation, Shrink/swell potential, and elastic modulus) Stress history (present and past vertical effective stresses) Chemical composition of soil Depth of seasonal moisture change Unit weights Geologic mapping including orientation and characteristics of rock discontinuities

Driven pile foundations

Pile end-bearing Pile skin friction Settlement Down-drag on pile Lateral earth pressures Chemical compatibility of soil and pile Driveability Presence of boulders/ very hard layers Scour (for water crossings) Vibration/heave damage to nearby structures Extreme loading

Subsurface profile (soil, ground water, rock) Shear strength parameters Horizontal earth pressure coefficients Interface friction parameters (soil and pile) Compressibility parameters Chemical composition of soil/rock Unit weights Presence of shrink/swell soils (limits skin friction) geologic mapping including orientation and characteristics of rock discontinuities

Drilled Shaft Foundations

Shaft end bearing Shaft skin friction Constructability Down-drag on shaft Quality of rock socket Lateral earth pressures Settlement (magnitude & rate) Groundwater seepage/ dewatering Presence of boulders/ very hard layers Scour (for water crossings) Extreme loading

Subsurface profile (soil, ground water, rock) Shear strength parameters Interface shear strength friction parameters (soil and shaft) Compressibility parameters Horizontal earth pressure coefficients Chemical composition of soil/rock Unit weights Permeability of water-bearing layers Presence of artesian conditions Presence of shrink/swell soils (limits skin friction) Geologic mapping including orientation and characteristics of rock discontinuities Degradation of soft rock in presence of water (e.g. rock sockets in shales)



Figure 6.3: An example of a subsurface profile at a bridge site.

The following site conditions will warrant special consideration during investigation: If soils are soft and compressible, it will be important to collect high quality,

relatively undisturbed samples for laboratory evaluations of compressibility and strength. This information may be critical for assessing issues including long-term settlement of spread footings, down-drag on piles and shafts, and settlement of piles and shafts.

Thin layers of soft soil can result in settlement of spread footings or down-drag on shafts and driven piles. These layers can also serve as a sliding surface for embankments and slopes, particularly during seismic loading. If movement occurs, foundations could be damaged.

The presence of liquefiable soils is also a concern. Conventional practice is to locate the spread footing below the deepest depth of liquefaction or to improve the ground so that the potential for liquefaction is mitigated. For deep foundations, the toe elevations should be founded below potential liquefiable soils. Liquefaction can result in loss of lateral support within the liquefied zone and down-drag loads on the pile as the liquefied soil settles.

The presence of problem (collapsible, expansive, dispersive) soils both at the foundation and in its surroundings should be identified. The presence of sinkholes and problem rocks such as those highly susceptible for solution and slaking should also be investigated.

Evaluate the groundwater conditions in borings. When feasible, install piezometers to monitor the level of water. The existences of fluctuating groundwater, perched water tables, and artesian conditions beneath the structure should also be investigated. In the highlands of Ethiopia where differential weathering of basalt led to the formation of clays in between fractured rocks, perched and artesian conditions are common. Artesian conditions can reduce the load carrying capacity of the soil and alter the effective stress distribution.



6.4.3 Footings The depth of footings for bridges should be determined in consideration of the character of the foundation materials and the possibility of undermining. Footings not exposed to the action of stream current should be founded on a firm ground.

In cases where spread footings are being considered, consider the following guidelines: On Soil: the bottom of footings on soil shall be set at least 3.0m below the channel

bottom and below the total scour depth for the design flood. On Rock: avoid keying into the rock at shallow embedment depths. Keying into

the rock typically involves blasting or other destructive methods that frequently damages and renders the rock structure more susceptible to scour. If footings on smooth massive rock surfaces require lateral restraint, drill and grout steel dowels into the rock below the footing level. The bottom of the footings shall be at least 1m below the surface of scour-resistant rock with the top of the footings at least below the rock surface.

On erodible rock: many sedimentary rocks such as marl, shale and mudstone and pyroclastic deposits, colluviums and weathered basalts are less resistant to erosion. These materials are common in many river valleys in Ethiopia and therefore, careful assessment of potentially erodible rock formations is essential to assess scour potential. The foundation decision should be based on an analysis of intact rock cores, including rock quality assessments and local geology, hydraulic data, and anticipated structure life. An important consideration may be the existence of a high-quality rock formation above a thin weathered zone in many regions of the country. For deep weathered deposits, estimate the potential scour depth for the design flood and locate the footing so that its top is below the estimated scour.

6.4.4 Driven Piles Driven piles are not often used as a single or individual foundation element to support a structure. Consequently, settlement analyses of single piles are not commonly conducted. Normally, an empirical approach known as the equivalent footing method is typically used to calculate the settlement of a group of piles. The pile group is treated as an equivalent footing that is founded at an effective depth below the ground surface. For uniform clays, the effective depth is two-thirds of the pile embedment in the bearing stratum. For sand sites, the effective depth depends on the soil conditions below the toe of the pile group.

The term “pile drill and socket” applies to a pile that is bored into the underlying rock a distance of 1 or 2 pile diameters. For pile foundations that are driven to rock, the exact area of contact with the rock, the depth of penetration into the rock and the quality of rock are largely unknown. Therefore, the determination of load capacity of driven piles on rock should be made based on driving observations, local experience and load tests.

In determining the spacing of piles, give consideration to the characteristics of the soil and to the length, size, driving tolerance, batter and shape of the piles. If piles are spaced too closely, the axial capacity and lateral resistance of each pile will be reduced. In addition, piles must be spaced to avoid toe interference due to specified driving tolerances. Piles are usually driven at minimum spacing of 3 pile diameters. Closer spacing minimizes the cost of the pile cap. However, driving piles at closer spacing in dense sands and saturated



plastic soils can cause heave or lateral ground displacements that may damage previously driven piles. 6 .4.5 Drilled Shafts The following factors should be considered during the investigation for drilled shafts:

Cobbles and Boulders: Construction of a shaft can be affected by the presence of cobbles and boulders and, therefore, the site characterization effort should try to quantify these effects through the review of the drilling information and the geology of the area. Because of the importance of cobbles and boulders to the shaft construction process, normal practice is to conduct a geotechnical exploration at the centre of the location of each shaft.

Gravels and sands: Identify the presence of open gravel and sand layers, as these materials may require the use of casing or special drilling mud to avoid hole collapse or excessive loss in drilling mud during construction.

Explorations shall extend at least 6m or 5 shaft diameters below the likely toe of the shaft. If hard bearing material or rock is located less than 6m, the depth of exploration can be stopped 3m into the hard bearing material.

Socketed: If the shaft is going to be socketed in rock, the exploration shall extend at least 2 shaft diameters below the planned elevation of the shaft toe.

Spacing: The centre-to-centre spacing of drilled shafts shall be greater than of 3.0 diameters or the spacing required to avoid interaction between adjacent shafts. Larger spacing than 3.0 diameters may be necessary when drilling operations are anticipated to be difficult.

Table 6.9 should be used to determine the depth and locations of borings for bridge foundations at the start of the final phase of site investigation. This exploration programme should be adjusted based on the variability of the anticipated subsurface conditions. Geophysical testing, engineering judgment, and pit and trench excavations during reconnaissance survey and the preliminary site investigation may be used to guide the planning of this programme.

If conditions are determined to be variable, the exploration programme should be increased relative to the requirements in Table 6.9 such that the objective of establishing a reliable longitudinal and transverse substrata profile is achieved. If site conditions are observed or previous local construction experience has indicated that the subsurface conditions are homogeneous, or otherwise, and are likely to have minimal impact on bridge performance, then a reduced programme relative to what is specified in Table 6.9 may be considered.

Laboratory testing should be used to augment the data obtained from field investigation and to refine the soil and rock properties selected for design. Foundation design typically relies upon the SPT results obtained during the field exploration through correlations to shear strength, compressibility, and the visual descriptions of the soil and rock encountered, especially in non-cohesive soils. The information needed for the assessment of ground water and the hydrogeological properties needed for foundation design and constructability evaluation is obtained mainly from field instrumentation (e.g. piezometers).



Index tests such as soil gradation, Atterberg limits, and water content are used to confirm the visual field classification of soils, but may also be used directly to obtain input parameters for some aspects of foundation design (e.g. soil liquefaction, scour, degree of over-consolidation, and correlation to shear strength or compressibility of cohesive soils). Laboratory tests conducted on undisturbed soil samples are used to assess shear strength or

Table 6.9: The Minimum Number and Depth of Exploration Points for Bridge Foundations

Application Minimum number of

investigation points and their location

Minimum depth of investigation

Shallow Foundations

For substructure widths (piers or abutments) less than or equal to 30m, a minimum of one investigation point per substructure is necessary. For those greater than 30m, a minimum of two investigation points per substructure.

Additional investigation points (boreholes) should be provided if erratic subsurface conditions are observed at or near the bridge location.

Sufficient to fully penetrate unsuitable foundation soils into competent material of suitable bearing capacity (stiff to hard cohesive soil, compact to dense cohesion-less soil or bedrock).

At least to a depth where stress increase due to estimated foundation load is less than 10% of the existing effective overburden stress at that depth.

If bedrock is encountered, investigation depth shall be a minimum of 1.5m into the bedrock, but rock investigation should be sufficient to characterize compressibility of infill material in bedding planes.

Deep Foundations

For substructure widths (e.g. bridge piers or abutments) less than or equal to 30m, a minimum of one investigation point per substructure is needed.

For substructure widths greater than 30m, a minimum of two investigation points per substructure is required.

Additional investigation points should be provided if erratic subsurface conditions are encountered.

Due to the large expense associated with construction of rock-socketed shafts, conditions should be confirmed at each shaft location during investigation.

In soil, the depth of investigation should extend below the anticipated pile or shaft tip elevation a minimum of 6m, or a minimum of two times the maximum pile group dimension.

All borings should extend through unsuitable strata such as unconsolidated fill, peat, highly organic materials, soft fine-grained soils, and loose coarse-grained soils to reach hard or dense materials, a minimum of 5m.

For piles bearing on rock, a minimum of 3m of rock core shall be obtained at each investigation point location to verify that the boring has not terminated on a boulder.

For shafts supported on or extending into rock, a minimum of 3m of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the maximum shaft group dimension shall be extended below the anticipated shaft tip elevation to determine the characteristics of rock within the zone of foundation influence.



compressibility of finer grained soils, or to obtain seismic design input parameters such as shear modulus. 6 .4.6 Potential scour depth The investigation of bridge foundations and the selection of boring depths for this purpose must consider the potential scour depth. Scour is a localized erosion of the channel bed that occurs around flow obstructions (at pier and abutments), at channel contractions (bridges), and on the outside of channel bends. It can also be the result of long-term erosion of the channel that has occurred during the life of a structure. In Ethiopia, a number of bridge collapses can be attributed to scour around bridge foundations. In view of the potential implications, the evaluation of scour potential is a particularly important part of a design process.

Scour is a site-specific process that is a function of the following: Flow velocity and duration; The geometry of the structural elements exposed to the flow of water; The geomorphology of the channel; and The properties of the foundation and channel bed materials.

A multidisciplinary team of hydraulic, geotechnical and structural engineers should evaluate the risk of scour.

A scour assessment during site investigation requires the determination of the cumulative effects of the three main components of scour:

The total aggradation (deposition) and degradation (erosion) process of the river; The contraction scour at bridges;: and The local flow obstruction causing scour at piers.

Scour assessment also requires an evaluation of potential changes in channel geometry and location that is anticipated to occur during the structures design life. The amount of scour depends on factors that include the hydrological characteristics of the site, the hydraulics of the flow, and the properties of the streambed materials.

During boring or pit excavation, soil strata should be summarized to a depth beyond the probable limit of scour. The logs should then be used to establish the D50 values at the streambed, and to verify depth to bedrock, competency of the bedrock, and material conditions. The hydraulic engineer evaluates scour potential based on idealized soil that uses the D50 of the streambed material. D50 is the median size of the sediment particle. A minimum of four soil samples (two upstream and two downstream) need to be collected to obtain the grain size distribution of streambed materials. However, for long bridges, an appropriately greater number of samples depending on the site conditions and variation need to be collected. 6 .4.7 Inspection of existing bridges During upgrading, reconstruction or rehabilitation, the inspection of existing bridges is essential. The inspection allows for the evaluation of the condition of the bridge waterway opening, substructure, channel protection, and scour countermeasures. Both existing and



potential problems with scour should be reported so that a scour evaluation can be made. During inspection the following items need special consideration:

Evidence of movement of piers and abutments (rotational movement and settlement, check sub- and superstructure for discontinuities, structural cracking or spalling);

Damage to scour countermeasures (riprap; guide banks, sheet piling, sills, etc.); Changes in streambed elevation (undermining of footings, exposure of piles, etc.); Changes in streambed cross section and the location and depth of scour holes.

Perhaps the single most important aspect of the inspecting the bridge for actual or potential damage from scour is by measuring and plotting of stream bottom elevations in relation to the bridge foundations. The stream bottom should be accurately measured by rods, poles, sounding lines or other means, such as the use of divers.

Table 6.10 summarizes the most important aspects that should be considered during field inspections to investigate the degree of scour along stream banks, at the main channel and throughout the floodplain of a certain river or stream for rehabilitation, reconstruction and upgrading of bridges.

Table 6.10: Indicators of Active or Potential Scour at or around Existing Bridges

Location Indicators

Banks Bank sloughing, undermining, evidence of lateral movement, damage to stream stabilization measures, etc.

Main Channel

Meandering or braided with main channel at an angle to the orientation of the bridge;

Existence of islands, bars, debris, fences that may affect flow; Aggrading or degrading streambed; Evidence of ponding of flow; Extent of debris in upstream channel.

Flood Plain

Evidence of significant flow on floodplain; Extent of floodplain development and any obstruction to flows

approaching the bridge location and the surrounding areas; Evidence of overtopping approach roads (debris, erosion of

embankment slopes, etc); Evidence of ponding of flow.

6.4.8 Culverts

Culverts are open-ended conduits used in place of a bridge to carry surface water under roadways at stream crossings. They are also used to facilitate crossings of ditches or to transfer the drainage run from one side of the carriageway to another. Culverts are the most common drainage structures that are appropriate both for low and high volume roads.

The type and detail of investigation for culverts depend on the size and shape of the structure, the design need and the drainage system. For small watersheds, a short exploration could be enough. For larger culverts, however, the exploration must consider the watershed and channel characteristics, high water levels, local rainfall, foundation conditions, and the design requirements. The shape of the culvert itself, such as a round



pipe, pipe arch, structural arch, or box, depends largely on the information from the surface and subsurface investigation.

Flow from outfalls or culverts will generally have a higher velocity than that of the receiving watercourse and this can result in erosion of the bed and banks of the receiving channels. When the depth and/or extent of the scour hole is such that it undermines the foundations of the outfall structure or its outlet wing walls, structural damage can occur leading to collapse. Hence, they need to be properly sized and installed, and protected from erosion and scour.

The position of a culvert is dependent on its intended purpose. Culverts are most often placed along natural drainage courses to limit grading and disruption of the existing groundwater regime. The alignment of the culvert relative to that of the road should aim to minimize its length. Culvert system design shall include an investigation of the groundwater levels and soil conditions at the site. The practice is to investigate culvert sites with at least three pits or trenches, including one near each end of the structure and another around the centre. All pits should be logged, and if necessary, samples should be taken for laboratory tests.

Footings for culverts shall be carried to an elevation sufficient to secure a firm foundation. In any location subject to erosion, the use of aprons or cut-off walls at both ends of the culvert shall be considered and this has to be investigated early in the design phase.

Detail investigation with more pits is necessary when culverts are founded on problem soils, and colluvial deposits. This is because of the potential for erosion, scour, collapse, cracking and instability. For the effect of these soils, the reader can refer to Chapter 7. 6 .4.9 Low water crossings

Low water crossings, fords, or drifts, as they are commonly called, can offer a desirable alternative to culverts and bridges for stream crossings on low-volume roads where road use and stream flow conditions are appropriate. Like other hydraulic structures for stream crossings, they require specific site considerations and hydrologic and hydraulic analyses. Ideally, they should be constructed at relatively narrow and shallow stream locations, and should be in an area of bedrock or coarse soil for good foundation conditions.

Key factors to consider for the design and investigation of low-water crossings include the low and high water levels, foundation conditions, scour potential, channel cross-section shape and confinements, protection of the downstream edge of the structure against local erosion, stream channel and bank stability, and locally available construction materials.

Unlike bridges and culverts, low water crossings don't normally need detailed investigations. In most cases, a reconnaissance survey should be enough to get adequate information for design. Consider, however, digging some pits at the streambed and banks to determine the size and gradation of materials useful to evaluate stream stability and scour problems.

Chapter 7 Site Investigation Manual – 2013 Special Investigations


7 SPECIAL INVESTIGATIONS 7.1 Introduction

The existence of diverse topography, geology and climatic conditions in Ethiopia often creates different geotechnical problems specific to certain regions. These geotechnical problems range from landslides common in rugged mountains to problem soils observed in flat areas. The list also includes the effect of fluctuating groundwater, wetlands and degradable rocks. Although the techniques used to investigate these problems are similar to any other effort of exploration, their probable occurrence at the time of road construction and subsequent long-term effect on the road often demand special attention during exploration, sampling and testing. This special attention could include additional pits and borings, or it may be related to the problem of retrieving undisturbed samples for better analysis and understanding. New tests may also be needed other than those routine to road design and construction. The following sections contain descriptions on these specific geotechnical issues with recommendations to identify and analyse them during site investigation.

Design aspects related the specific geotechnical investigations are covered in the ERA Geotechnical Design Manual, which should be read in conjunction with this section.

7.2 Landslides

Landslides occur frequently in the highlands of Ethiopia. With the exception probably of Afar and Somali regions, the presence of steep cliffs and susceptible stratigraphy in other areas produce conditions favourable to the occurrence of landslides along road alignments.

The susceptibility of a slope to the occurrence of failure is often related to an increase in shear stress (driving force) or a reduction in shear strength (resisting force). Factors that increase the shear stress or decrease the shear strength in a slope are normally termed as landslide causes or triggers. Landslide causes are considered as factors that made the slope susceptible to failure. Triggers are events that finally initiated landslides. Thus, geological and topographical parameters are usually regarded as landslide causes. The triggering factors include rainfall, groundwater, earthquake and manmade activities such as road cuts and embankments.

The causes and triggers of landslides can also be divided as external or internal. External causes and triggers are often responsible for an increase in the shear stress. This may include an increase in the height and steepness of the slope due to road cuts, a structural load or embankment placed on a slope, or an earthquake. Internal causes and triggers are those which occur without any change in the external conditions of the slope. They are associated with a loss of the shear strength of the slope materials. An increase in pore pressure or a reduction of cohesion due to weathering are considered as internal causes or triggers of landslides.

Chapter 7 Special Investigations Site Investigation Manual – 2013


7 .2.1 Types of landslides

Landslides can be classified into different categories on the basis of the type of material involved and their nature of movement. Materials in a landslide mass can be either rock or soil. The soils are described as earth if they are mainly composed of finer particles and debris if there are coarser fragments. The type of movement explains the actual internal mechanics of how the landslide mass is displaced. There are about five types of movements: fall, topple, slide, spread, or flow. Thus, landslides are described using two terms that refer respectively to material and movement. They are fall or slide.

Figure 7.1: Schematic Illustrations of Slope Failure - Fall and Topple.

A fall begins with the detachment of soil or rock, or both, from a steep slope along a surface on which little or no shear displacement has occurred (Figure 7.1). The material subsequently descends mainly by falling, bouncing, or rolling. A topple is recognized as the forward rotation of a displaced mass around a point below the centre of gravity (Figure 7.1).

A slide is a downward movement of a soil or rock mass on a surface of rupture or slip plane, or on a relatively thin zone of intense shear strain. On the basis of the shape of the surface of rupture, slides are further divided into two: rotational and translational (planar).

A landslide on which the surface of rupture is curved upward and the slide movement is more or less rotational about an axis that is parallel to the contour of the slope is called



rotational landslide (Figure 7.2). The displaced material moves as a coherent mass along the rupture surface with little internal deformation. The upper part of the landslide or the head of the displaced zone may move almost vertically downward. If a rotational slide has more than one parallel curved planes of movement, it is called a slump. Rotational slides or slumps tend to occur in concave slopes where there is enough moisture and deep soil profile.

The mass in a translational landslide moves down and outward along a relatively planar surface where the soil profile is shallower. This type of slide may progress over considerable distances if the slip plane is sufficiently inclined. Translational slides commonly fail along faults, joints, bedding surfaces, or the contact between rock and soil (Figure 7.2).

Figure 7.2: Rotational (slump) and Translational (planar) Landslides.

The term “spread” describes sudden lateral movements of relatively homogeneous clays. Spread is often accompanied by tension cracking, subsidence and liquefaction. A flow is a spatially continuous movement in which the surfaces of shear are short-lived and usually not preserved. Often, there is a gradual change from slides to flows along the way, depending on the water content, mobility, and evolution of the movement (Figure 7.3). Creep is a type of failure where movement is imperceptible without careful observation (Figure 7.3).



Figure 7.3: Examples of flow and creep.

Almost all types of landslides mentioned above can occur in Ethiopia. Those most frequently observed are rotational slides and translational slides. The materials involved in these landslides range from loose, unconsolidated clayey soils to large blocks of rock. In addition to slides, rock falls and rock topples are common in river gorges and steep escarpments where long columns of bedded and jointed sedimentary rocks, such as sandstone and limestone, are undercut by stream erosion or road excavation. Debris and mudflows derived from liquefied slides also occur in steep hills and swales during intense rainfall.

7 .2.2 Depths of landslides

Slides with a sliding depth of less than 3m are considered to be shallow. Often, these shallow landslides displace the top part of the ground surface known as the soil mantle, which has a natural angle at which it is relatively stable (natural angle of repose). When hill-slopes or road cuts evolve to be steeper than the natural angle of repose, they become less stable and more prone to shallow landslides, especially with the addition of water. The combination of steep slopes and concave topography has the highest potential to initiate shallow landslides.

Deep seated landslides are those slides in which the slide plane is well below the maximum rooting depth of big trees (generally greater than 3m). Deep seated landslides can occur almost anywhere on a hill-slope or road cut and are typically associated with the hydro-geologic responses in permeable materials that overly impermeable rocks. Deep-seated landslides characteristically occur in thinly layered rocks, unconsolidated sediments,



deeply weathered rock, or rocks with closely spaced fractures. They can also occur when a weak layer such as ash and tuff is present in otherwise strong rocks like basalt.

A deep-seated rotational landslide has often three parts to identify: the scarps (head and side) along which marginal streams can develop; the body, which constitutes the central part of the landslide; and the toe (Figure 7.2). When small landslides are found nested within larger envelopes, a landslide may have several scarps, bodies and toes. The head and side scarps together form an arcuate or horseshoe shaped feature that represents the surface expression of the rupture plane. The body and toe area are usually hummocky and the flow path of streams on these sections may be displaced due to differential movement of landslide blocks. 7.2.3 The role of groundwater

Some landslides in Ethiopia are triggered by the rise in groundwater level. The role of groundwater in initiating landslides can come in several ways. The water may increase the weight of materials on a slope above their point of gravitational equilibrium. This is an increase in shear stress or driving force. Alternatively, it may increase pore pressures within a zone of weakness in the materials underlying a slope or decrease the coefficient of friction on a potential sliding surface. This leads to a decrease in shear strength. Groundwater can also cause clays to hydrate and expand and make them susceptible to slope failures.

A common observation in many areas is that a road cut or a hill-slope may be perfectly stable during the dry season but may slide after the rains begin. This seasonal change in stability is due mainly to the rapid change in the amount and level of groundwater in local and shallow perched aquifers. Often, the presence of weathered, unconsolidated soils between the top soil and impermeable rocks can form a perched water table (Figure 7.4). This is common in the highlands of Ethiopia where basaltic rock masses are present in association with weak volcanoclastic rocks. A key predictive observation for the existence of such kind of geologic configuration is to note the presence of horizontal line of springs.

Figure 7.4: Landslides Related to a Perched Water Table

Groundwater may also contribute to slope instability through the seepage force. This is the drag force that moving water exerts on each individual soil particle in its path in an attempt to disperse them. Therefore, the seepage force contributes to the driving force that tends to drive soils down-slope. The concept of the seepage force may be visualized by noting how easily portions of a coarse-textured soil may be dislodged from a road cut when the soil is transmitting a relatively high volume of groundwater.



7 .2.4 Landslide mapping

Landslides are often mapped by interpreting the distribution of contours on topographic maps and analysing landforms on aerial photographs. Typically, large-scale photography is necessary and the photo scale depends on the size of landslides common in the project site. The range of useful scales of aerial photography for landslide inventory work is limited to about 1:40,000. Depending on vegetative cover, photo quality, and the skill of the interpreter, the overall identification accuracy can go up to 75%. Generally, an intermediate inventory map can be prepared at this stage. This map would show the expected landslide types and distinguish between areas of landslide origin and accumulation.

During topographic map or aerial photo interpretation, understanding and recognizing the differences in slope form is the key in the recognition of potentially unstable slopes. There are three major landforms to observe when looking across vertically in the contour direction. These are: divergent (convex), planar (straight), and convergent (concave-shaped) landforms. Landslides can occur on any of these landforms but convergent slopes tend to be more unstable than planar and divergent slopes because of the concentration of water.

As part of the field study in the preliminary site investigation phase, rocks and soils should be described, accurate stratigraphies prepared, landform boundaries around the route alignment precisely delineated, potentially unstable slopes identified, and the landslide inventory updated. The most common stratigraphies susceptible to landslides in Ethiopia include:

A hard fractured rock such as basalt and limestone overlying a highly weathered or weak rock such as marl, shale, and mudstone;

Hillsides made up of weak rocks such as ash and tuff and weathered basaltic rocks; Hillside deposits such as colluvium and talus resting on beds of firm rock; Rock and soil layers exposed by toe erosion; Residual soil formation with an inclined inherited structure.

Factors that need to be taken into account when undertaking road alignment investigation related to landslides in the field include:

Topographical o The steepness and shape of the slope; o The location of tension cracks and other signs of movement.

Topographical factors that assist in recognising landslides in project sites are deformations and irregular landforms and bulges; cracks; and indications of seepage. A complete survey of cracks usually provides a good indication and description of the slope failure to occur in the area. Other topographic and environmental indicators for the presence of active landslides are:

o Bare or raw, exposed, un-vegetated soil on the faces of steep slopes; o Hummocky or benched surfaces, especially below crescent-shaped

headwalls; o Ponding of water in irregular depressions or undrained swampy areas on the

hill slope;



o Cracks in the surface (across or along slopes, or on existing roads); o Seepage lines or springs and soil piping; o Rapid increase in creek water levels, possibly accompanied by increased

turbidity; o Deflected streams that have moved laterally to accommodate landslide

deposits; o Back-rotated, bowed, kinked or leaning trees; o Offset or cracked retaining walls along existing roads.

Hydrological

o The presence of a river or stream at the base of the slope, particularly if this causes toe erosion during periods of flood or high flow;

o The presence of a drainage course at or above the crest of the slope; o Any indications of a high or temporarily perched water table within the

slope (e.g. seepages and springs); o In upgrading and rehabilitation projects, the effectiveness and condition of

the existing drainage measures.

Climate o The pattern of rainfall in the immediate locality, particularly periods of

prolonged and/or intense rainfall that could lead to saturation of the slope.

Geological o Rock type, weathering grade, jointing and fracture patterns; o Presence of faults or shear zones; o The direction and angle of dip and joints in underlying bedrock compared to

the angle and orientation to the slope, particularly if bedrock is exposed or is at a shallow depth beneath the surface. The extent of the joints and the presence of clay filling also has an influence;

o The sequence of the underlying strata, particularly if this includes weak or impermeable layers;

o The presence of colluviums and unconsolidated materials.

Land Use o Forest clearance and the extent and type of cultivation; o The presence of irrigation channels, ditches and water pipes; o The presence of wetlands; o Excavations and fill slopes associated with commercial and residential

developments adjacent to the road. 7.2.5 Exploration and sampling

Once landslides are identified and mapped, further field investigations using pits and borings might be required. This depends on the type of landslide, the extent of the problem, the type of the road project, the presence of access, and the amount of safety needed.

Often, the extent of rock falls and topples is determined by discontinuity surveys and drilling is not essential. A number of empirical methods have been developed to predict the stability of rock slopes based on discontinuity data and determine support requirements.



Flows often start as slides in the upper part of mountains and hills, and their general outline and type of material involved can be verified by simple field observations.

Translational slides can be explored using test pits and trenches since the soil profile involved is shallow and the direction of movement is usually dictated by the inclination of bed rocks. In this case, trial pits are useful to determine the nature and composition of soils, foundation conditions, the depth to slip surfaces and the presence of water seepages. In situ tests such as the DCP can also be used to obtain an estimate of soil strength near the slip surface. The use of these tests is often limited in gravelly soils and boulder containing colluviums.

Generally, boring is needed when information on ground conditions at depth is critical to the design of a road near a deep-seated landslide. In this case, the information is useful to determine the type and degree of disruption of materials, the depth to the slip surface, and the thickness and geometry of the landslide mass. There is also a need to drill boreholes where there is an indication that a creep is slowly developing into a major slump that can affect the roadway. Boring is also needed for installation of some monitoring instruments.

In general, a minimum of two borings should be drilled along the cross-section of the slide during final site investigation as shown in Figure 7.5. Larger slides may require more borings to adequately define the failure zone. Borings shall extend through the full depth of the landslide material, terminating at least 3m into the underlying stable stratum. Drilling could proceed even deeper to obtain an accurate interpretation of the depth of failure and identify any underlying zones of weakness that may affect the design of mitigation measures.

Geophysical techniques (resistivity and seismic) can be used to determine some subsurface characteristics such as the depth to bedrock, zones of saturation, and sometimes the ground-water table. They can also be used to determine porosity, and degree of consolidation of subsurface materials and the geometry of the landslide. In most instances, geophysical methods can best be used to supplement boring information, spatially extending and interpolating data between boreholes. They can also be alternative information source if boring is not possible.

Sampling in landslide areas often does not follow standard procedures because of the difficulty of identifying shear zones and the need for strength testing at different intervals. The location of slip planes usually coincides with the depth of thin layers of impermeable plastic clays. Hence, if many of these layers are observed in boring logs, then continuous sampling and testing is needed to establish the depth to the critical slip plane. Often, determining this critical slip plane is more of an art than a science and requires an experienced practitioner. Generally, shear strength tests such as triaxial and direct shear, require undisturbed samples. If undisturbed samples are not possible to obtain from suspected depths of shear zones, disturbed samples could be remoulded in the laboratory prior to their use for testing.



Figure 7.5: Illustration of Borehole Locations to Investigate a Failed Slope.

7.2.6 Monitoring

The monitoring and interpretation of the patterns of movement associated with landslides is important to accurately define the critical depth and suggest proper remedial measures. The most common instruments for short term monitoring include survey stakes, extensometers, inclinometers, tiltmeters and piezometers. For long term inspection and change detection of slowly moving landslides, remote sensing techniques such as analogue and digital photogrammetry and synthetic aperture radar interferometry (InSAR) are often helpful.

The two most important parameters that can be monitored during the investigation of a landslide are groundwater levels and displacement. When monitored over several months or years, the data from these two parameters can be very valuable in determining the behaviour of the landslide and the relationship between seasonal groundwater levels and periods of active sliding. Piezometers allow the determination of groundwater levels. Slope displacement is characterized by the depth of failure plane(s), direction, magnitude, and rate. Normally, one or all of these variables may be monitored. Surveying stakes, extensometers, inclinometers and tiltmeters allow for the determination of the direction and rate of movement, and the depth of the failure plane. Extensometers provide an indication of the magnitude of displacement.

Survey stakes are normally the cheapest and simplest ways of monitoring susceptible road cuts. The stakes should be placed along the axis of the slide and extend beyond the interpreted limits of movement. A line, perpendicular to the slide axis, can also be used as shown in Figure 7.6. The stakes should be surveyed on a regular basis with increased frequency in the rainy season; and immediately before and after the season. Movements should be recorded in the X, Y and Z directions. The results can help defining the type of slide, the rate of movement, changes in the slide limits, and areas of greatest activity. The vector sums of the X, Y, and Z movements can be plotted and used to help model the actual shape of the failure surface.



Figure 7.6: Distribution of Survey Stakes and Inclinometers in a Landslide.

Piezometer instrumentation should be designed to accurately record specific groundwater heads that exist along the slip plane and within the slide mass. Normally, enough piezometers shall be installed at different depths to accurately model the groundwater conditions. It could be possible that the disturbed materials may create a less permeable zone, which can lead to the build-up of an artesian pressure along the surface of failure which can decrease stability. A piezometer should be installed in that zone to determine if this condition is present.

Landslides often occur after periods of prolonged and heavy rainfall. In Ethiopia, this is most likely to happen during the wet season between July and the end of September. In the southern part of the country, this period may extend to October and November as severe and localized rainfall can occur during this time. In general, monitoring of roadside slopes where landslides are expected should be carried out:

Shortly before the onset of the rainy season to see the presence of any signs of instability even at dry conditions,

During the rainy season, to monitor and investigate how the landslide is initiated and the rate of displacement,

Immediately after the rainy season to ascertain the extent of damage and movement.

7.3 Expansive soils

Expansive soils are typically clayey soils that undergo large volume changes in direct response to moisture changes in the soil. Unlike collapsible soils, expansive soils tend to increase in volume (i.e. swell) as the moisture content of the soil increases and decreases in volume (i.e. shrinkage) as the moisture content decreases. Although the expansion potential of a soil can be related to many factors (soil structure, environmental conditions, etc), it is primarily controlled by the clay mineralogy and moisture. Soils that contain kaolinite will tend to exhibit a lower shrink/swell potential than soils containing montmorillonite.



Known as vertisols in agriculture (see Section 2.4 and Figure 2.5), expansive soils are found in the central, north-western and eastern highlands of Ethiopia, in western lowlands around Gambella, and in some parts of the rift valley. Local deposits of these soils are also present throughout the country near rivers; water logged areas; and in drainage restricted localities. Damage caused by expansive clays is particularly prevalent around Addis Ababa.

The pattern of swell and shrink is attributed to the dry and rainy seasons in a year. During the dry period, desiccation cracks are formed because of shrinkage. In the rainy season, on the other hand, water enters the cracks and forces the soil to swell. These cycles of swelling and shrinkage can be detrimental to the performance of pavement structures.

Unlike collapsible soils, deep-seated volume changes in expansive soils are rare. More common are volume changes within the upper few meters of a soil deposit where seasonal moisture content changes due to drying and wetting. The zone of seasonal moisture variation over which volume changes are most likely to occur is defined as the active zone. The active zone can be evaluated by plotting the moisture content with depth for samples taken during the wet and dry seasons. The depth at which the moisture content becomes nearly constant is the limit of the active zone (which is also referred to as the depth of seasonal moisture change).

The active zone is an important consideration in road design and construction. In the design of bridge piles or drilled shafts for example, it is important to recognize that full side friction resistance may not be realized in this zone. As the soil undergoes cycles of shrinkage, it may lose contact with the pile or shaft. Alternatively, as the soil swells, it may impose significant uplift pressures on the foundation element. Similarly, in this zone, a reduction in shear strength, elasticity and bearing capacity may occur as a consequence of expansion. 7.3.1 Identification

In the field, the presence of surface desiccation cracks or fissures on a clay deposit are indications of expansion. The most problematic expansive soils are typically highly plastic, stiff, fissured, over-consolidated clays. In Ethiopia, these are characteristics of the black clays (commonly known as "black cotton soils"), found in the central highlands and some other areas. However, red clays in the wetter regions of Ethiopia (central and western highlands) also show the nature of having high plasticity (Figure 7.7), and should be subjected to laboratory tests. The clay mineral present in these soils is usually kaolin of the halloysite type, not montmorillonite. But, kaolin in the form of halloysite has a disordered structure, which gives rise to a soil of high potential plasticity than well-ordered kaolinites. Red soils remain plastic until hydrated halloysite is converted to meta-halloysite upon drying. This is accomplished in short period of time compared with black clays which remain wet longer.



Figure 7.7: Red Clays with Significant Plasticity around Bako in Wellega.

To identify expansive soils, several empirical relationships have been developed. Currently, a standard classification procedure doesn't exist. Generally, soils with a plasticity index (PI) of less than 15 percent will not exhibit expansive behaviour. For soils with a PI greater than 15 percent, the clay content of the soil should be evaluated in addition to the Atterberg limits. Activity can be computed from the plasticity index and clay fraction to evaluate the swell potential as shown in Figure 7.8.

Figure 7.9 relates expansion potential and collapsibility to liquid limit and in-situ dry density. Additional tests for the qualitative assessment of expansion potential include percent swell calculated from the CBR test (ASTM D4429), the free swell test, and the expansion index test (ASTM D4829). Such correlations are semi-empirical and should only be used for an initial assessment of the expansion potential of a soil. If any of the above empirical relationships or tests indicate a potentially expansive soil, laboratory testing should be conducted on undisturbed samples to determine the potential swelling pressures.



Figure 7.8: Classification Chart for Swelling Potential (after Seed et al, 1962)

Figure 7.9: Guide to Collapsibility and Expansion (after Mitchell and Gardner, 1975 and Gibbs, 1969).



7 .3.2 Laboratory tests

For road construction in areas with expansive soils it is necessary to estimate the magnitude of swell (surface heave) and the corresponding swelling pressures that could occur when the soil is wet. A one-dimensional swell test can be performed in an oedometer on undisturbed or recompacted samples according to AASHTO T256. In this test, the swell potential is evaluated by observing and measuring the swell of a laterally confined specimen when it is surcharged and flooded with water. Alternatively, after the specimen is soaked, the height of the specimen is kept constant by adding load. The swelling pressure is then defined as the vertical stress necessary to maintain zero volume change over a certain period of time. Swelling pressures in some expansive soils may be relatively large such that the static loads imposed from the structures of roads do little to counteract heave.

The US Bureau of Reclamation developed a correlation between observed volume change and colloidal content, plastic index, and shrinkage limit. The measured volume change is taken from oedometer swell tests using a surcharge pressure of 7KPa from air-dry to saturation conditions. Table 7.1 summarizes the result of this correlation.

As noted previously, potentially problematic expansive soils are located near the ground surface. These soils have natural moisture contents equal or less than the plastic limit of the soil. Practically, the shear strength of these materials is relatively high and therefore presents no major concerns to shallow foundation bearing capacity or embankment slope stability.

However, if the road design doesn't include systems that reduce the potential for large moisture changes, then laboratory strength testing needs to be performed for the most critical saturation condition the soil is expected to incur in the field. If the near-surface soils undergo relatively large seasonal changes in moisture content, then shear strength testing should be performed on specimens at the anticipated highest moisture content.

7.4 Collapsible soils Collapsible soils are described as soils that undergo a significant, sudden and irrecoverable decrease in volume upon wetting. These types of soils dominantly contain silt and sand with some clayey material and rock fragments. They are usually associated with areas of moisture deficiency, such as those in arid and semi-arid regions. In Ethiopia, they are present in the southern part of the Omo River and in the central and southern part of the rift valley. Often, their existence around Zeway, Shashemene, and Awassa is manifested by the occurrence of ground cracks and potholes during heavy rains or floods due to hydro-compaction (Figure 7.10). In the Afar region, collapsible soils are present in the form of sand dunes.

Table 7.1: Classification of Expansive Soils according to US Bureau of Reclamation

Colloid content % - 1µm PI (%) SL (%) Potential expansion (%)

Degree of expansion

<15 <18 >15 <10 Low 13-23 15-28 10-16 10-20 Medium 20-31 25-41 7-12 20-30 High >28 >35 <11 >30 Very high



Collapsible soils usually exist in the ground at low values of dry unit weight (density) and moisture content. In their natural conditions, collapsible soils can support moderate loads and undergo relatively small settlements. They are also moderately strong and exhibit a slight but characteristic apparent cohesion. Usually, this cohesion is the result of calcareous clay binder that holds the silt particles and rock fragments together. The clay coating and the silt create a very loose soil structure with little true particle-to-particle contact. Upon wetting, however, the cohesion is lost and large settlements can occur even if the load remains constant.

Figure 7.10: Collapse holes near Shashemene

7.4.1 Identification

For rapid identification, liquid limit values can be used. If, under natural circumstances, the void ratio of a given soil is higher than that at its liquid limit, the soil will lose strength on absorbing water. Before saturation is achieved, the soil will undergo considerable structural collapse accompanied by an appreciable reduction in volume. If this is observed, then laboratory testing of undisturbed samples should be performed to quantify the magnitude of volume reduction. Silt containing collapsible soils is also extremely erodible.

Typical pit excavation and disturbed sampling procedures can be used to obtain soil samples for sieve analysis, hydrometer, soil classification, and Atterberg limits. For samples to be collected at shallow depths, it may be prudent to obtain block samples from trenches or test pits. In the rift valley, there are indications that the thickness of these soils is greater than 8m. In this case auger sampling or shallow boring can be considered. 7.4.2 Strength

The shear strength of collapsible soils is greatly affected by the degree of saturation of the soil. Therefore, it is essential to develop an accurate estimate of the depth to groundwater and assess whether the degree of saturation for the deposit is likely to change during the design life of the road (i.e. if there will be a change in the position of the groundwater table).



Typical in-situ testing procedures such as DCP and CPT can be used to assess the current shear strength of the soil deposit only if the condition will be maintained during the project design life. If the soil eventually becomes saturated in-situ tests alone are not enough to develop design parameters. In-situ tests should only be used to assess shear strength properties if the results are correlated to laboratory values performed on samples at similar moisture contents and saturation conditions. The potential for collapse can also be evaluated in the field by performing standard plate load tests under varying moisture conditions.

Although not a design parameter, tensile strength is also important in collapsible soils to analyse their ability to support overlying loads and withstand tectonic forces. Studies on soil samples taken from different places in the central rift valley indicated that the tensile strength reached a maximum value at the optimum moisture content and decrease thereafter. Many ground cracks observed in the rift occurred two or three hours after flooding and inundation. In this case, collapse might be initiated at shallow depth and propagated upward leading to sudden and non-uniform settlement and cracking of the ground surface, including the road layers. 7 .4.3 Collapse potential

For situations in which it is necessary to construct a road on collapsible soils it is of primary importance to estimate the magnitude of potential collapse that may occur if the soil becomes wet. The amount of collapse normally depends on the initial void ratio, stress history of the soil, thickness of the collapsible soil layer, and magnitude of the applied stress.

To estimate the magnitude of potential collapse in an area, a one-dimensional collapse potential test can be performed in an oedometer on undisturbed or recompacted samples according to ASTM D 5333. For this test, a sample is placed in an oedometer and the vertical pressure on the sample is increased to the anticipated final loading in the field. At this load level, water is introduced into the sample and the resulting deformation due to collapse is recorded. The percent collapse (%C) is defined as:

%퐶 = ∆퐻푐퐻표

Where ΔHc is the change in height upon wetting and Ho is the initial height of the specimen. Collapse is also described using void ratio in the form of the following:

%퐶 = ∆푒푐

1 + 푒표

Where Δec is change in void ratio upon wetting and eo is the void ratio before saturating the soil.

The collapse potential (CP) is calculated as the percent collapse (%C) of a soil specimen. The CP is an index value used to compare the susceptibility of collapse for various soils. Table 7.2 provides a relative indication of the degree of severity for various values of CP.



Table 7.2: Qualitative Assessment of Collapse Potential

Collapse Potential (CP) Severity of Problem

0 - 1% None

2 - 5% Slight

6 - 10% Moderate

11 - 20% Severe

> 20% Very severe

7.5 Dispersive soils

Soils in which the clay particles will detach from each other and the soil structure and go into suspension without a flow of water are termed dispersive clays. These soils deflocculate in the presence of relatively pure water to form colloidal suspensions and are therefore highly susceptible to erosion and piping. Normally, they contain a higher content of sodium in their pore water than other soils. However, there are no significant differences in the clay contents of dispersive and non-dispersive soils although soils with high exchangeable sodium such as Na-montmorillonite clays tend to be more dispersive than others.

In Ethiopia, dispersive soils exist in the rift valley, the southern and eastern lowlands, and Afar, Somali and Tigray regions. Isolated occurrences of these soils can also be found in other parts of the country. They are common in areas where pyroclastic deposits and weak sedimentary rocks outcrop. Normally, dispersive soils tend to develop in low-lying areas with gently rolling topography and relatively flat slopes. Their environment of formation is also characterized by an annual rainfall of less than 850mm. Dispersive soils have low natural fertility. Often, they are calcareous with a PH value of about 8. Suspicion of their presence is indicated by the occurrence of erosion gullies and piping as shown in Figure 7.11.

Unlike expansive and collapsible soils, it is difficult to identify dispersive soils using conventional engineering index tests such as Atterberg limits, gradation or compaction characteristics. Hence, engineering properties may be used to evaluate the degree of expansion and collapse, but chemical properties determine the amount of dispersion in soils. This means a soil classified as expansive or collapsible based on engineering indexes can be dispersive if it contains high content of dissolved sodium in its pore water.



Figure 7.11: Erosion Gulies at Roadcuts in the Rift Valley near Arsi Negele. Generally, exchangeable sodium and cation exchange capacity are frequently used in countries like South Africa, and Australia to help distinguish dispersive soils as shown in Table 7.3. The exchangeable sodium percentage (ESP) in soils is expressed as follows:

퐸푆푃 = 퐸푥푐ℎ푎푛푔푒푎푏푙푒 푠표푑푖푢푚

퐶푎푡푖표푛 푒푥푐ℎ푎푛푔푒 푐푎푝푎푐푖푡푦 푥 100

High ESP values and piping generally exist in soils with a clay fraction composed largely of smectite. Some illite and halloysite containing soils are also dispersive. On the other hand high values of ESP are rare in clays composed of kaolinites (Bell and de Bruyn, 1997).

Table 7.3: Relationship between the Degree of Dispersion and % of Exchangeable Sodium

Rating Exchangeable sodium percentage Soil dispersion test

Non-sodic <6 No dispersion evident after 24 hours. Aggregates slaked but not dispersed (milky) clay.

Slightly sodic 6 - 10 Dispersion (milky halo) evident after 24 hours. Soil aggregates slightly disperse

Moderately sodic 6 - 10 Dispersion (milky halo) evident after several hours.

Soil aggregates partially disperse.

Highly sodic >15 Dispersion (milky halo) evident in less than 30 minutes. Soil aggregates completely disperse.

7 .5.1 Laboratory tests

In addition to the chemical tests needed to determine the ESP there are other laboratory tests which can indicate the potential for dispersion of a soil. Commonly, dispersive



characteristics are determined by performing three standardized tests and adopt a rating system.

The soil crumb test is an Australian standard for the prediction of the dispersive behaviour of clay soils. The test is simple and can be used in the rapid identification. It involves the immersion of an air dried aggregate in a beaker of distilled water, remoulding at near maximum field capacity and re-immersion, and shaking. The potential for dispersion is judged on the basis of the degree and speed with which the soil forms clouds in the beaker.

The dispersion test is an indicator test widely used in Australia to evaluate the susceptibility of soils to erosion. In this test, soil is shaken end over end in a dispersant to ensure complete dispersion. A second suspension is then prepared in a distilled water without using either dispersant or mechanical agitation. The difference between the amount of dispersion (measured as the % particles <2 microns) between the two tests is used to infer dispersion risk. The dispersion index is very similar to the Double Hydrometer test (ASTM D4211-83) routinely used in the USA for predicting dispersive behaviour of soils.

The pinhole test (ASTM D6572-06), originally developed to evaluate piping erosion, is performed by causing water to flow through a small hole punched in the specimen and observe the extent of dispersion. The method involves packing a 38mm long sediment sample into a 38mm diameter metal cylinder, attaching it to a base plate, creating a horizontal 1mm hole through the sample, and running distilled water through the hole for 5 minutes at increments of head from 50mm to 1,020mm. The water flow through the pinhole simulates the movement of water through a crack. Erosion in dispersive soils can occur at a head of 50mm, intermediate soils erode slowly, whereas non dispersive soils are supposed to produce no colloidal erosion even when the head is close to 1,020mm. 7.5.2 Field identification

While laboratory tests are a useful way of identifying dispersive soils, much can be determined by observing the behaviour of the soils in the field. For example, the occurrence of deep erosion gullies, ‘worm channels’, and piping failure in existing embankments indicates the presence of dispersive soils. Besides, erosion of road cuttings along ditches or gully lines, and weathered rock joints is a sign of potentially dispersive soils. Besides, cloudy water or high turbidity in ponds after rain is linked to the effect of dispersive soils.

The geology of the area can also be a guide to the presence of dispersive soils. Many dispersive soils are of alluvial origin. Soils derived from shale and claystone in sedimentary areas and pyroclastic sediments in volcanic regions are also dispersive in nature.

A simplification of the crump test can also be used in the field to identify dispersive soils. Collect soil aggregates (1-2 cm diameter) from each layer in the soil profile. If moist, dry the aggregates in the sun for a few hours until air-dried. Place the aggregates in a shallow glass jar of distilled water or locally available small bowl of rain water. Leave the aggregates in the water without shaking or disturbing for 2 hours. Observe and record if you can see a milky ring around the aggregates. Classify the soil on the basis of Figure 7.12. The soil is highly dispersive if discoloration and cloudiness extends throughout the jar or bowl.



Figure 7.12: Test for the Dispersive Nature of Soils.

Dispersive soils should be properly compacted to avoid piping. This is especially true near drainage structures. Care should be taken at the downstream end of culverts. As it is suggested in the Pavement Design Chapter, it is generally recommended to compact these soils at a moisture content above the OMC so as to form a flocculated soil structure. A moisture content between optimum and optimum plus 2% and a density ratio of greater than 97% is desirable. On no account should the moisture content be more than 1% below optimum. Most dispersive soils can be rendered non-dispersive by the addition of lime or gypsum. This process is one of cation exchange with Ca replacing Na. Laboratory tests should be carried out to determine the required amount of lime or gypsum. Commonly one would require from 2 to 3%.

7.6 Colluvial soils

All soils which have been transported by gravity forces and deposited in valleys, swales, or other low-lying topographic features, often with the aid of water flow, are called colluvial soils. They include slope-wash deposits, scree (talus), and landslide debris. Often, they are a result of a two-stage process: in place weathering of the parent rock and subsequent down-slope migration primarily by gravity forces. The soils are usually characterized by being mixtures of particles of contrasting sizes from highly plastic clays to boulders.

Slope-wash deposits are admixtures of clay, sand and gravel, which moved down-slope by the combined actions of soil creep and running water. Near the base of steep slopes, slope-wash soils often overlie or are inter-stratified with alluvial deposits. The thickest deposits are developed in depressions or gullies. Scree and talus are deposits of rock fragments which detach from cliffs or areas of steep outcrops and fall by gravity and roll or slide down-slope. The deposits are not water-sorted and are usually very loose and just stable at natural angles. When the deposits contain 30% or more of fine-grained soil, they are called talus.

Landslide debris constitutes materials which have moved downward during a slope failure. In most cases, the soils are very variable vertically and laterally and it is not uncommon to find large boulders embedded in clay matrices. Deposits of landslide debris are often underlain by a sheared or slicken-sided zone (the slide surface) and there may be several sliding and shear surfaces at different depths within the debris. In many cases, the main slide surface may be in a zone of material which appears to be residual soil or extremely weathered rock and is characterized by higher clay content than that of most of the debris.

In Ethiopia, colluvial soils are present in the middle slopes and foothills of mountainous areas. They are especially common in northern, western and eastern highlands, and in river basins such as the Blue Nile (Figure 7.13), Tekezze and Gibe. Colluvial soils are also



present near the bases of isolated hills and domes scattered in the rift valley and other areas.

Figure 7.13: Colluvium from Basalt and Volcanoclastic Rocks - Blue Nile basin

7.6.1 Exploration techniques

During road construction, cut slopes often have to be made in colluvial deposits located near the base of a slope. In many cases, the cut slope exposes the colluvium. Because these materials are formed by migration and sliding along the slope, they are often only marginally stable in their natural state. Therefore, the cut slopes made in these deposits tend to disrupt the natural equilibrium, thus requiring special attention during investigation.

Typical boring and sampling techniques can be utilized in colluvium to obtain samples for evaluating the physical characteristics of the material, provided it consists primarily of fine-grained particles and relatively thin lenses. In many cases, however, colluvium exists as a massive deposit more than 5m in thickness with boulders and cobbles. Drilling and sampling in colluvial soils with large rock fragments can prove to be difficult. In addition, for new roads, potential access problems due to steep slopes and marginal stability typically make investigation in colluvium difficult and expensive.

Test pits and trenches represent economical alternative exploration methods for colluvium. This approach allows visual observation of the subsurface conditions in these materials. Often they are backhoe-excavated. However, pits can also be excavated by hand in inaccessible areas. Undisturbed block samples of colluvium can be cut from the sidewalls of pits and trenches. In cases where rock fragments do not preclude sampling, colluvial soils may be sampled by manually advancing Shelby tubes. Test pits and trenches can also be used to evaluate the depth to bedrock and groundwater. Usually, the colluvium/rock interface provides an impermeable layer that allows water to accumulate in the colluvium.

In an extended investigation, small inclinometers socketed into the underlying bedrock can be used to monitor whether the colluvium is actively creeping down the slope. The



measured displacements provide an indication of the relative stability of the colluvial soil slope.

Exploration in talus is more demanding than any other colluvial deposit. This is because talus slopes includes particles that range from silt and sand to boulders. Although individual samples may be recovered, they are often of little use because these small samples are indicative of only the matrix component of talus, and are not at all representative of the mass itself. Thus, characterizing the properties of talus using conventional boring and sampling is extremely difficult. Similarly, any in-situ test method like DCP or CPT is often not useful because the penetration would likely meet refusal on a large rock or boulder.

Hence, the occurrence of talus can probably best be identified by the use of aerial photographs and site reconnaissance. This can be followed by geophysical study to determine the depth to the bed rock, especially where a study over great lengths is required. 7 .6.2 Engineering characteristics

Colluvial soils are likely to be highly permeable, and compressible. As they are sorted, they are normally poorly graded. As these materials occur close to their natural angle of repose, excavation into scree or talus slopes usually causes ravelling failures extending upslope. Entry of excessive water into talus materials can cause them to develop into debris-flows.

Landslide debris is often only marginally stable and slope instability may be initiated by minor changes to the surface topography or to groundwater conditions. When considering the possible use of landslide debris as fills, the critical issues are the potential variability of the soil and the possible need to remove large boulders and cobbles.

The compressibility of many colluvial soils can generally be assessed in the laboratory using the standard oedometer test, provided that undisturbed samples can be obtained in the field. However, the mixture of granular material and boulders in talus often leads to differential settlements, and compression properties are difficult to assess due to this heterogeneity. Because of this, the design of the road should not depend on the strength of talus deposits.

Shear strength of colluvium can be assessed from laboratory tests on undisturbed samples. In some cases, however, the colluvium is relatively strong. It is the colluvial material and potentially weathered rock at the colluvial soil/rock interface that represents the weakest point. It is here that the greatest potential for sliding exists especially when there is an accumulation of water. The shear strength of the material at this interface can be evaluated by performing laboratory direct shear tests on remoulded samples at the expected in-situ moisture content. In the test, residual conditions should be evaluated, especially if there is evidence that the material had been previously displaced as a result of major landsliding or creep.

A reliable method to evaluate the strength of colluvial soils along road cuts is to back-analyse a failed or failing slope in close proximity to the area of interest. Back-analysis involves performing stability analyses where the values of the cohesion intercept and friction angle are modified to achieve a factor of safety of 1.0. This method for



determining shear strengths is only accurate if a thorough assessment of the slope geometry and groundwater table is made. Additionally, the depth to competent rock and the location of a known or anticipated slip surface should be assessed via borings or by geophysical methods.

7.7 Lateritic soils

Laterites are the products of intensive and long lasting tropical rock weathering assisted by high rainfall and elevated temperatures. In practice, laterite formation requires particular conditions which concentrate the iron and aluminum rich weathering products sufficiently to allow concretionary development, resulting in a cemented horizon within the profile. Hence, the process involves tropical weathering to produce the minerals of laterite, concentration of these minerals in a discrete horizon, and concretionary development within the horizon.

The factors that affect the development of laterites are climate, topography, and drainage. Laterites can be formed from any rock, but the speed with which they are formed is to a certain extent governed by the availability of iron and aluminum, and the amount of silica. Hence, basic igneous rocks, which contain high amount of iron and aluminum, can easily form laterites under oxidation. Rocks rich in quartz, on the other hand, resist weathering and may not degrade into laterite easily and quickly. Instead, the end product in these rocks is a granular soil with fine materials forming the matrix around quartz nuclei.

Climate controls the formation of laterites more than any other factor as laterites in general require hot, humid conditions. A mean annual temperature of 25oC has been suggested for their formation. The minimum annual rainfall is thought to be around 750mm. The higher the rainfall above this value, the greater is the leaching effect, which removes free silica.

Topography affects the amount, rate and direction of water flow in an area. On steep slopes, weathering products are quickly removed by running water before they are changed into laterites or red soils. On gentle slopes, erosion is limited and long uninterrupted weathering provides time to produce laterites. On level ground, on the other hand, drainage is impeded and the area becomes waterlogged to form black soils at the expense of laterites.

In addition, the concentration of weathering products in distinct horizons in a laterite profile can be linked to fluctuating groundwater following seasonal changes in climate.

However, many laterites are ancient (Tertiary) deposits which can now be found in very different climatic and landscape conditions from those in which they were first formed. In Ethiopia, they are distributed in the north-western, western, and southern part of the country. This includes the areas around Assossa (Figure 7.14) and many places in western Gojam.

7.7.1 Identification

The description of laterites in the field should include their strength, colour, degree of cementation, grading, amount of thickness and variation with depth, lateral continuity, and ease of excavation. Laterites may contain quartz gravels which can be an important factor



in pavement performance. This should be assessed and reported. For agricultural purposes, red soils and laterites are often classified as Ferralsols, Nitosols, or Luvisols (see Figure 2.5).

Aerial photographs are useful to identify and map laterites in remote areas. The landforms where they occur include flat hilltops with sharp edge, gentle foot-slopes, rounded hill on hillcrest or plain, and edge of flat valley floor. Often, laterite zones can be identified by elevated slopes since the hardening of laterite protects the area from erosion. There are few or no streams on lateritic slopes, soils are less fertile, and the vegetation cover is characterized by shrubs or grassland with scattered bushes. On black and white aerial photos, the natural strong dark red colour appears as dark grey with some masks where there is vegetation.

Figure 7.14: Nodular Laterite - Assossa-Kurmuk Road Project.

Since the laterite horizon that can be used for road construction is located at shallow depths, they can be investigated by trial pits. The spacing of pits depends on the extent of the horizon and it is advisable to first start with a few pits and decrease the spacing later. Like any material, the deposit is likely to vary in thickness, depth and quality both at an angle and vertically.

Careful attention should be given to sampling. Laterites are distinct horizons and the whole profile should be logged, with the samples clearly defining the potentially useful horizon. Sufficient and representative material should be taken to allow for compaction and CBR testing in addition to classification tests and natural moisture content. 7 .7.2 Special properties

In many places, concretionary laterite has been a traditional source of road aggregate, not only in the form of nodular laterite (lateritic gravels), but also as a crushed derivative of



old lateritic profiles (honeycomb and hardpan laterites). Nodular laterites commonly comprise a gravel fraction of rounded concretionary nodules in a matrix of silt and clay, often without sand particles. Hence, when their natural grading is close to a mechanically stable particle size distribution, they perform well both as a base and sub-base of low volume roads. They can also be used as gravel wearing course. Sometimes, however, the silt and clay content renders them moisture sensitive and this has to be checked in the field.

Laterites show an irreversible change in plasticity on drying. This is partly because dehydration of alumn-oxides creates a stronger bond between the particles, which is resistant to water. The process cannot be reversed by rewetting. The effect takes place during air-drying but becomes more pronounced on oven drying at higher temperatures.

Another aspect of lateritic materials which is relevant for road construction is their self-hardening nature that increases their strength. This is seen as a time-dependent improvement in performance. Any potential improvement in performance as a result of self-hardening is dependent on the proportion of oxides present in the matrix.

Immature or relatively young laterites, known as plinthite, are the most likely to exhibit self-hardening. However, they lack a mechanically stable grading for use in road construction. Immature nodular laterites may also have self-hardening properties and could undergo significant improvement with time. Hence, the laterites which are normally considered mechanically unstable and too plastic using standard requirements have been known to give satisfactory performance in service. For this reason, many nodular laterites can be used as base, sub-base and gravel wearing courses, especially for low volume roads.

7.8 Saline soils

Soluble salts covering large areas of lower ground often occur in arid regions. These salts are normally chlorides and sulphates of calcium, magnesium, sodium and potassium. In Ethiopia they cover the northern portion of the Afar region in and around the Dallol depression (Figure7.15). The presence of small amounts of salt does not have a major effect in soil embankments and road-bases. But when the quantity is high as is the case in Dallol, salt can produce rapid corrosion of metal reinforcement in concrete. Hence, salt bearing sands should be washed before they are used for concrete making. As much as possible, the concrete should also be impermeable.



Figure 7.15: Salt Deposits in the Dallol Depression (Northern Afar region).

Salts have no deleterious effect on bituminous materials but blistering of the surface as shown in Figure 7.15 can occur where salts are present below the road. Such blistering occurs when evaporation exceeds precipitation, bringing saline groundwater upwards to the surface. This process is hygroscopic in nature, so that when evaporation brings saline water upward during the heat of the day, more salt comes from below throughout the cool of the night.

It has been suggested that a dense bituminous surface of about 30mm in thickness can inhibit the day time evaporation and reduce the rate at which the salt crystals develop. In some cases, however, there might be a need to use coarse grained aggregates at the sub-base level to limit evaporation. When water exists, ponding can also be used to remove salts from soils.

The occurrence of saline soils in an area and their geographic distribution is almost always controlled by climate. Hence, aerial photographs are usually enough to map their spatial extent. Pits might only be needed to assess the concentration of salts with depth.

7.9 Degradable rocks

On many road projects, construction activities involve the use of potentially degradable rocks. Although these materials may at first exhibit rock-like characteristics, they have the ability to degrade to soil-size particles. Often, the gradual but ultimate degradation occurs quickly. Shale is the most common member of this family, but includes clay-stone, siltstone, marl and mudstone. Pyroclastic deposits such as ash and tuff are also easily degradable.

In some regions of Ethiopia, where high-quality granular soil is not locally available for use in the construction of embankments or rock-fill, degradable materials such as shale and



pyroclastic deposits that at first appear to be suitable are often used. However, once in contact with water, these materials may degrade causing problems or failures during the service life of the road. Deep bridge foundations, especially rock-socketed drilled shafts, which may be designed to provide intimate contact with and support from the rock interface, may fail as the materials degrade and contact is lost. In shallow foundations, the bearing capacity may decrease as the materials degrade resulting in settlements.

Additionally, in road cuts, the exposure of the cut to the air may result in significant degradation during or soon after construction ends (Figure 7.16). The degradation can take the form of swelling, weakening, and disintegration. The effect of degradation on slope stability can range from sloughing at the surface and the gradual retreat of the face, leading to catastrophic slope failures as a consequence of the significant loss in strength. In sedimentary formations comprising alternating beds of resistant sandstone and degradable shale, the weathering process can develop overhangs in the sandstone and produce rock falls as observed in the Blue Nile basin, Dire Dawa area, and in the Tigray region where these rocks are present.

When potentially degradable materials are encountered, it is essential to establish the anticipated performance of the materials over the design life of the road. An assessment of the time required for significant degradation relative to the service life of the structure should be evaluated. Commonly, the point load test, the slake durability test (ASTM D4644), and the jar slake test are used to make this assessment. The jar slake test is somewhat easier and less quantitative than the first two and can be used for rapid evaluation for use in LVRs.

Figure 7.16.: Degradable Shale Underlying a Sandstone Layer - Road Cut near Kulbi. The procedure for the jar slake test is summarized below:

A piece of oven-dried material is immersed in enough water to cover it by 15mm. After immersion, the piece is observed continuously for the first 10 minutes,

followed by an additional 20 minutes of discontinuous, but careful, monitoring. If a



reaction between the water and the rock is to occur, it will usually happen during this time.

A final observation of the condition of the rock is made after 24 hours. Based on the visual observations, the Jar Slake Index is established using the

criteria described in Table 7.4.

The result from the jar slake test can be used to roughly assess whether any potentially degradable material may be considered as a rock-fill, soil, or as intermediate material between soil and rock-fill. Intermediate materials are considered to be non-durable materials and need to be conditioned to be soil-like prior to any use as a construction material.

Table 7.4: Guide to Interpret of the Result of the Jar Slake Test

Jar Slake Index General behaviour during test Category

1 Degrades rapidly to a pile of flakes or mud Soft, non-durable materials treated as soil. 2 Breaks rapidly and/or forms many chips

3 Breaks slowly and/or forms few chips Hard, non-durable intermediate material 4 Breaks rapidly and/or develops several fractures

5 Breaks slowly and/or develops few fractures

6 No change

Durable rock-fill materials, if the material finer than gravel-sized fraction is less than 20 to 30%

7.10 Groundwater

The impact of groundwater on the long term performance of a road requires thorough investigation during exploration. Unidentified subsurface water has been responsible for pavement failures in many places. If groundwater is not identified and adequately addressed early, it can significantly impair constructability, road performance and slope stability. Claims related to unforeseen groundwater conditions often form a significant proportion of contractual disputes. Many of these claims originate from a failure to record adequate groundwater information during site investigations.

As shown in Figure 7.17, groundwater can affect the roadway when contact springs or seeps are intercepted in cut sections, when fault or artesian springs appear either above or below the road, and when there is a seasonally fluctuating groundwater table. A spring or seep is a place where water from an aquifer discharges naturally onto the land surface. There are two types of springs: gravity and artesian. Water may flow by gravity from a water table aquifer or by pressure from an artesian aquifer. Thermal springs are considered as artesian springs. Gravity springs result where water moves from the water table aquifer through a permeable formation to the land surface because of an elevation gradient. They are normally low yielding, but may supply enough water that can destabilize the road cut and the pavement structure.



There are three principal types of gravity springs: depression springs, contact springs, and fracture springs. A depression spring is formed when the land surface intercepts the water table in permeable material at low topographic spots. A contact spring is developed when the downward movement of water is restricted and deflected laterally to the land by a layer of impervious material. Fracture springs occur when water emerges from joints of a rock.

Sinkhole springs appear from solution channels in limestone or gypsum. Faulting can also form a boundary to ground water flow and force water in the aquifer to discharge as a fault spring. Generally, spring flows vary considerably throughout the year, depending on the rise and fall of water in the water table or the variation of pressure in an artesian aquifer.

In the field the presence of wetlands and thick vegetation, seepage and springs from natural slopes, free flowing wells, evidence of slope instability such as vertical scarps, old landslides and inclined trees, fractured rocks, transported sandy soils, and unlined irrigation canals and dams, indicate the occurrence of groundwater. The geology of the area, as described in Table 7.5, can also suggest whether a substantial amount of seepage is expected.



Figure 7.17: Illustration of the Movement and Occurrence of Groundwater Near Roadways



Table 7.5: Characteristics of Rocks as Potential Sources of Seepage at Road Cuts Types of rocks Occurrence of seepage or springs

Metamorphic and intrusive igneous rocks

Fresh and strong bedrocks of metamorphic and intrusive igneous rocks contain very little or no primary porosity and yields water only from fractures within the rock (secondary porosity). Typically, well yields are very low. Springs go dry after flowing for only a short period of time. In general, these units neither store nor transmit much water and are of only minor importance as sources of seepage.

Sandstone and limestone

Typically, the primary porosity in sandstones is very significant but it is negligible in limestones. However, the secondary porosity in limestones may be considerable. Sandstones are most important as sources of groundwater. Limestones are also the sources of some of the largest well and spring yields. Both karstic and fractured limestones are capable of producing very high amount of seepage.

Shale, clay-stone, marl, siltstone and mudstone

Thinly bedded weak sedimentary rocks contain some secondary porosity along fractures and minor primary porosity along bedding planes. These bedded clays and silts generally do not yield adequate water to supply springs.

Sand and gravel

Unconsolidated mixtures of sand and gravel contain varying amounts of fine materials. When the latter is a small amount, their ability to transmit groundwater can be high, and springs from them may come with a high amount of water.

Volcanic rocks

Extrusive igneous rocks contain secondary porosity along fractures. When well fractured, they often have high spring yields. Basalts, rhyolites and other volcanic rocks are among the most productive water-bearing formations in Ethiopia.

Weathered rocks

Unconsolidated materials, commonly termed regolith or saprolite, are derived by weathering of the underlying bedrock, and contain only primary porosity. Water generally moves readily in these materials, but well and spring yields are commonly low because the available thickness is often insufficient for an adequate supply.

In upgrading or rehabilitation projects, distress or patch patterns on existing pavements, wet spots or sub-grade material piping through cracks or joints, and undulating road surfaces may suggest the presence and seasonal fluctuation of groundwater in the area.

In all phases and types of investigation, an appropriate record of the groundwater condition should be prepared. The record should include the source of groundwater, the location of seepage points or springs with respect to the road alignment, the amount of groundwater inflow, the elevation of the water table and its variation with changes in river discharges, seasonal effects, and pumping from nearby wells. The source of seepage may be from water stored in the aquifer or an adjacent stream or lake. This depends on the geological and environmental features of the area and the permeability of the rock. Measurement of seepage flow rates gives a broad indication of the proportion of rainfall which infiltrates, the response of the groundwater to storms, and areas where high groundwater flow rates may be expected.

If borings are drilled along a road alignment where groundwater is expected, groundwater levels should be monitored for a sufficient period of time to establish the seasonal variations likely to occur during the construction period and beyond. Both short and long term methods can be used to observe the groundwater level. Short-term methods rely on observing drilling tools and cuttings for water contact and measuring the depth from the



ground surface to the water level. These observations are more reliable in sands and gravels. Long-term measurements need the installation of piezometers and the assessment of historical data.

Groundwater must also be investigated to determine the occurrence of a perched water table and its level and the presence of sub-artesian conditions. When water is encountered in a boring, it should be reported as a "water level" unless its specific occurrence is known (i.e. water table, perched, or artesian). The chemistry of the groundwater should also be checked because seepage from some rocks may be highly corrosive to metals and concrete. This is especially true in broad flat areas of Afar and Somali regions where soils contain abundant sulphates and have a widespread surface or near-surface crusts of halite. Changes in the temperature of the groundwater should also be monitored and recorded.

Groundwater is frequently encountered along road cuts in many parts of Ethiopia. In areas where springs and seepages are present, there are several good indicators that may be used to determine the height that groundwater may rise in a slope and roughly how long during the year that the slope remains saturated. In the highland areas where weathered basaltic lava flows are common for instance, iron compounds within the slope usually oxidize when in contact with groundwater and turn rusty red or bright orange and give the soil a mottled appearance. The depth below the ground surface where this effect first occurs indicates the average maximum height that this fluctuating water table rises in the slope.

At locations where the water table remains for long periods during the year, the iron compounds are chemically reduced and give the soil layer on the slope a grey or bluish-grey appearance. The occurrence of these soils indicates a slope that is saturated for much of the year. Occasionally, a mottled appearance may appear above a grey subsoil. This indicates a seasonally fluctuating water table above a layer that is subjected to a prolonged saturation. The geotechnical engineer should be aware of the significance of mottled and grey soils that are going to be exposed during road construction. These soil layers give clues to the need for drainage or extra attention concerning the stability of the road cut.

In general, the Geological Survey of Ethiopia or water drilling agencies should be contacted if boring and long term surveys are needed. The installation and closure of wells and standpipes penetrating the groundwater table is regulated by government laws and regulations and should only be pursued by qualified institutes in compliance with all applicable regulations.

7.11 Wetlands

Wetlands are dynamic ecosystems that have characteristics between deep-water bodies and uplands. Some wetlands are wet all year-round, and others are wet only seasonally. There are different types of wetlands in Ethiopia. These include:

Seasonal and permanent riverine wetlands along many of the minor and major rivers such as Awash;

Palustrine wetlands (swamps and marshes) in the highlands and adjacent lowlands; Lacustrine wetlands in the surroundings of lakes in the rift valley; Montane peat bog wetlands near towns and villages; Man-made wetlands around dams such as Koka and Metahara.



The design and construction of roads across wetlands usually has a significant impact on the surrounding ecosystem. Hence, care needs to be taken during site investigation or earlier in the route selection process to locate the road alignment away from wetlands. However, when wetlands must be crossed, it is advisable to minimize the total wetland road distances and their width. The design should also include upland road approaches to wetlands so that surface runoff is diverted before entering the wetland. Similarly, it is worthwhile to avoid using fills in wetlands if other alternatives exist. However, if a fill is necessary, then the design should include wide culverts in the fill to prevent constriction of expected flood flows.

During site investigation, the survey should, as a minimum, determine the type and depth of wetland soils. Normally, there is a need to prepare a location map that shows the boundaries of wetlands, and profiles and cross sections which describe the existing landscape. These might include channel grades, existing drainage systems, bridges, culverts, channel restrictions, existing ground elevations and topographic information needed for the design of structural components, and the location and elevation of necessary soil borings.

When borings are needed, their intervals shall be determined by the size of the wetland, the type of road, and the characteristics of subsurface materials. A minimum of three shallow borings is required when relatively uniform wetland bottoms are encountered. These borings are usually taken at the centreline and on each side of the road, halfway between the shoulder and the toe of the slope. At least one boring should extend about 3m below the apparent wetland bottom to provide adequate evidence against a false rock stratum. In the absence of borings, the degree of bottom resistance may be assessed by advancing augers. Any CBR and swell test for wetland soils shall be conducted under long (ten-day) soaked conditions.

Scheduling of work in wetlands is very important. In some swamps and marshy areas, it may be easier to access the site during dry periods. The size of land covered by water in these areas varies significantly from year to year. Looking into the condition of the local climate may help deciding the time of access that best suits site investigation.

Permits may be needed when a road crosses wetlands. Hence, it is recommended to check regulations and get advises from the relevant local governmental departments.

7.12 Disposal sites

Site investigation is also sometimes necessary for selection of sites in advance to dispose of excess materials from excavations and grading, and for long and short-term stockpiling of these materials so that they can be used during maintenance. The general watershed criterion for selecting any disposal site is an area where the material will not erode into any part of the channel network, and where it will not initiate a formerly dormant landslide. The site investigation process is useful to determine the location of existing disposal sites, potential disposal sites, and locations of significant spoil generation along roads.

Site investigations should include the disposal area size, distance to watercourses, potential slope instabilities, listed species habitat, archaeological sites, nearby residential areas, access, and other limiting factors. Preparing a map as early as possible that indicates sites



(existing and potential) with acceptable site characteristics will help later during construction. The objective of the site investigation shall be the following:

Seek a stable site where sediment cannot reach the stream during any high water event.

Avoid adjacent riparian corridors or any area within the 100-year floodplain. Avoid all wetland sites as these sites are protected from disposal activities and

permits will be required and may not be granted. Avoid placing spoil on unstable slopes, where the added weight could trigger a land

movement. Excessive loading of clay or silt soils could also trigger a failure. Use wide, stable locations such as rock pits, ridges, and benches as places to

dispose of fill. Avoid locations where ground water emerges or a thick organic layer is present. Avoid sites with endangered or threatened plant species.

Test pits and trenches are normally sufficient to investigate potential disposal sites. Sometimes, the pits should be deep enough to better define the underlying materials and their stability, and determine the level of groundwater and any seasonal fluctuation.

Chapter 8 Site Investigation Manual – 2013 Construction Material Surveys


8 CONSTRUCTION MATERIAL SURVEYS 8.1 Introduction

One of the principal concerns of road design and construction is the availability of sufficient suitable construction materials in the vicinity of the alignment. Material availability is critical to the road design and the design is not complete if it does not contain a complete list of the borrow pits and quarries from which soils and rocks are to be taken. Generally, apart from the quality and volume, borrow pits and quarries must be:

Accessible and suitable for efficient excavation; As close as possible to the site to minimize costly hauling of materials; Economical for use in the construction with little or no treatment; and Located to ensure that their exploitation will not lead to any complicated legal

problems and will not unduly affect the local inhabitants or adversely affect the environment.

Often, exploration of an area to establish the source of construction materials has the objective of determining:

The nature of the deposit, including its geology; The history of previous excavation and possible mineral rights; The depth, thickness, extent and composition of the strata of soil or rock that are to be

excavated; The condition of groundwater including the position of the water table, its variations,

and possible flow of surface water into the excavation ground; and The property of soils and rocks, for the purposes intended as well as the purposes for

which they had been used previously.

8.2 Investigation procedures

Table 8.1 presents the techniques that are needed to identify and locate the source of construction materials. Normally, the search for materials in road design start during the desk study phase of site investigation. At this stage, the location of existing and previously used quarries and pits in the project area and estimates of quality and quantities in existing sources and any encountered problems should be established. It is also important to identify likely areas for further exploration, the range of sub-grade conditions along the proposed alignment, climatic details including rainfall, rainfall intensities and evaporation, the geology of the area, project materials required in terms of quality and quantity, project economic, contractual and time-related constraints, proposed road design standards, and likely soils and aggregate testing requirements.

Records of roads already built in the area can be a valuable source of data, not only on the location of construction materials, but also on their excavation, processing, placement and subsequent performance. Records of material usage are an essential part of efficient construction materials management. In Ethiopia, road design and construction records are kept by different departments of the Ethiopian Road Authority (ERA), the Regional Road Authorities, or by road design consultants and construction supervising organizations and contractors.

Chapter 8 Construction Material Surveys Site Investigation Manual – 2013


Table 8.1: Techniques that Assist the Investigation of Construction Materials Field activity Description

Surface mapping

Both geological and geomorphological mapping and may include the acquisition of hydrological, vegetation and climatic data and the mapping of earthwork exposures. Surface data gathering can comprise a formal materials inventory approach.

Aerial photo interpretation

The systematic recording of material characteristics including the use of in-situ behavioural testing. All samples and exposures should be logged using these standard guidelines

Filed survey and sample description

The systematic recording of material characteristics including the use of in situ behavioural testing. All samples and exposures should be logged using these standard guidelines.

Trial pits and trenches

May be hand or machine dug; particularly cost effective in the examination and logging of material fabric and the delineation of mass structure. Caution should be exercised in geotechnical interpretation of areas where weaker materials underlie strong rocks. Pits and trenches are very useful for obtaining bulk samples.

Boreholes

These may be sunk by a number of percussion, or rotary methods. The techniques employed should be chosen to take into account the type and condition of material involved. In some locations options may be restricted by economic or access constraints.

Augering Can range from hand augering to machine driven hollow stem augering with undisturbed sampling.

Probing

Relatively inexpensive procedure that can be effective in delineating boundaries to soft or weak materials and in the recording of general in situ material condition. Quarry drills may be used in conjunction with cored holes for correlation.

Engineering geophysics

Seismic refraction is the most generally used procedure; best utilized to interpolate or extrapolate in situ conditions in conjunction with boreholes. Caution required in environments where stronger material may overlay weaker formations.

In situ tests Utilized for calculation of CBR values for in situ sub-grade. Hydrology data Installation of piezometers to measure water table depths. Climatic data Possible set-up of thermograph, hygrograph an anemometer.

During the reconnaissance survey, the assessment of the area should aim to describe the general location of resources that were identified earlier from maps in the desk study, to provide recommendations of areas for further investigation. Access to these areas and environmental aspects should also be evaluated. In addition, the quality and quantities of available materials for different layers of the road should be defined at a basic level.

At the preliminary site investigation stage, the options available for selection of construction materials within a road project should be finalised. Sufficient information should be recovered to establish a material supply strategy and ensure that minimum costs are associated with providing suitable quality materials. A materials supply strategy requires factors such as:

Cost per cubic metre of extraction and processing to comply with quality requirements, including cost of land, royalties, quarry/pit preparation, extraction costs, processing costs and reinstatement costs;

Cost of haulage from source to site on road; largely a function of distance, although the relative state of quarry access roads and socio-economic factors are also important;



Resource size (e.g. the need to balance the economics of developing a number of small cheap quarries compared with one larger quarry);

The relative quality of all resources being considered; Environmental impacts and associated costs (e.g. costs over and above normal re-

instatement, cost benefits in terms of the utilization of quarry sites for waste disposal, etc.).

The final site investigation phase should aim at providing sufficient information that allows final decisions on the resources within the framework of the material supply strategy. At this stage, the three dimensional shape of deposits, the nature of their contact with surrounding materials, and the thickness of overburden and underlying strata must be known. Thickness and variation in thickness, together with removal and storage of overburden and waste are critical features that need to be assessed at this stage. The structures of hard rock resources need to be developed and the approach to extraction (i.e. blasting or mechanical excavation) should be worked out. The state of weathering or alteration should also be established as this may define the use for materials for different pavement layers. The principal outputs from this phase of investigation are detailed reports confirming construction material quantity and quality. 8.2.1 Aerial photographs

For many years, the search for construction materials were based on ordinary methods of exploration (location survey), from the simple examination of the ground to the use of open pits. In recent years, geophysical methods and aerial photo interpretation have gradually been added, saving a great deal of time, effort and exploration costs. Standard surface and sub-surface site investigation techniques such as boring are also employed on large road construction projects. These may be augmented by the use of quarry drills or borrow pit excavation plant at sites where extensions to existing reserves need to be proven.

In Section 3.2, reference was made to the use of photo interpretation for site investigation. Photo interpretation is a method useful to explore large areas at a low cost. It is known that the location and abundance of construction materials primarily depends on geology. In addition, topography plays a role to limit the extent of construction materials in an area. For example, conspicuous landforms are useful to find small occurrences of granular deposits that are difficult to locate in the field due to lack of exposure.

Also, the gradients of hills are easily determined from aerial photographs and provide important indicators in searching for suitable construction materials. For example, in flat terrain where the gradient is less than 5o, weathering is deep and rocks are highly weathered to form thick layers of soils. When the slope is more than 30o, very little soil is retained on the surface and the thickness of weathered rock is also greatly reduced. With further steepening of the slope, increasingly more fresh rock appears on the surface and, when the slope is more than 60o, there will usually be outcrops of predominantly fresh rock.

Drainage patterns are also useful to identify different types of construction materials. For instance, in regions where rocks are bare, drainage patterns are decidedly more different than those in an area with thick alluvial deposits. Likewise, drainage patterns develop differently in horizontally layered rocks compared to tilted beds. A dense and finely textured drainage pattern with rounded ridges and gullies may indicate impermeable, highly plastic clayey materials. This pattern contrasts with those observed in areas where there is high infiltration, indicating the existence of permeable and well-drained gravel or sand materials.

Chapter 8 Construction Material Surveys Site Investigation Manual –



Table 8.4: The requirements for unbound granular pavement materials Key engineering

factor Material requirements Investigation requirements

Strength

Aggregate particles need to be load resistant to any loads imposed during construction and the design life of the pavement.

Aggregate impact and strength testing.

Mechanical Stability

The aggregate as a placed layer must have a mass mechanical interlocking stability sufficient to resist loads imposed during construction and the design life of the pavement.

Particle size distribution, particle shape, mass strength (CBR) at appropriate density and moisture condition.

Durability

Aggregate particles need to be resistant mineralogical change and to physical breakdown due to any wetting and drying cycles imposed during construction or pavement design life.

Aggregate durability test., petrographic examination.

Haul distance Reserves must be within physically and economically feasible haulage distance.

Mass-haul analysis. Examine potential for alternative use of sub-standard sub-base reserves.

8.3.4 Bitumen-Bound Granular Layers and Surfacing Aggregates

The general requirements for aggregate to be used as a bitumen-bound granular material (BBGM) and surfacing aggregate are that it should be durable, strong and should also show good adhesion with bituminous binders. If aggregate is to be used in a surfacing layer, it should also be resistant both to the polishing and abrasion action of traffic. The main requirements to search for BBGM aggregate are summarised in Table 8.5.

Basic igneous rocks have better adhesion properties than acidic rocks. The comparatively poor performance of acid rocks may not only be related to the high silica content but to the formation of sodium, potassium and aluminium hydroxides. Hence, coarse granite with large feldspar inclusions is for instance likely to experience bitumen adhesion difficulties.

The surface course is the layer in contact with traffic loads and normally needs high quality materials. It provides characteristics such as friction, smoothness, noise control, rut and shoving resistance. In addition, it serves to prevent the entrance of excessive quantities of surface water into the underlying base, sub-base and sub-grade. This top structural layer is sometimes subdivided into two: wearing and intermediate or binder course.



Table 8.5: Requirements for Bitumen-Bound and Surfacing Aggregate Materials Key engineering

factor Material requirement Investigation requirements

Strength

Aggregate particles need to be load resistant to any loads and abrasion imposed during construction and the design life of the pavement.

Aggregate impact and strength testing, particle shape.

Durability

Aggregate particles need to be resistant mineralogical change and to physical breakdown due to any wetting and drying cycles and abrasion imposed during construction or pavement design life.

Aggregate durability tests, aggregate abrasion tests, petrographic examination.

Skid resistance (Surface aggregate only)

Aggregate particles must be resistant to polishing.

Polishing tests, petrographic examination.

Adhesiveness

Aggregate must be capable of adhesion to bitumen and sustaining that adhesion for its design life.

Bitumen stripping tests, petrographic examination

Haul distance Reserves must be within physically and economically feasible haulage distance.

Mass-haul analysis, examine potential for alternative use of sub-standard sub-base reserves.

The wearing course is the layer in direct contact with traffic loads. It is meant to take the brunt of traffic wear and can be removed and replaced as it becomes worn. A properly designed pavement management programme should be able to identify pavement surface distress while it is still confined to the wearing course. In this way, the wearing course can be maintained and rehabilitated before distress propagates into the underlying intermediate or binder course. The latter provides the bulk of the top part of the pavement structure. Its chief purpose is to distribute load.

8.4 Sources of materials

Generally, there are five ways of extracting materials from the ground: Borrow pitting which is suitable for excavating unconsolidated material (e.g. gravels

and weak rocks); Quarrying (which involves blasting of hard rock); Cut to fill operations along a road alignment; Mining (which refers to underground material extraction either by shaft or adits); and Dredging (the extraction of unconsolidated material from under water).

Materials for road construction are very rarely derived from mining operations. Dredging is also not common in developing countries due to its cost. Materials are generally obtained from pitting, quarrying and cut to fill operations. Table 8.6 presents borrow pitting and quarrying methods useful to extract different types of materials.



Table 8.6: Borrow pitting and quarrying methods

Source Excavation type Typical materials

Borrow pitting

Wet pit Alluvial sands and natural gravels below the water table.

Dry pit Alluvial, terrace, fan, beach, natural gravel deposits, above the water table

Quarrying

Hard rock; predominantly by drill and blast methods.

Unweathered strong igneous, sedimentary and metamorphic rocks

Weak rock; mechanical excavation; may be aided by light blasting

Mudstones, shales, weak limestones and sandstones and weathered hard-rock materials

8.4.1 Borrow pits

Borrow pits are sources of materials for fill, selected sub-grade, sub-base, base and surface courses of a pavement structure. A single borrow pit may often meet several of these requirements. When compared to quarrying, borrow pits require a higher rate of land use owing to the shallower depth and the need to exercise greater environmental control. In many cases, materials from borrow pits are capable of excavation without the aid of explosives.

Because of the large volumes frequently required, borrow pits producing fills should not be too far from each other to minimise hauling costs. In cases where resources are scarce there is an emphasis on using any locally available material rather than source selection. In most practical cases, the resulting distances should not usually be more than 5km. Special considerations can be taken in agricultural areas where land expropriation costs can be high.

The distance between borrow pits for sub-grade and capping layer materials can be as high as 10km. For low standard roads, the sub-grade or capping layer can be made of the same material as the common fill, the only difference being that it should be compacted better. Borrow materials for sub-grade and capping layers are usually found in broad hills, in highly altered basic and acidic igneous rocks, in silty-sandy sections of river bar deposits, in areas where there are pyroclastic deposits, and in stratified sedimentary rocks.

Materials for sub-base and base course depend largely on mechanical treatments to bring their quality up to the required levels. This often requires special equipment and processing plants, which ideally should not be moved too far. For these reasons, borrow pits are usually widely spaced. In the current practice, distances between them of about 50km are not unusual.

The minimum thickness of a deposit normally considered workable for excavation to select materials for sub-grade, sub-base, and basecourse, is in the order of 1m. However, thinner horizons could also be exploited if there are no alternatives. The absolute minimum depends on material availability and the thickness of the overburden. If there is no overburden as may be the case in arid areas, horizons as thin as 30cm may be excavated.

Borrow operations can be divided into dry pits and wet pits, the difference being the working method employed in the presence of water. In wet pits, working depth is restricted and the recovery of material is less than 100%. Dry pits are more common than wet pits and a wider variety of materials can be produced. However, in dry seasons it is important to provide



measures to minimize dust generation to reduce the impact on the local environment. The main geotechnical problems in dry pit operations are face collapse, variability of the deposit, the occurrence of large boulders, and the presence of cemented layers.

In any site investigation, the location of each proposed borrow pit should be indicated on a key plan. The plan should also show the position of test pits; trenches and borings within and around the borrow pit; the characteristics of the site; and the means of access that can be used. The site plan of borrow pits shall be included in the soils and materials report. 8.4.2 Quarry materials

Quarrying involves the extraction of useful natural stone or aggregate from a hill, cliff or mountain by a process that involves cutting, wedging or blasting. Rock is often quarried either as building and dimension stones or crushed aggregates. The dimension-stone process comprises the quarrying of solid blocks. In the practice of crushed-stone, on the other hand, rocks are broken to form aggregates for a variety of purposes. Usually, they are the main sources of materials for basecourse, gravel surfacing, bituminous mix, and asphalt wearing course. Sometimes it may be more convenient to use crushed stones for base course rather than natural gravels that may contain too many plastic fines. Quarries also provide large rocks that can be used for masonry and gabion structures, and aggregates for concrete.

Quarries are common in different parts of Ethiopia. Many quarries have been opened to supply aggregates for buildings and roads. The production in many places commonly involves basalt, but roadside quarries in the rift valley are also sources of rhyolite, ignimbrite and cinders. Limestone quarries are common in Dire Dawa, Harar and Tigray regions.

Similar to borrow pits, potential sources of stone in the vicinity of the road alignment are often identified by using aerial photos during the desk study phase. During the preliminary investigation, the regional and local geology will be accessed, and the quantity and quality of materials will be visually determined. In upgrading and reconstruction projects especially, the preliminary investigation can help to obtain information on known existing quarries and produced rock gradations and amounts of rock waste, blasting patterns, types of explosives and blasting procedures, and required processing and processing equipment (crushers, screens, etc.). Those visually considered suitable, in terms of stone quality and quantity should be further investigated in the final phase of site investigation.

For large production, investigation may need a series of boreholes. Consideration should also be given to the use of a bulldozer or a mechanical excavator to prove the availability of solid rock. However, in many cases, this is only possible during construction when equipment is readily available. Costs relating to the haulage and processing of materials have also a considerable impact upon the economics of a rock deposit for quarrying. Hence, material searches are generally restricted to about a 10km corridor centring on the road.

Moreover, the cost of constructing access tracks to quarry sites offset from the road may very well be prohibitive to development. This is especially true in areas where land is used for agriculture. Hence, viable sources will require some kind of existing track, unless the terrain is accessible. In addition, the material sources should have little overburden and low extraction costs; quarry sites must be located at sites suitable for the erection of crushing plants; blasting should be minimised to produce rock fragments suitable for crushing; and materials should require minimal processing to achieve a suitable specification.



The location and physical size of each potential source of stone or quarry site shall be indicated on a plan and included in the soils and materials report. The site plan should show the characteristic features of the site (including outcrops) and the means of access and duration. Like any other investigations, information on the geology and topography is desirable in order to assess quantities of rock material with a potential for improved exploitation.

8.5 Laboratory tests

Materials testing programmes vary greatly in size and scope depending on the type of the road project and associated works. Laboratory tests needed to evaluate soil and rock materials vary according to their intended purpose. Table 8.7 summarizes the tests used to verify the suitability of borrow and quarry materials to produce fill, sub-grade, sub-base, unbound base course, aggregates for bitumen-bound premixes and chippings for surface treatments. These tests can be divided into basic engineering and detailed aggregate tests. Basic tests are associated with defining the inherent physical properties. Aggregate tests are performed by simulating some form of geotechnical processes or engineering behaviour. In some cases, some chemical and petrographic tests may also be needed for detailed examination. 8.5.1 Basic engineering tests

Table 8.8 presents the types of basic tests that borrow and quarry materials should be subjected for analysis. These tests are useful for identification and classification purposes and to establish the quality of materials according to the specifications and design procedures. Most basic physical tests for fill, sub-grade, sub-base and base are similar except that they are conducted in greater number and detail when used to analyse materials for unbound granular materials. In the majority of cases no single test procedure will satisfy specification and design requirements and a combination of test procedures will be needed.



Table 8.7: Types of Tests required to Analyse Materials for various purposes

Material usage Standard specification compliance testing Further tests recommended

Common fill (soil)

Heavy compaction MCV Moisture content Atterberg limits

Common fill (rock) Moisture content Atterberg limits

Sub-grade

Moisture content PSD Plasticity Light and heavy compaction CBR (soaked & unsoaked) Swell

Dispersion [pin hole etc] Collapse

Drainage-filter Atterberg limits PSD

Capping layer

Heavy compaction CBR (soaked) Moisture content Atterberg limits PSD

Unbound gravel (UBG) sub-base or road-base

Atterberg Limits PSD CBR (soaked) Heavy compaction 10% FACT AIV (modified)

Unit weight of aggregate Linear shrinkage Petrographic examination Durability (Mg and Na Sulphate soundness) Finer than No. 200 sieve Sand equivalent Durability Mill (some base materials only)

Bitumen bound base and surfacing aggregate

Sedimentation Flakiness index ACV AIV LAA AAV PSV Mg/Na sulphate soundness Water absorption Immersion tray test

Linear shrinkage Chemical tests Full petrographic examination For weaker aggregates: 10% FACT For fine aggregates: Sand equivalent, Plasticity

Cement and lime stabilized aggregate

Plasticity PSD Heavy compaction Strength (UCS) CBR (soaked)



Table 8.8: Basic Engineering Tests needed for Material Analyses

Tests Standard Procedures Advantages Disadvantages AASHTO ASTM

Moisture content T 265 D2216

Simple and widely accepted test.

Misleadingly high moisture contents in halloysitic and allophane rich soils.

Liquid limit (WL) T 90 D4318 Well established soil index

and classification test.

Influence of greater than 425m particles; moisture condition and mixing time. Correlations between procedures require caution.

Plastic limit (Wp) T 89 D4318

Well established soil index test. Plasticity index (Ip = WL-Wp) used as a key defining parameter in many specifications.

Influence of particles greater than 425m; moisture condition and mixing time. Poor reproducibility and repeatability.

Shrinkage limit (Ws) T 92 D427 &

D4943 Yields index information on volume change potential.

Initially intended for undisturbed samples although remoulded material can be used.

Linear shrinkage (Ls)

Can give an estimate of Ip for soils where WL and Ws are difficult to obtain. Better repeatability and reproducibility than plasticity test.

Established relationships between Ls and Ip may not hold true for some tropical soils.

Particle size distribution (PSD)

T 88 D422

Simple and widely accepted test incorporating both sieving and sedimentation. A fundamental soil classification tool.

Interpretation problems with aggregated particles or weak clasts. Requires particle density values.

Swell pressure T 258 D4546

Undertaken on undisturbed or recompacted material to determine pressure to minimize swell.

Only measures swelling pressure not swell amount. Soil or fine aggregate only.

Water absorption C127 &

C128

Simple test with correlations established with bitumen bound material design.

Variability in multi-clast type deposits.

Compaction T99 & T180 D698 & D1557

Simple test. Basis of control on site compaction of fill and pavement materials.

Zero air voids a function of particle density- highly variable in tropical soils.

CBR D1883

Quick and simple to perform. A convenient and widely established test for defining material suitability for road construction and subsequent quality control.

An empirical test only. Correlations with other parameters may be material-specific. Dependant on transient soil moisture-density-void ratio conditions. Three test points are recommended.

Generally, the procedure is that one set of tests is made during the preliminary or final site investigation and other sets are performed subsequently during construction. The first set of test results will permit the selection of the most promising zones within the borrow area. The final sets of tests are needed to establish the extent of the mineable portion of the deposit.

Sometimes, highly weathered rocks tend to break down during compaction. For gravels derived from these rocks, the grading of the specimen compacted closest to 95% MDD shall



be determined after compaction and CBR testing, and compared with the grading before compaction. To assess the suitability of materials in the field, other simple tests than those listed in Table 8.8 are necessary. These tests are summarized in Table 8.9.

Table 8.9: Field tests useful to identify engineering properties of soils and rocks

Field test Description

Fines content Relative percentages of silt/clay: dilatancy test. Schmidt hammer

Use of Schmidt hammer on solid rock exposure or large boulder can be correlated to estimated compressive strength.

Hand sample index strength

Use of small geological type hammer on hand or core sample: Very weak: easily broken in hand. Weak: broken by leaning on sample with hammer. Moderately weak: broken in hand by hitting with hammer. Moderately strong: broken against solid object with hammer. Strong: difficult to break against solid object with hammer. Very strong: requires many blows of hammer to break sample. Extremely strong: sample can only be chipped with hammer.

Point load strength

Simple test with portable equipment. Correlates with unconfined compressive strength (UCS).

Field durability or dispersion test

Immerse samples in still water for 30 minutes and observe behaviour: No effect. Noticeable drop in strength. Slowly breaks into pieces under light finger pressure. Slowly crumbles to small blocks under light finger rapidly breaks into pieces.

under light finger pressure. Rapidly crumbles to small blocks under light finger pressure. Rapidly crumbles to small blocks. Disintegrates to sediment.

Field plasticity

Prepare a ball 2 or 3 cm in diameter. Moisten so that it can be modelled without being sticky. Roll to a 3mm thread adding water if necessary. At 3mm the material should start to break, then remould into a ball and carry out the following: Ball is hard to crush – does not crack/crumble = high clay content. Tends to crack/crumble = low clay content. Impossible to make a ball = high sand or silt content, very little clay.. The ball has a soft or spongy fell = organic soil.

8.5.2 Aggregate tests

In addition to basic engineering tests, crushed stone aggregates are sampled so that their potential as construction materials can be assessed in the laboratory. Testing is aimed at verifying the suitability of the rock to produce unbound basecourse materials, aggregates for bitumen-bound premixes and chippings for surface treatments. The engineering properties which make rock important for all these purposes include its resistance to crushing and abrasion, specific gravity, water absorption, property to polish, and the size and shape of the crushed rock chipping. Tests are normally needed first during site investigation to evaluate the behaviour of the rock from a potential quarry site or crushed aggregate source, and later during construction when a fresh face of rock in the quarry is exposed for work.



Table 8.11 gives the list of tests required to assess aggregate strength and durability. The strength tests involve evaluating the resistance of selected aggregate to either impact or load, whilst durability procedures deal with assessing the performance of aggregate when subjected to some form of artificially imposed degradation or weathering. Some test procedures, such as Los Angeles Abrasion, encompass elements of both strength and durability testing.

Aggregate crushing value is used to evaluate the crushing strength of available supplies of rock and to make sure that minimum specified values are maintained. The Los Angeles Abrasion value gives an indication of the impact strength in combination with the abrasion resistance of the aggregate. The soundness tests are useful for the evaluation of aggregates suspected of chemical decomposition. Specific gravity and water absorption assist the interpretation of compaction tests and in the design of bituminous mixtures. Affinity for bitumen is the ability of rock fragments to form a permanent bond with the latter.

The slake durability index test, in addition to being a useful performance indicator can perform a significant role in indexing materials in the rock to hard soil range. The test requires competent lumps of material. The combination of slake index with plasticity has been suggested as a useful means of presenting results for argillaceous materials.

The tests given in Table 8.10 are also considered appropriate to assess the potential of the quarry to produce coarse and fine aggregates for bituminous mixes and surfacing. In these cases, such characteristics as flakiness, resistance to polishing, sand equivalent and plasticity index of material passing the 0.425 mm sieve are usually reserved for verification during extraction. It should also be known that considerations related to gradation is governed to a large extent by the crushing process and may be left for verification during construction.



Table 8.10: Aggregate strength and durability tests

Tests Standard Procedures

Advantages Disadvantages AASHTO ASTM

Aggregate grading (sieve), aggregate sedimentation T 27 C136 &

C117 Simple and widely accepted test for defining aggregate size distribution.

Differing usage of "coarse" and "Fine". Wet sieve unless little or no fines

Flakiness index (If), elongation index (Ie) D4791 Standard gauge methods of ascertaining particle shape.

Parameters incorporated into coarse aggregate specifications Use restricted to coarse aggregate only

Aggregate particle density (Bulk particle or relative density)

C127/128 Required in bitumen bound granular material design calculations

In aggregate the procedure will give an "apparent" rather than an "absolute" value. Not directly related with soil specific gravity

Moisture condition value (MCV) Assessment of material suitability. Easy to perform. Simple

apparatus Methodology not yet proven effective for fabric-sensitive residual soil materials.

Aggregate impact value (AIV) Simple test with inexpensive portable equipment giving a

basic index parameter for aggregates Flakiness, elongation can influence results as well as base-floor condition. Tests limited grading.

Aggregate crushing value (ACV) Gives basic index parameter for aggregates commonly used

in specifications.

Flakiness, elongation can influence results as well as base-floor condition. Tests limited grading. Requires compression test machine.

10% Fines aggregate crushing tests (FACT) Modification of ACV test, more generally used, particularly

for weaker materials. As for ACV

Sulphate Soundness T 104 C88

Assesses aggregate durability as a response to repeated crystallization and rehydration stresses. Incorporated in many specifications

Time consuming. Poor repeatability and reproducibility unless great care taken over procedures.

Slake Durability D4644 Simple assessment of durability of rock-like material. Not generally used a suitability parameter in specifications. Fragile materials require careful handling

Los Angeles abrasion (LAA) T 96 C131/535

Standard combined impact and rolling abrasion test. Commonly used as a specification parameter

For aggregate <37.5mm. Tests a specified grading only. Measures breakdown in terms of material passing 1.68 mm sieve only

Accelerating polishing Test E303

Means of assessing the tendency for aggregate to polish. Polished Stone Value (PSV) commonly incorporated into surfacing aggregate specifications.

Difficult and time-consuming test not normally carried out in standard laboratories. Selected aggregate pieces only.

Aggregate –bitumen adhesion D1664 Tests for assessing adhesion of bitumen to aggregate in water Observational test only. Takes account of stripping only and

not prior coating difficulties. Aggregate abrasion value (AAV) Means of assessing surface wear in surfacing aggregates. Selected aggregate pieces only.

Water absorption T 84 & T 85 C127 & C128

Simple test with correlations established with bitumen bound material design Variability in multi-clast type deposits

Sand equivalent value T 176 D2419 A rapid site-lab means of determining relative fines content Dispersion problem in agglomerated minerals. Relative proportions only



8.5.3 Chemical and petrographic tests

Whilst the detailed chemical composition of materials may be of limited interest for road engineering, the presence of some constituents can be of great significance. Especially, the determination of organic matter, sulphates, chlorides and carbonates is very important.

Petrographic examinations are conducted to describe, classify, and determine the relative amounts of the sample constituents, identify the sample lithology, to determine the sample fabric, and to detect evidence of rock alteration. The identification of rock constituents and determination of fabric and micro-structural features assists in the recognition of properties that may influence the engineering behaviour of the rock. Complete petrographic examination may require the use of such equipment as light microscopy and X-ray diffraction.

8.6 Sampling

A sufficient number of samples, each of sufficient quantity, are to be taken to carry out tests to determine the main materials and processes. All layers visually considered as suitable for use shall be sampled. When possible, the samples shall be taken over the full depth of the layer using vertical slices. In borrow pits, the number of samples is determined by the heterogeneity of the subsurface and the characteristics of soils. At least one sample should be taken per test pit or trench. When there is a major change in material property, the number of samples should increase to include as many layers as possible. The quantity of material in each sample must also be sufficient to carry out different types of tests.

In the case of quarries, at least 3 samples shall be taken from each potential material source. Hand samples from existing faces or outcrops are sufficient to conduct many of the tests. The position of each sampling point shall be accurately determined and reported on the site plan. Each sample shall be accurately described from a geological and mineralogical viewpoint. Great care shall be taken to ensure that the samples are obtained from sound rock and not from a superficial horizon of weathered rock. Care is also needed to ensure that samples are representative of the rock mass that is going to be used for different pavement layers.

The sample size that is extracted will depend on the purpose for which the material is intended. Table 8.11 indicates typical sample sizes for common test procedures. For fill materials, grain size distribution and plasticity limit tests are carried out for soil identification and classification followed by compaction and swell tests. These tests in general require samples weighing between 30kg and 40kg. For sub-grade, sub-base and base course materials, California Bearing Ratio or similar strength tests should be conducted in addition to identification tests. There is, therefore, a need to take about 100kg of sample.

However, up to 150kg of sample may be needed to carry out a combination of tests for the purpose of identification, classification and design. The demand for such large sample sizes sometimes makes the appropriate assessment of the materials very difficult during site investigation. This is especially true for new construction projects where access is limited. Hence, it may only be possible to take incremental samples from the production site at the time of construction to build up the required quantity for testing. In addition, large representative samples may be obtained by sampling from existing pits or trenches at a later time, or by sampling additional quantities when construction begins.



Table 8.11: Sample sizes needed for different tests

Test Procedure Minimum sample required

Fine Medium Coarse Moisture content 0.05kg 0.35kg 4.00kg Liquid limit 0.50kg 1.00kg 2.00kg Plastic limit 0.05kg 0.10kg 0.20kg Shrinkage limit 0.50kg 1.00kg 2.00kg Linear shrinkage 0.50kg 0.80kg 1.5Kg Particle size distribution 0.15kg 2.50kg 17.00kg Specific gravity 0.30kg 0.60kg 0.60kg Point load test Schmidt hammer test

10 identical samples 20 tests on each sample

Aggregate crushing value (ACV) Aggregate impact value (AIV) Los Angeles abrasion (LAA)

2.00kg 2.00kg

5.00-10.00kg Slake durability 10 lumps, 40-50g each AAV PSV

24 aggregate particles 4 x 35-40 aggregate patches

8.7 The Geological Background

The geological background of natural road building materials has a profound effect on their engineering performance. Hence, knowing the local geology before starting a site investigation can provide a useful framework for identifying material sources easily. In addition, a clear understanding of the geological processes that lead to the formation of the various rocks and soils in the region may help to determine the suitability of materials for road construction purposes, and their likely behaviour under expected traffic loads. A graphical illustration of the performance of rocks for road building purposes is given in Figure 8.1. Engineering concerns consider the natural state of rocks while excavation concerns are related to their unweathered state. Generally, those rocks which can be excavated easily such as marl and tuff are known to have a high degree of engineering concerns.



Figure 8.1: The Relative Engineering and Excavation Concerns for Different Rocks

8.7.1 Sedimentary rocks

In addition to clastic, chemical and organic, sedimentary rocks can be divided into arenaceous, argillaceous and carbonates based on their composition. Arenaceous rocks include all those clastic sedimentary rocks whose particle sizes range from 2 to 0.0625mm. The vast majority of arenaceous rocks are sandstones. The strength of sandstones depends upon the strength and durability of the cemented grains. Strong sandstones are used as rock-fill and as a base if properly graded. They are also used as surfacing aggregates.

When sandstones are used for aggregate production, attention should be given to the alkali-silica reaction. Sandstones are sometimes extremely strong and this together with the high content of quartz makes the rock highly abrasive. This can result in high quarrying and handling costs. Also if the rock is not well graded, it can be difficult to compact, as little breakdown occurs under the roller. Weak and highly decomposed sandstones are easily compacted and can be used as fills. Disintegrated sandstones, on the other hand, are useful as sub-base materials, but they are usually excellent as sub-grades. Siliceous sandstone performs much better than other sandstone types.

Argillaceous rocks include shales, siltstones, claystones and mudstones. They are clastic sediments whose constituent particles are less than 0.0625mm in size. These rocks include sedimentary rocks which are predominantly clay-silt admixtures. In the extremely weathered conditions, all argillaceous rocks are clays and silts. Intermediate weathering conditions are often difficult to define in these rocks. Weathering profiles are more uniform, gradational and not deep. Most argillaceous rocks are unsuitable as materials for pavements except as sub-grades for low standard roads due to their low strengths and slaking properties.

Sedimentary rocks which contain calcite as a main constitute are known as carbonates. Limestone and dolomite are the most common calcite containing sedimentary rocks. When fresh, these rocks have very low porosities and their permeability is negligible. Groundwater



flow is confined to joints and other defects which are widened by solution to form cavities. In areas where there are cavities, the construction of roads can initiate sinkholes.

Dense limestones in the strong to extremely strong range have been used extensively for the production of crushed stone aggregates for concrete; bitumen bound granular materials; and unbound road bases. Limestones can produce good road aggregates because they are relatively soft and easy to crush, and usually form rounded edges when broken. Limestone aggregates also bond well to bitumen. Disintegrated limestone is useful as a sub-base and sub-grade material. However, limestone is rarely used as a road wearing course because it does not perform well under traffic and often has poor resistance to polishing.

Impure black limestone found in Mekele and the surrounding regions is used as a source of crushed stone for road bases. The rock is also very practical for use in dry masonry walls and gabion structures. However, a few dolomitic varieties, if present, can react with the alkalis in Portland cement causing expansion and cracking as displayed in alkali-silica reactions. 8.7.2 Volcanic rocks

The most common rock types in this group include basalt, andesite, dacite, trachyte and rhyolite. All are extrusive igneous rocks, fine grained and strong when fresh. In this condition, the rocks generally are very durable and could be good sources of materials for road-bases. Where disintegration is high, volcanic rocks break into natural gravels and may be used as basecourse materials. Moderately weathered volcanic rocks can be excavated for sub-base and gravel wearing courses, and drainage filters.

Basalt is frequently used as road construction material in Ethiopia probably because of its availability and high affinity to bitumen. It is followed by rhyolite and andesite. Most lava flows show hexagonal columnar joint pattern, with the columns being interrupted by near-planar or saucer shaped cross joints. These joints are developed as a result of shrinkage on cooling. Basaltic rocks with well-developed columnar joints are common in the highlands of Ethiopia. They are ripped easily with an excavator and used as a source for crushed stone.

Although all unaltered volcanic rocks are highly durable within the life-span of a pavement structure, the basic varieties, particularly basalt, are quite susceptible to chemical weathering. Weathering in volcanic rocks is governed by the distribution of any previously altered material, as well as by patterns of joints and vesicular zones. When they are extremely weathered, all volcanic rocks are clayey soils in the engineering sense. 8.7.3 Plutonic rocks

Plutonic rocks are intrusive igneous rocks which include granite and other coarse grained varieties, formed by the cooling and solidification of large masses of viscous magma beneath the ground surface. When fresh, plutonic rocks are durable and strong. Fresh granitic rocks are commonly quarried for rock fill and road aggregates. Mica-rich granites are however unsuitable for aggregates in concrete due to excessive amounts of platy fines. In addition, granites and other acidic igneous rocks containing feldspars have a poor bitumen adhesion and need a stripping test before they are used as bitumen bound granular aggregates. Decomposition of feldspars and ferromagnesian minerals in granites is rare. However, disintegration can occur leaving the quartz grains essentially unaffected. These quartz gravels can make good quality road bases for asphalt concrete roads, or base courses for unsealed, gravel roads.



8.7.4 Pyroclastic rocks

Pyroclastic rocks are those which have been formed by the accumulation of solid fragments of volcanic rock extruded into the air during eruption. The rock fragments include dense, solidified lava, highly vesicular lava (cinder, red ash or scoria) and light vesicular material (pumice). Pumice is formed from acidic lavas while scoria is largely associated with basic lava flows. It is common for lava flows to be inter-bedded with pyroclastic materials (ash, tuff and lava fragments or agglomerates) in rock sequences derived from volcanic explosions.

Pyroclastic deposits are characterized by extreme variability in engineering properties over short distances laterally and vertically. This variation in properties resulted from differences from the ways in which they were initially deposited and the processes to which they were subsequently exposed. When the fragments are sand size or smaller, they form ashes, which create tuffs when compacted. The strongly welded varieties of tuffs are called ignimbrites.

Relatively fresh ash and tuff can only be used as fills or sub-grade materials in low standard roads. Volcanic breccias and agglomerates are useful to obtain good sub-grade materials if they do not contain high amounts of plastic clay. Scoria (cinder gravels) is widely used for sub-bases in many parts of Ethiopia, notably in the rift valley, and can be tried for bases if it meets the specification requirements. Ignimbrite is as good as some volcanic rocks such as rhyolite and trachyte to be used as a source of road-bases and sub-bases. 8.7.5 Metamorphic rocks

Metamorphic rocks are formed by the prolonged action of physical and chemical forces (heat, pressure, moisture, etc.) on sedimentary, igneous or pre-existing metamorphic rocks. Foliated metamorphic rocks (gneiss, schist, etc.) represent an advanced stage of metamorphism on a large scale (regional metamorphism), and the peculiar schistose or foliated structure is due to the more or less parallel arrangement of their mineral components. The non-foliated metamorphic rocks (quartzite and marble) are results of alteration of sedimentary rocks without affecting the structure and chemical composition of the original rock.

Metamorphic rocks have, as a rule, a low binding power, owing to regeneration of secondary minerals and to the effects of heat and pressure. With the exception of gneiss, foliated metamorphic rock such as schist and phylite are composed of some platy minerals and can part readily along planes of schistosity. They are, therefore, not ideal for road construction. Gneiss, owing to a high degree of crystallization and a preponderance of silicate minerals, offer a greater resistance to abrasion. It is usually low in toughness on account of its granular structure, but invariably shows a high resistance to wear and compressive strength. Slates, as low grade metamorphic rocks, may be used as sources of sub-base materials.

When gneiss and associated granites are disintegrated, they break up into granitic gravels, which have proved satisfactory for road construction. This is due to the fact that the pebbles are sharp and angular which creates a good interlock. They also contain some binding clay which is produced by the breaking down of feldspar minerals.

Non-foliated metamorphic rocks are rarely used as road building materials. Marble is usually used as building stone and quartzites are too hard to work. When weakened and sufficiently disintegrated, quartzite gravels are useful as a source of road-base and sub-base materials.



Figure 8.2: Schematic Illustration of a Cross Section in a Quarry

8.8 The influence of weathering The quality and durability of road construction materials can greatly be affected by weathering or alteration. Hence, each source of gravel or crushed rock should be examined to assess the effect of weathering in the outcrop. The type and rate of weathering vary from one region to another. In areas with high temperatures and high humidity, weathering produces physical and chemical changes (disintegration and decomposition) to a considerable depth from the ground surface. In dry areas, physical weathering dominates and rocks could disintegrate by alternate heating and cooling, but still keep their general appearance. In more humid regions where moisture exists, chemical weathering proceeds quite rapidly and many of the rock minerals will decompose fully or partially towards their ultimate clayey form.

Figure 8.2 presents an illustration of the reduction in the degree of weathering with depth. Generally, fresh rocks that can be used as sources of crushed stone are found at some depth from the ground surface. On the other hand, soils and weathered rocks at the top part of an exposure can be used as common fill and selected sub-grade. The material in the middle part of the two extremes is often the sources of unbound sub-base and base courses.

However, exceptions exist to the observation and it is not uncommon to see highly weathered soil profiles intercalated with relatively hard rocks at considerable depth. This is can happen because of differential weathering due to compositional or textural differences, differential weathering of contact zones associated with thermal effects within volcanic rocks, directional weathering along permeable joints, faults, or contacts where weathering agents can penetrate more deeply into the rock mass; differential weathering within a single rock unit due to relatively high permeability, and differential weathering due to topographic effects.



Generally, apart from geology and topography, weathering can be affected by both macro and micro climatic features. Hence, any local variation in climate could have a major influence on the final product of weathering. Mineralogical analyses can help a lot in this regard.

Table 8.12 presents a system for describing and classifying weathering grades in a quarry or borrow site. As shown, the degree of weathering is divided into five categories that reflect definable physical changes due to the chemical and physical processes. This table summarizes general descriptions which are intended to cover ranges in bedrock conditions. It is an extension of Table 4.8 given in Chapter 4 without the last category corresponding to residual soils, which are normally considered as soil in the case of construction materials.

Weathering tables are generally applicable to all rock types. However, they are easier to use with igneous and metamorphic rocks, especially in rocks that contain ferromagnesian minerals. Weathering in many sedimentary rocks will not always conform to the criteria shown in Table 8.12, and weathering categories may have to be modified for particular site conditions. However, the basic classes and descriptors given in the table can be used. Site-specific conditions, such as fracture, openness and filling should be described for each project.



Table 8.12: Weathering Grades for Describing and Classifying Road Construction Materials

Grade Descriptive term

Chemical weathering Texture General characteristics

(strength, excavation, etc) Body of rock Fracture surfaces

I Fresh (un-weathered) No discoloration, not oxidized. No discoloration or

oxidation. No change

Hammer rings when crystalline rocks are struck. Almost always excavation involves rock except for naturally weak or weakly cemented rocks such as siltstones or shales.

II Slightly weathered

Discoloration or oxidation is limited to surface of, or short distance from, fractures; some feldspar crystals are dull.

Minor to complete discoloration or oxidation of most surfaces.

Preserved

Hammer rings when crystalline rocks are struck. Body of rock not weakened. With few exceptions, such as siltstones or shales, classified as rock excavation.

III Moderately weathered

Discoloration or oxidation extends from fractures, usually throughout; Fe-Mg minerals are “rusty,” feldspar crystals are “cloudy.”

All fracture surfaces are discoloured or oxidized.

Generally preserved

Hammer does not ring when rock is struck. Body of rock is slightly weakened. Depending on fracturing, usually considered as rock during excavation except in naturally weak rocks such as siltstone or shales.

IV Highly weathered

Discoloration or oxidation throughout; all feldspars and Fe- Mg minerals are altered to clay to some extent; or chemical alteration produces in situ disaggregation, see grain boundary conditions.

All fracture surfaces are discoloured or oxidized, surfaces friable.

Texture altered by hydration

Dull sound when struck with hammer; can be broken with moderate to heavy pressure or by light hammer blow without reference to planes of weakness such as incipient or hairline fractures, rock is significantly weakened. Usually easy for excavation.

V Completely weathered (decomposed)

Discoloured or oxidized throughout, but resistant minerals such as quartz may be unaltered; all feldspars and Fe-Mg minerals are completely altered to clay.

----

Resembles a soil, partial or complete remnant rock structure may be preserved; leaching of soluble minerals usually complete.

Can be granulated by hand. Always easy for excavation. Resistant minerals such as quartz may be present as grains.



8.9 Local sources of rocks and soils

The types and distribution of rocks and soils in Ethiopia has been discussed in Chapter 2. It is known that the oldest rocks in the country are metamorphic rocks of Precambrian age. They are exposed in parts of Harar, Dire Dawa, Southern region, Welega, Gojam, and Tigray. An accumulation of sedimentary rocks was formed farther north and east in Mesozoic, and much of the Blue Nile basin and Tigray, and places in Dire Dawa and Harar are covered today by limestone and sandstone. The Blue Nile in particular provides thick deposits of sandstone, limestone, and gypsum intercalated with relatively soft units of mudstone, shale, and marl.

There was a widespread volcanic activity in the Tertiary period. This resulted in the outpouring of vast quantities of basaltic lava flows known as the Trap Series over much of the country, accompanied by the eruption of large amounts of ash and tuff. Most of the highlands in the northwest, west, central and south-eastern part of the country are now covered by these rocks. Cenozoic volcanics (basaltic lava flows, rhyolites, and ignimbrites intercalated with pyroclastic deposits) and recent sediments are common in the rift valley.

Table 8.13 gives a summary of the location and distribution of rocks and soils, their characteristics, problems, and performance as pavement materials. As shown in the table, suitable materials exist in every region of the country. Normally, the geology of Ethiopia indicates that, even in areas with thick soil cover, a relatively fresh rock must be encountered at a certain depth as there is a gradual transition from one weathering state to the other. The extraction of a suitable material for fill, selected sub-grade, sub-base, base and surface courses is, therefore, a matter of understanding the geological history of the project site.

8.10 Sources of sand

Sand is needed during concrete and mortar production for bridge and drainage structures. Sand may also be used as part of bituminous mixes, and may also be considered as an additive for mechanical stabilization. The sand particles used for these purposes should be predominantly angular in shape and be devoid of soft particles. Also, the material should be non-plastic in nature. Although samples need to be checked by test pits, the use of aerial photos to identify the location of sand deposits such as alluvial plains and dunes is often very successful. When necessary, trenches up to 2m can be dug in sands very easily.

The construction industry in Ethiopia utilizes sand mainly from streambeds, which are commonly derived from the weathering of quartzo-feldspathic acidic igneous rocks, sedimentary rocks and alluvial deposits. Sand size grains of basalts and pyroclastic deposits are also common in many river beds. The major sand supply for the construction works in and around Addis Ababa is the Awash River basin. The alluvial deposits in other river basins and in the Afar region can also be used as sources of sand for roads in the vicinity.

8.11 Sources of water The needs for water during road construction must be considered in the design phase. Water is added to soil to bring the water content to optimum moisture content. In areas where water is



scarce, such as the Afar and Somali regions, additional search is to be planned as required. Data from a review of topographic maps and the field reconnaissance can indicate if surface water for construction is a crucial problem. Major water sources need to be sampled and chemically analysed for the presence of chloride, sulphate, and organic content in order to check their suitability for use in construction of pavement and concrete structures.

The Ethiopian Geological Survey has a collection of hydrogeological maps for different parts of Ethiopia. There are also some private and state agencies which are responsible for water resources studies in the country. Additional information can be obtained from these sources. Of primary interest in hydrogeology is the ability of the various rock units to store and transmit water and act as water-containing bodies (aquifers). Understanding geological conditions is the cornerstone of any ground water evaluation as geology forms the physical framework for the flow of groundwater. Primary and secondary porosity, storage properties, and transmitting properties are largely a function of the geological materials present.

Similar to other site investigations, hydrogeological studies start with the interpretation of aerial photographs. The study aims at determining the location of potential sites for drilling water supply wells, and analysis of regional or local ground water flow systems. Methods employed in such investigations include analysis of soil patterns that may reflect on infiltration potential; drainage characteristics that suggest rock type and soil/rock permeability; mapping and interpretation of joints and fractures; land form analysis; and observation of vegetation patterns or types that provide inferences about the presence or preferential movement of water or its chemical quality. Other related uses of aerial photographs in the assessment of hydrogeology include the interpretation of channel geometry and the identification of fluvial or lacustrine sediments and bedrock contacts which can be potential sources of water.

When water is a great concern, it is advisable to contact the Geological Survey of Ethiopia or the Regional Water Resources Bureau. All studies should be carried out by a hydro-geologist.



Table 8.13: The Local Distribution and Usage of Materials for Road Construction Material type and location Material description Typical problems Performance as pavement material

Metamorphic rocks (present in southern and western part of Ethiopia as well as Dire Dawa, Harar, Tigray regions and northern part of Afar)

Massive to closely jointed strong rocks, which may produce poorly graded materials upon crushing that comprise a significant proportion of flaky and elongated particles

Poor particle shape; High proportion of flaky particles (> 40%) in road-bases will lead to poor particle interlock, compaction difficulties and relatively low in situ dry densities High mica content which can lead to difficulties with compaction in the laboratory and on site. May also affect liquid limit determination and unrealistically high PI that bear little relationship to field performance.

Materials with poor particle shape tend not to satisfy laboratory CBR required for “standard” road-base materials. May be satisfactory for low standard road-base in low volume roads. Can be improved by mechanical stabilization (blending) with well-shaped angular materials designed to improve particle interlock, reduce voids and produce a smooth curve within the desired grading envelope. Densification of layers cannot occur easily due to the presence of excess mica, particularly when using vibratory compaction equipment.

Volcanic rocks (basalt, rhyolite, trachyte, andesite, dolerite, available in the highlands of Ethiopia and the rift valley)

Strong massive to closely jointed strong rocks which can typically be processed by crushing and screening to produce desirable grading.

Apparently sound strong rock aggregate, may deteriorate after processing and in the road pavement although this has yet to be reported in Ethiopia. This is especially true for aggregates from basalt and gabbro.

Provided that secondary mineralization is not significantly developed, these hard rocks can produce good quality crushed road-base, sub-base and sealing aggregate. Basic rocks like basalt are considered to have better adhesion properties to bitumen than acidic rocks such as rhyolite.

Plutonic rocks (granite, grano-diorite, gabbro common in southern and western Ethiopia, Dire Dawa, Harar and Tigray)

When fresh, granitic rocks are highly durable and strong. Often, they contain widely spaced tectonic joints.

Mica-rich granites are unsuitable for use as aggregates due to excessive amounts of fine and platy particles in the crushed products.

Quartz gravels remained after prolonged weathering in granite and gneiss can make good quality road bases for asphalt concrete roads, or base courses for unsealed, gravelly roads. Fresh granitic rocks are commonly quarried for rock fill and road aggregates.

Weak volcanic agglomerates and breccias (often associated with basalts in the highlands)

May comprise poorly consolidated, rippable deposits that when excavated produce variably graded clayey or silty angular to sub angular gravel and cobbles with some boulders.

Poor “as dug” grading. Frequently gap graded with a high proportion of oversize material. Near surface deposits may be weathered but often appear well consolidated rock at depth. Unsound stone content. Rippable materials could be weathered and basaltic inclusions may have undergone secondary mineralization.

Rarely suitable for use in pavement construction without some processing to reduce oversize content and improve grading. Cobble and boulder size fragments are typically strong and may be difficult to treat with a grid roller. Screening alone is likely to be wasteful and the use of quarry crushing and processing may be required. High variability within the outcrop. Often inter-bedded with finer ash and tuff deposits, which may have high PI.

Scoria or cinder cone materials (available in the main rift valley and surrounding areas, also common in the highlands)

Unconsolidated pyroclastic materials which vary in colour from red to dark brown, and in size from sands to large gravels.

Characterized by a wide range of particle hardness and relatively low particle strength, high porosity, and poor grading.

Used as sub-base materials in the rift valley and in Bahir Dar area. A recent (2000) survey confirmed excellent performance with little deterioration in 20 years. They can be used for road-base in low volume roads, especially with the help of mechanical stabilization and selection of appropriate compaction procedure.

Volcanic ashes and tuffs (present throughout the rift valley and in some places in the highlands)

Weakly consolidated rock or soil comprising silt and sand size particles sometimes with gravel size inclusions.

Low particle strength. Often associated with poor cementation and low density. Poor “as dug" grading. Aggregate can decompose rapidly to produce clays if used for pavements.

Little reported construction experience. May have characteristics similar to highly weathered rocks.

Sandstones (common in the Blue Nile basin and the surrounding areas; also in Tigray, Afar and Dire Dawa regions)

Strong rock comprising mainly sand size particles dominated by quartz and feldspar and cemented by iron oxide (the red color) or silica.

High permeability and loss of strength on saturation High PI in material when feldspars decay to kaolin.

Selected deposits can supply good road-base and sub-base materials for both high and low volume roads in all climatic conditions. Good aggregate durability if not associated with argillaceous (clayey) materials.

Limestones (common in the Blue Nile basin, Tigray Afar and Dire Dawa areas)

Fractured and bedded but very strong carbonate rock.

High PI carbonate fines, associated with high degree of weathering along joints and fractures, and the presence of intercalated marl.

Well graded (suitably processed) limestone typically provides high soaked CBR strengths and can be a good source of road-base aggregates. It may not be used as wearing course because it often has poor resistance to polishing.

Marls (commonly exist Inherently weak calcite that Poor “as dug” grading. Very low strength, inherently weak. Selected low plasticity deposits may supply sub-grade materials



Table 8.13: The Local Distribution and Usage of Materials for Road Construction Material type and location Material description Typical problems Performance as pavement material associated with limestone) produce silty and clayey angular

gravels. All marls decompose quickly while in service to produce plastic fines.

for low volume sealed roads.

Argillaceous materials (shale, siltstone, claystone, and mudstone) found in association with sandstones)

Fine grained weak rocks that may be fissile, typically produce silty to clayey angular or platy gravel.

Low particle strength, inherently weak rock types. They will tend to “slake” after extraction and in the road to produce plastic fines.

Some materials may be suitable for use as sub-base for low volume roads in well drained and dry conditions. They will tend to soften rapidly in wet areas. If well compacted, they can be used as sub-grade materials.

Weathered rocks (present throughout Ethiopia, susceptible rocks include basalt, ash, tuff, shale, marl, mudstone)

Many partially weathered rock types (whether igneous, sedimentary or metamorphic) may produce sandy gravel materials. Fracture spacing and or bedding planes facilitate extraction of well graded materials by ripping.

Variability within the outcrop, expect considerable and sometimes unpredictable lateral and horizontal variation in aggregate quality. Presence of deleterious secondary minerals, low particle strength. Poor “as dug” grading. High plasticity fines.

Some rippable partially weathered and fractured rock types can supply road-base material for low volume roads. Aggregate quality will vary according to degree of alteration (i.e. depth below ground). Selection and mixing during extraction may be critical to obtain a satisfactory material. A wider range of weathered rock types will be suitable for supply of sub-base and selected sub-grade aggregates.

Lateritic gravels (available in western and northwestern part of Ethiopia, abundant in Assosa and the surrounding areas)

Laterites are known to be the last products of intensive weathering, can be loose or strong.

Immature or relatively young laterites known as plinthite show low particle strength, low compacted strength, lack of mechanically stable grading and high plasticity, but can undergo self-hardening and improve with time.

Satisfactorily used as sub-base materials in many countries including the western part of Ethiopia. Mature laterites can also be used for road-base. Trials successful on trunk roads carrying traffic up to 1.0 x 106 esa. Crown height and provision of good drainage essential component of performance.

Alluvial sand deposits (can be found along major and minor rivers)

Typically silty non plastic to low plasticity sand deposits because of the presence of some fines.

Uniformity of particle size, poor performance in pavement layers is associated with sand deposits comprising a high proportion of single size particles Poor particle shape, angular particles provide good interlock and improved engineering properties.

Well graded materials may be suitable for sub-base construction.

Alluvial clayey deposits (present along major and minor rivers)

Clayey (low to high PI).

Poor grading, these deposits often lack large size fraction, materials with good engineering properties usually have a considerable amount of coarse grained particle sizes. Moderate to high PI values.

Unstabilised materials can be used for sub-grade construction for very low volume sealed roads in low rainfall areas (< 500 mm/year).

Alluvial gravel deposits (river bed deposits, river terrace deposits, exist along major rivers in low land areas such as Afar and Somali regions)

Typically moderately to well graded silty sand and rounded to sub-angular gravel with a variable proportion of cobbles and boulders.

Poor particle shape, rounded particles have poor interlocking properties, hence “as dug” alluvial deposits tend to be difficult to compact and produce low dry densities. Alluvial gravels comprise a mix of rock types that reflect the geology of the drainage catchment, and may contain a significant proportion of unsound aggregate fractions.

Alluvial gravels typically require crushing and screening in order to satisfy “standard” road-base specification requirements. Crushed gravels for use in bituminous surfacing should be investigated to determine their unsound stone content and adhesion characteristics. Use of the sodium sulphate or magnesium soundness test is recommended in conjunction with standard strength testing (Aggregate crushing value and Los Angeles Abrasion).

Colluvial deposits (fans, scree deposits or talus, and landslide debris, found near mountains and hills)

Typically coarse angular sand and gravel deposits with a variable cobble and boulder content in a matrix of silty sand or sandy clay.

Poor grading, usually gap graded with a high proportion of oversize material, variability within the deposit is high. High amount of PI fines, performance is significantly affected by PI.

The character of these deposits is dependent on the nature of the parent rocks and terrain.

Residual clayey sand deposits

Clayey (low to medium PI) with silt and sand.

Poor grading. Poor particle shape.

It is very difficult to use these materials because of their high clay content, but sometimes they can be used as sub-grade materials for low volume roads.

Residual gravel deposits (quartz gravels, weathered granite gravels, other residual gravelly soils)

Variably graded clayey sandy, angular to sub-angular gravel.

Poor grading. Deposits tend to be variably graded within the exploitable horizon and frequently gap graded. In situ weathering can lead to mineralogical decay producing plastic fines. High proportion of partially weathered particles can be present that have not been subjected to sorting by water.

“As dug” residual gravel deposits will rarely be suitable for standard road-base construction, due to inherent variability in terms of grading, particle strength and plasticity. However, this group of deposits has been widely used as a source of sub-base and selected sub-grade materials.

Chapter 9 Site Investigation Manual – 2013 Construction Review


9 CONSTRUCTION REVIEW 9.1 Introduction

In Chapter 6, 7 and 8, the site investigation aspects required in the pre-construction phase are discussed. The construction phase, the last stage of work required to complete a roadway, is the emphasis of this chapter. Site investigation is an iterative process. Although much of the exploration is supposed to be finished earlier in the investigation phase for design, additional surveys may be required during construction to resolve unforeseen problems. This is especially true in new road construction projects where access is not present to transport site investigation equipment. Moreover, it will be difficult to mobilize investigation tools twice or three times and may be economical to do some exploration during construction.

The investigations during construction would be primarily composed of detailed observation of excavation faces, pit, trenches and borings with sampling and testing concentrated on specific features. They should be specifically planned to provide the geotechnical engineer with information to characterize the sub-grade, determine the availability and extent of construction materials, and estimate quantities of earthworks. If there have been changes in vertical and horizontal alignments since the final design, it will be necessary to undertake additional site investigation according to the new geometric design. Construction reviews are also useful to evaluate the type of materials encountered at road cuts and to refine earlier suggestions of cut-slope angles. In addition, the location of river crossings, landslides, and problems soils should be checked and potential places of disposal sites selected.

9.2 Subgrade conditions

Fundamental to any road construction is the preparation of the sub-grade to meet the pavement design requirements. Normally, the pavement engineer prepares a design based on the information obtained from the exploration programmes for design. However, characterising the sub-grade completely in the design phase is often difficult, and unexpected field conditions could appear later during construction. Additional investigations of the sub-grade conditions are, therefore, necessary to determine whether or not soil conditions encountered in construction correspond to those visualised in the original design; and to ensure that the pavement design is carried through in the construction phase.

Generally, sub-grade performance depends on three basic characteristics; strength, moisture content and swelling, all of which can be checked by trial pits and trenches. In some circumstances, such as soft deposits and deep sub-grade cuts, borings shall also be considered. The plan for sub-grade investigation at the time of construction should be related to the plan of exploration employed earlier during site characterization. Hence, the previous locations of pits and borings, the different logs and field memos, and the site investigation report for design should be thoroughly revised. The location of pits, trenches and borings should be such that the information obtained will assist in filling any gap that exists. The locations and sampling frequencies should also be at such intervals to allow the identification of all soil types, the level of the water table and the depth to the bed-rock.

Chapter 9 Construction Review Site Investigation Manual – 2013


Moreover, inherent to the construction of roads is the ability to inspect sub-grade properties to enforce quality control measures before the overlying structures are placed. This is especially true in the case of silty soils which can meet the moisture-density requirements during design, but may fail at the time of construction. In such cases, the DCP is a good indicator of sub-grade stability. As stated in the previous chapters, the DCP can be driven to a depth of 1 m at each test location without excavating any soil layers. The DCP can also be used to identify soft sub-grades in deep cut areas. In this case, extension rods might be used to conduct the DCP test to a depth of 3 m in a soil boring drilled by a hollow stem auger. In this way, the DCP allows frequent field testing of the sub-grade in a reasonable time.

Proof rolling is another economical method of identifying unstable or unsuitable soils during construction. It is not a direct test for evaluating sub-grade stability, as is the DCP, but is a highly recommended field procedure for high volume roads which should be used prior to additional in-situ tests or the excavation of additional pits and borings. Proof rolling involves driving a loaded truck, or heavy construction equipment, repeatedly over the sub-grade (especially in cut areas) and observing the surface deflections and the development of rutting. It is intended to distress the soil to conditions anticipated during construction. Since, proof rolling the entire section under construction may be time consuming, it is often preferable to proof roll only areas of potential problem, identified by the DCP investigation. Proof rolling is particularly useful in identifying silty soils and soft deposits. Repeated passes of truck loads cause moisture to move up from high groundwater tables, soften or remould the moisture-sensitive soils and cause excessive rutting.

Typically, proof rolling should be conducted as follows: The engineer should observe the earthwork at all times during construction to

identify weak areas prior to proof rolling. The sub-grade is prepared according to standard specifications, in which the sub-

grade shall meet the density requirement. If conditions change after sub-grade preparation, due to rain or construction traffic before determining the type and thickness of treatment, the sub-grade should be reworked.

The length of sub-grade, prepared for proof rolling, should be 150m to 300m at a time. If the section is too large, the period between truck passes will be too long to agitate the moisture sensitive soils, and may not exhibit excessive rutting.

The contractor should provide a fully loaded, tandem-axle truck, or loaded truck similar to those anticipated during pavement construction.

The number of truck passes in proof rolling is dictated by field conditions. Sometimes, for example, in cut or at-grade sections with high moisture soft deposits, one or two passes might be adequate to cause several centimetres of rutting, thereby indicating sub-grade instability. However, in fill sections where density and moisture can be controlled for each layer, five to six passes may or may not be adequate, depending on whether or not the underlying material is a compacted fill or in-situ soil.

The number of passes should be until the sub-grade rutting exceeds 12mm. This is particularly important in cut or at-grade sections with more than 75% silty material or soft deposits.

During proof rolling, the engineer should observe the sub-grade performance at all times. The last truck pass is usually performed at walking speed so that the engineer may follow to observe the rebound deflections, and rutting and/or pumping of the sub-grade. Immediately after the last truck pass, the inspector should test areas



showing more than 30mm of rutting and areas of high rebound deflections (pumping), with the DCP to determine the required treatment thickness. However, the engineer should ensure that the finished sub-grade does not exhibit more than 12mm of rutting.

When rutting and deflection under heavy equipment indicates a soft sub-grade, test pits up to a depth of 1.5m are excavated using a backhoe to further investigate the subsurface condition. At least two test pits are needed in any failed sub-grade during proof rolling. Excavate the test pits across the width of the sub-grade in the failed locations. Pick locations where the deformations are the highest to evaluate the site.

9.3 Road cuts

During the project design phase, the cut slope design recommendations are prepared, with slope inclinations required for stability, mitigation requirements if needed, and the usability of excavated cut materials. Additional investigation might, however, be needed during construction when there is a change in design requirements or route alignment. Road cuts are places where geotechnical problems are often encountered. For this reason, the existing natural and cut slopes in the vicinity of the project should be inspected repeatedly to evaluate the performance of the new cuts. The inclination and height of existing cut slopes should be measured and erosion or slope stability problems should be examined. Observation of existing slopes should include vegetation, in particular those that may indicate wet soil, as vegetative patterns may indicate subsurface drainage characteristics. Assessment should also include an indication of whether tree roots may be providing anchoring of the soil and if there are any existing trees near the top of the proposed cut that may become a future hazard.

As with the design phase, additional exploration programmes during construction should give consideration to the potential for use of the excavated material as a source for fill elsewhere in the project. If the construction contract is set up with the assumption that the cut material can be used as a source of fill material (or other uses in the project) it is important to have adequate subsurface information to assess how much of the cut material is useable for that purpose. A key to the establishment of exploration frequencies for embankments is the potential for the subsurface conditions to impact the construction of the cut, the construction contract in general, and the long-term performance of the finished project. Any additional exploration programme at this time should ensure that costs and time to complete the programme are reduced to an acceptable level.

If it is determined that slope stability analysis should be performed for some cuts, laboratory strength testing on undisturbed samples may be required. Slope stability analysis requires accurate information of soil stratigraphy and strength parameters, including cohesion, friction angle, undrained shear strength, and unit weight for each layer. Consideration should be given during stability analysis to adjusting strength parameters to account for future changes in moisture content, particularly if field testing was performed during dry months as it is possible that the moisture content of the soil will increase in the future.



9.4 Embankments

The key geotechnical issues for the performance of embankments include the stability and settlement of the underlying soils and the impact to adjacent structures, such as buildings and utilities. Therefore, additional site investigation may be needed during embankment construction. This investigation should extend to at least two to three times the width of the embankment on either side and to the top and bottom of adjacent slopes. Furthermore, areas below the proposed embankments should be fully explored if any existing landslide activity is suspected. Engineering parameters generally required for embankment design include:

Total stress and strength parameters; Unit weight; Compression indexes; Coefficient of consolidation.

The type (pits, in situ tests, and boreholes) and amount of investigation is determined by the extent of information needed. When existing data are insufficient, and pits and boreholes are needed, every effort should be made to site them so that maximum information is obtained where it is most relevant. All embankments over 3m in height, embankments over soft soils, or those that could impact adjacent structures (bridge abutments, buildings) will generally require deep pit excavations or borings. At critical locations, a minimum of two exploration points in the transverse direction to define the existing subsurface conditions for stability analyses should be obtained. More exploration points to investigate the subsurface stratigraphy may be necessary for very large fills or very erratic soil conditions.

In road upgrading projects, embankment widening will require careful consideration of exploration locations. Pits or borings near the toe of the existing fill are needed to evaluate the present condition of the underlying soils, particularly if the soils are fine-grained. In addition, pits through the existing fill into the underlying consolidated soft soil, or to define over-excavation, should be obtained to determine conditions below the existing fill. In some cases, the stability of the existing embankment fill may be questionable because raveling or slope failures have been observed. Embankments constructed of material that is susceptible to weathering or instability may require additional pits through the core of the embankment to sample and test the present condition of the existing fill.

Pits or borings are also needed near existing or planned structures that could be impacted by new fill placement. Soil sampling and testing will be useful for evaluating the potential settlement of the existing foundations of the structure as the new fill is placed.

The depth of test pits and borings will generally be determined by the expected soil conditions and the depth of influence of the new embankment. Explorations will need to be sufficiently deep to penetrate through problem soils such as loose sand, soft silt and clay, and expansive soils, and at least 1.5m into competent soil conditions. In general, all borings should be drilled to a minimum depth of twice the planned embankment height.

When groundwater is anticipated, water levels should be recorded in all test pits or borings. Information regarding the time and date of the reading and any fluctuations that might be seen should be included in field logs. For high volume roads that involve embankment widening, piezometers are needed in borings located at or near the toe of an existing



embankment, rather than in the fill itself. Exceptions are when the existing fill is along a hillside or if seepage is present on the face of the embankment slope. The groundwater levels should be monitored periodically to provide useful information regarding variation in levels over time.

The location of the groundwater is important during stability and settlement analyses. A high groundwater level results in lower effective stress in the soil affecting both the shear strength and the consolidation behaviour under loading. If there is a potential for a significant groundwater gradient beneath an embankment, or surface water levels are significantly higher on one side of the embankment, the effect of reduced soil strength caused by water seepage should be evaluated. Normally, more than one piezometer could be needed to estimate the gradient. Also, seepage effects must be considered when an embankment is placed on or near the top of a slope that has known or potential seepage through it. A flow net may be used to estimate seepage velocity and forces in the soil and to model pore pressures.

9.5 River crossings

Additional site investigations should be performed to provide the information needed for structural foundations at river crossings. The extent of exploration during construction should be based on any deviation of the subsurface conditions from which was considered in the design phase, structure type, and any new project requirements. The exploration programme should be designed to reveal the nature and types of soil deposits and rock formations; the engineering properties of the soils and rocks; the potential for liquefaction; and the groundwater conditions. It should also be sufficient to identify and delineate unforeseen problematic subsurface conditions such as soft deposits and swelling and collapsing soils. Boring logs should be prepared and cores retained and preserved for future reference and testing.

In general, the additional investigations required during construction at river crossings should have the objective of finalizing the following issues:

The anticipated structure type and magnitudes of settlement (both total and differential) the structure can tolerate.

At bridge abutments, the approximate maximum elevation feasible for the top of the foundation in consideration of the foundation depth.

For interior piers, the number of columns anticipated, and if there will be single foundation elements for each column, or if one foundation element will support multiple columns.

The depth of scour anticipated, if known. Any known constraints that would affect the foundations in terms of type, location,

or size, or the assumptions which need to be made to determine the nominal resistance of the foundation (utilities that must remain, construction staging needs, excavation, shoring and false-work needs, other constructability issues).

Often, foundation recommendations made during design are subject to change depending on the construction staging needs and other constructability issues that are discovered. Hence, during construction, the geotechnical engineer will need to check the foundation with regard to these changes and the structural design, with suggest modifications as required. For this purpose, frequent communication and team work with the structural engineer is required.



For drilled shaft foundations, it is especially critical that the groundwater regime is well defined at each foundation location. Piezometer data adequate to define the limits and piezometric head in all unconfined, confined, and locally perched groundwater zones should be obtained at each foundation location. Moreover, during excavation of the shaft hole, a project geotechnical specialist is usually on-site to provide support in the following areas:

Review of soil types with respect to the site conditions established during the exploration programme for design. If the soil type differs, there is a need to evaluate whether the capacity of the drilled shaft will be affected by the changed site condition.

To observe conditions during shaft drilling. These conditions include the type of equipment, progress of drilling, use of temporary and permanent casing, the depth to groundwater and the height of slurry in the hole if slurries are being used to maintain shaft-hole stability. Often, contract documents provide the contractor with requirements for monitoring slurries and hole-stability; the geotechnical engineer provides an independent confirmation that these requirements are being met.

To check the condition at the bottom of the drilled shaft and confirm that slough accumulated at the bottom of the shaft has been removed to the extent practical.

For culverts, revising the investigation conducted for design could also be necessary during construction. Where soil conditions are favourable, the data collected earlier is often sufficient. Additional pits or even borings are needed only for box culverts. In this case, the excavation should extend a minimum of 5m below the bottom of the culvert, or until 1.5m of firm ground. In normal circumstances, a minimum of two investigation points spaced adequately are needed to develop a subsurface profile for the entire culvert. Additional borings should be provided for long culverts or in areas of erratic subsurface conditions.

9.6 Landslides

Landslide areas should have been detected in the early stages of the design. If a landslide is identified during construction, inclinometers and piezometers should be installed in normal circumstances to accurately define the depth of movement and the role of groundwater. Surveying stakes can also be used for this purpose. When monitored over several months, this instrumentation can be very valuable in determining the behaviour of the landslide and the relationship between periods of active slide movement and seasonal groundwater levels.

Generally, in terrains where landslides are expected, the geotechnical engineer will often be requested to provide support during construction. This support could be in the form of selecting an appropriate remedial measure or confirm that the method suggested earlier during design will not lead to additional failures or result in long-term maintenance requirements. In this case, the field inspection should include the following activities:

Review the method of excavation and the sequence of work; If groundwater is observed, analyse plans for collection and disposal of

groundwater. Assess whether any of these plans could result in further instabilities of the landslide.

Confirm that the lines and grades for the excavation and backfill are being satisfied. Check backfill material to confirm that they meet repair requirements, particularly

relative to any drainage collection layers or trenches.



Oversee installation and monitoring of any instrumentation installed to monitor movement during construction and beyond.

Review results of instrumentation immediately after installation to confirm that performance is acceptable.

9.7 Retaining walls

If a slope failure is detected, then one of the methods to mitigate its effect on the road would be to construct a retaining wall. The site investigation activity for the construction of retaining walls should aim at establishing the suitability of the site for the type of structure being considered, the overall stability and suitability of the foundation, and the availability of suitable building stones for the wall. The design of the proposed works is often helpful in identifying parameters that need to be obtained from the ground investigation. The investigation should identify specific groundwater and surface drainage conditions in the vicinity of the site and their likely response to heavy rain. In general, the following are the general investigation requirements for retaining wall design and construction:

Take a minimum of 2 pits per wall. At retaining wall locations, pits should be taken at a maximum interval of 30m,

with a minimum of two pits that are dug as close to the wall alignment as possible, and with locations alternating from in front of the wall to behind the wall.

One pit should be located near the expected highest portion of the wall. For wall heights greater than 6m, use a maximum pit spacing of 15m. Retaining structures with tiebacks or soil nails will need an additional row of pits or

shallow borings spaced at 30m to 60m, where the anchor load zone is anticipated. Pits or borings should continue to depths where all unsuitable foundation materials

are penetrated, and the proposed stress increase due to the retaining wall will be less than 10% of the original overburden pressure. Alternatively, boring may be completed at a depth of 2/3 of the anticipated wall height or a minimum of 1.5m into the bedrock.

Exploration depth should be great enough to fully penetrate soft, compressible soils (peat, organic silt, soft fine grained soils) into competent material of suitable bearing capacity (stiff to hard cohesive soil, compact dense cohesionless soil, or bedrock).

9.8 Construction materials

The investigation for borrow and quarry materials during construction is aimed at finalizing the options available within a road project. Often, the details of the original exploration determine whether a site merits additional investigations. A common construction request involves guidance on the suitability of borrow and quarry materials for different parts of the pavement structure. Questions on material suitability require the project geotechnical specialist to review how the material will be used for construction and whether the proposed material meets engineering design requirements. In contrast to the design phase, the requests during construction will need to be determined on a case-by-case basis.



In general, a material source investigation during construction should provide the following minimum information.

Expected quality of processed materials and procedures necessary to obtain that quality.

The boundary limits of proven materials and limits of previously used areas. Specific areas and elevation of non-usable materials. Previous uses of material from the source. Recommendations on uses and limitations for processed materials.

At a minimum, the quality of material reserves should be known during construction. The structures of the hard rock are necessary to develop an approach for extraction (i.e. blasting or mechanical excavation). The state of weathering or alteration also needs to be established, as this may define the use of the materials. Weathered materials may be designated for fill.

For quarry site investigations in large projects, wet rotary rock coring methods are used to determine subsurface conditions and obtain samples for testing. Triple-tube core barrels are commonly needed to maximize core recovery. For riprap sources, fracture mapping includes careful measurement of the spacing of fractures to assess rock block sizes produced by blasting. Every rock formation requires its own blasting method determined by a complete discontinuity survey. Also, identification of the type and amount of joint infilling is needed. Core samples are reviewed to test the quality of riprap or aggregates. If assessment is made on the basis of existing quarry faces, the use of core or geophysical techniques might be needed to verify that the nature of the rock does not change behind the face or at depth.

The indicated quantity of material that is available in the potential material source should also be further evaluated. The geologist or the engineer in charge uses the mode of occurrence of the deposit in conjunction with test pits and borings to determine the surface plane area and volume of usable materials. The quantity of material reported should be confirmed as the amount of material estimated to be present at the site. Extrapolation beneath the depth of test pits and borings should not be made for the calculation of indicated quantities unless well supported by geological information. Other tasks to consider include:

The surface drainage at the site, noting areas of ponding water, swamps, sloughs, or streams. It is important to determine flooding possibilities or surface flow after periods of heavy rainfall, and from artesian conditions.

The location of the groundwater table along with seasonal variations. Identify any springs in the area that will affect the development of borrow or quarry sites, or if production operations can impact the water source.

The degradation and wear characteristics of aggregate sources. The history of use of the aggregate is especially important for this purpose.

An estimate of oversize material (greater than 254 mm) determined in percent by volume is necessary. The estimate is given in a percent range such as 15% to 25%. Also describe the largest size cobble or boulder observed during site investigation.

In some projects, pilot material production trials may be necessary to establish requirements for production at the required quality and quantity. Materials extraction and production trials should be well planned and supervised with samples taken from the end products of all accepted and rejected deposits. Further laboratory investigations should be



made on samples collected from all stages of extraction. This information can then be used to review and determine optimum extraction and production procedures at the required standards or specifications.

For each source, the site map developed during design should be updated or a new map should be prepared showing the location of the source in relation to natural landmarks, property lines, existing roads in the area, and the new alignment. The map should include a plan view of the property with all test pits, trenches and borings (including identifying numbers). In large projects, the map should be part of a report that provides detailed site exploration, sampling and laboratory testing, along with the subsequent development of a pit or quarry site.

9.9 Pavement condition survey

The purpose of pavement condition survey is to evaluate the functional and structural aspects of the road right after construction or during rehabilitation and reconstruction. Functional evaluations identify the capability of the pavement structure to provide a comfortable and safe service. In a newly constructed road, the primary parameters determined in functional evaluations are the riding quality and skid resistance. For rehabilitation and reconstruction, this may include the evaluation of aspects such as potholes, cracks, and deformations. Structural evaluations are needed to determine whether the pavement will carry the traffic it has been designed for. During rehabilitation and reconstruction, structural evaluation can also help in assessing the status and integrity of all pavement layers and their structural capacity to carry the expected traffic over the remainder of their life.

Functional evaluation is usually performed by visual inspection. Visual inspection requires the rating of the degree and extent of the various distresses. Typical pavement conditions evaluated visually include surface conditions (roughness), potholes, deformations (ruts), cracks, edge-breaks, ravelling, bleeding (flushing), and patching. The roughness of a road pavement is the major parameter used to determine of the functional conditions.

The structural aspect of a pavement can be evaluated using different ways. The most common methods involve the following:

Probing the pavement (by DCP or any similar method), measure the strength characteristics of each layer, and correlate this information to data on similar types of pavement under similar physical conditions.

Measurement of the surface deflection and shape of the deflection bowl under loading, and relate this information to empirical data on similar types of pavement under similar physical conditions.

Excavation of trial pits, measure layer thickness and obtain laboratory data to characterise the properties of the materials in the pavement and sub-grade. Apply this information to current pavement design methods or relate the information to empirical data on similar types of pavement under similar physical conditions

Chapter 10 Site Investigation Manual – 2013 Reports and Checklists


10 REPORTS AND CHECKLISTS 10.1 Introduction

Upon completion of the site investigation and laboratory testing programme, the data should be compiled, evaluated and interpreted, and a report or reports should be prepared for use in the design process. Interpretation of the site investigation information provides essential inputs into the design and construction recommendations. Checklists are useful to review the entire process of site investigation and ensure the inclusion of all necessary information at each stage.

10.2 Reports

Two types of reports can be prepared at the end of the site investigation. These are the site investigation report and the soils and materials report. A site investigation report is a document used to communicate the findings, recommendations and the site conditions to the road design and construction personnel. It contains all available information on the characteristics of the sub-grade, road cuts and embankments, bridge foundation conditions, and special problems such as landslides and expansive soils. The data and information on construction materials, on the other hand, their engineering characteristics, local distribution, amount and quality can be summarized in the soils and materials report.

10.2.1 The site investigation report

While the content and format may vary from project to project, all site investigation reports should contain certain basic essential information including:

Summary of all subsurface exploration data, including subsurface soil profile, exploration logs, laboratory or in situ test results, and groundwater information;

Interpretation and analysis of the subsurface data; Specific engineering recommendations for design; Discussion of problem conditions and possible solutions; Recommendations on special geotechnical provisions.

The initial sections of the report describe the type of the road project, the regional location and limits of the project site, and the purpose and scope of the site investigation programme. A summary that highlights the methodologies used; the findings obtained; problems encountered; and the suggested recommendations, may also be added.

The discussions in the main part of the document identify the types of investigation methods used, the number, location and depths of borings, exploration pits and in situ tests, the types and frequency of samples obtained, the types and number of laboratory tests performed, the testing standards used, and any variations from conventional procedures. The station-to station descriptions of the alignment divided on the basis of differences in soil and rock conditions, terrain characteristics, and other factors, should be included in this part. The analysis of data obtained from the investigation of specific problems, and the subsurface profiles developed from the field, should also be given in this portion.

Chapter 10 Reports and Checklists Site Investigation Manual – 2013


The final sections of a report typically includes conclusions, recommendations, and appendices which contain pit and boring logs, water level readings, data plots from each in-situ test hole, summary tables and individual data sheets for all laboratory tests performed, rock core photographs, and geologic mapping data sheets and summary plots. Often, the site investigation report will also include copies of existing information such as boring logs or laboratory test data from previous ground surveys at the project site.

Generally, the outline given in this section addresses the minimum requirements needed for writing a site investigation report. Depending on the scope of investigation, it is advisable to include additional topics to ensure the coverage of all available information. General information A report needs a specific title that can describe its contents and can be used for its identification. Report titles should normally be short and concise. A brief summary of the findings is needed in the first part of the report. This includes explaining the specific recommendations and limitations that will result in deviations from standard road design practices. The introductory part describes the project, including the project limits, and the purpose and scope of site investigation. The project description consists of:

Project description: A detailed explanation of the project (including type of road, roadway length, width, paved or unpaved, important design features, etc.), the purpose of the investigation, dates of field exploration, and identification of personnel and companies involved in the project should be presented. A list of previous explorations or reports for the project and whether they have been supplemented by the current investigation should also be explained. The investigation techniques and exploration methods used (e.g. review of published data, site reconnaissance and mapping, equipment types, method of subsurface exploration, laboratory testing, analyses, etc.) should be described.

Location: The location of the project (including the beginning and ending stations, centreline alignment, and station equations). A location map showing villages, towns, and city (in urban areas), all cultural, environmental, and natural features, and rivers and lakes should also be included. The maps will show existing roads, major topographic and drainage features, materials site locations, and utilities and buildings. If material sites are not near the alignment, separate sheets are recommended.

Climatic conditions: A description of climatic conditions of the project site and the weather characteristics during site investigation is required. Describe climatic conditions that will have an effect on the project design and construction. Note seasonal conditions such as temperature extremes; heavy rain or fog that could limit construction seasons; the ability to reduce the moisture content of construction materials; and the effect traffic control. State the mean annual temperature, the temperature extremes, the mean annual precipitation, and the heaviest rainfall months. In some highland (Wurch) areas, a description on the characteristics of freezing and thawing is also needed.

Topography and drainage: Provide a description of the landforms and drainage characteristics through which the road will pass. Note topographic highs such as hills and ridges that will require cuts. Also describe topographic lows such as valleys, swales, marshes, and minor creeks that embankments will traverse. Discuss the depth below or height above the profile grade. Measure and describe steepness



of slopes along and perpendicular to the alignment. Describe slopes that will receive side hill cuts or fills. Note drainage patterns including creeks, intermittent streams, rivers, erosion patterns, and high water elevations. Report vegetation types, sizes, and density. Include special notes of vegetation that indicates subsurface conditions (such as the presence of shallow groundwater or leaning trees that indicate ground movement).

Geological information: An explanation of the regional geologic setting of the project area, geomorphic provinces and major characteristics such as depth of weathered profile, bedrock formations, and rock types should be provided. Include discussion on known or documented geologic hazards such as landslides, earthquakes and flooding. Describe the geology of the project site by dividing the road alignment into different sections or stations. Preparation of a geological or geotechnical map in hillsides or mountainous terrain is helpful. Emphasize properties or conditions of the soil and rock materials that will impact design or construction of the project.

Findings for Field Explorations This section of the site investigation report should start with an explanation of what was accomplished during the field explorations. Problems encountered that may have design or construction implications should be added. The report should contain an account of the geotechnical conditions revealed by pits, borings or other exploratory methods. It must also have a description of the outcrops of bedrocks and superficial deposits; evidence of current or past landslides; groundwater conditions (e.g. springs and streams); wetland locations; areas that may involve sub-excavation; stabilization or drainage measures; locations that may need rock excavations including areas that require blasting; areas that will involve extensive excavations or fills; and roadway conditions that may indicate sub-grade problems. Generally, exploration findings can be presented in the following forms:

Station to station descriptions: The road alignment is divided into logical intervals based on differences in soil and rock conditions, terrain differences, and other factors. The description of these intervals includes all information noted from visual inspections of the terrain conditions and factual engineering geological and geotechnical information obtained during surface mapping and from test holes. The discussion may also include carefully identified geotechnical interpretation of the data. The interpretation is made to increase the usability and reliability of the information inferred between observation points. Obviously, geologic interpolation cannot provide certainty regarding subsurface conditions, but it is useful to define assumptions for analytical purposes.

Horizontal plan: A plan showing the areal distribution and location of borings along the centreline of the proposed alignment should be presented in a site investigation report. Briefly describe the number and type of borings, trenches, and test pits. The boundaries of all soil and rock units should be shown on the plan, and properly designated by a geological name or other symbolic notation, which should be explained in a legend. Include surface contours to indicate ground elevations.

Subsurface profile: A complete description of each soil and rock unit encountered in pits and borings (including in-situ test results) should be used to prepare cross sections to show subsurface conditions across the centreline. Subsurface profiles and graphical boring logs are normally placed in appendices and may contain a depth and elevation scale; indication of stratum change; description of material in



each stratum; depth of bottom of boring; depth of boulders or cobbles; caving depth; static and free water level observations; artesian water and height of rise; and sampling type, depth and interval.

In-situ tests: Describe the in-situ tests performed with an explanation of why a specific method was chosen. Where the in situ test results lend themselves to a concise summary (i.e. general data in the form of result ranges) include the summary in the main part of the report. Otherwise summarize as appropriate in an Appendix.

Laboratory testing: Describe laboratory tests performed in the project by referring to the standard methods used in the testing programme. Include modifications of existing test methods or unpublished local practices in the Appendix. It is advisable to provide the laboratory test results in an approved soils testing report form.

Instrumentation: Describe any instrumentation installed during the field exploration. Indicate their locations on the plan view map or location drawing. State why each was installed and present summaries of the data in an appendix. If a monitoring programme must continue beyond the time of the site investigation, provide a schedule and duration. Present and discuss relevant data from instrumentation monitored during original construction or from previous exploration programmes of relevance to the project.

Data analysis: This is performed in order to develop recommendations regarding stability, settlement, any other problem. Use the geotechnical data and information obtained from the various field investigations, the laboratory testing data, and geologic interpretations of site conditions to determine and characterize the relevant engineering properties of the rock and soil materials encountered at the project site. Incorporate drawings related to the analysis that can create a clear understanding. The results of data analysis alerts designers and contractors to potential problems, and may provide the basis for the selection of the appropriate design solution. The analysis is also helpful in assessing risks associated with different design options.

Design recommendations: Preliminary recommendations related to the design and construction of the roadway including any remedial measures that may be necessary to complete the project. If comparing different vertical or horizontal alignments, list any concerns for each alignment and present a cost summary.

o Sub-grade characteristics: Prepare recommendations related to sub-grade characteristics. If soft sub-grades are encountered, recommend practical treatment options. Recommendations should be concise and directed to the preferred alternative. Note the areal extent of treatment (by station and width), and depth of required material excavation.

o Roadside conditions: Prepare recommendations related to the design of the roadside that include any remedial measures necessary to complete the project. Provide cut slope angles and suggest specific designs for road-cuts and embankments including an assessment on stability and settlement, the construction sequence, field controls and instrumentation, and any other related issue affecting the project. Be specific regarding cut and fill slope recommendations.

o Structural foundations: Consider all available site investigation information, including previously performed explorations, in making recommendations for types of structural foundations, treatment of the foundation, allowable bearing, design of piles and their estimated length, footing elevations, construction sequence, instrumentation, and any other



design factors affecting the project. Give the logs of all structural foundation borings in the Appendix.

o Landslides: Present a list and description of alternatives considered for the remediation of landslides observed on roadsides.

o Problem soils: Give the location of problem soils and provide economical solutions to mitigate their short and long term effects on the road.

Appendices Present the following information in the Appendices of the site investigation report:

Boring Plan: Present a site plan showing all boring locations. The plan should have a legend to identify all features drawn on it.

Boring Logs: Include boring logs in the report. Present colour pictures of representative rock cores and soil samples along with log descriptions.

Graphical analyses: Any repeated graphical outputs such as gradation curves from particle size analyses or stability analyses should be put in appendices.

Extended test results: Some long tables of laboratory results should be given in appendices and linked to the analytical descriptions in the main part of the report.

Descriptions of methods or guidelines: Put extended description of methods and guidelines used for laboratory testing, data analysis or recommendations.

10.2.2 Soil and materials report

The soils and materials report will generally contain information on the availability and local distribution of construction materials. In this document, the number and location of sites investigated and those selected for possible use, their characteristics, interpreted quantity and quality of material should be reported. A note is also necessary on rejected sites and the reasons for rejecting them. A description of laboratory tests performed, including controlling standards, sampling methods and test procedures is necessary. There should also be a map and photos which show the dimensional characteristics such as thicknesses and extent of resources. Generally, the report should have the following information:

Location of existing or previously used quarries and pits in the project area; Estimates of quality and quantities in existing sources; Previously encountered problems with the above sources; Climatic details; including rainfall, rainfall intensities and evaporation; Project materials required in terms of quality and quantity; Project constraints; eg economic, contractual, environmental or time-related; Proposed road design standards; Likely soils and aggregate testing requirements; Likely sub-surface extent and nature of deposits; As-dug properties of deposits; Excavation limitations; Likely processing requirements; Access requirements; Suitability for various road-building applications; Sketch maps of each source;



Cost of haulage from source to site on road; Cost per cubic meter of extraction and processing; Borehole or drill hole logs; Trial pits or trench logs; Typical cross sections based on the above to indicate volumes available; Quality assured laboratory test results; Comment on the field and laboratory data, which should highlight not only the

quality and quantities of the various materials but also the potential variability; Clear definition of problems associated with non-standard materials; Comment on suitability of stabilization, if required; Likely socio-environmental impacts of resource development.

10.3 Checklists

Checklists are charts or questions that are developed to aid engineers in the review of the site investigation process. In this manual, a set of questions have been developed and summarized in Table 10.1 so that they can be used to identify gaps during exploration. The advantage of using these lists is to ensure that pertinent data are not forgotten or overlooked; and to identify those items that need to be investigated further. Generally, Table 10.1 can be used as it is or can be populated further by adding other questions relevant to road projects. Upon completion of the questions, the engineer in charge of the site investigation should summarize the negative responses and discuss with others to decide what should be done next.



Table 10.1: Site Investigation Checklist Site investigation activities to check

Y N General: 1. Check if the scope and purpose of the investigation have been summarized 2. Verify if the location of the investigation has been described and a map is included 3. Confirm if the field explorations and laboratory tests are listed 4. Check if the following general information has been provided

Brief description of the project Brief presentation of geological and topographical information Brief presentation of boring and sampling methods Summary of general soil, bedrock, and groundwater conditions, including a

generalized interpretation of findings

Statement of where original drawings and data may be inspected Statement of where soil or rock samples may be inspected Initials of personnel and dates they performed field reconnaissance,

subsurface exploration and preparation of the soil profile

5. Verify if all the roadway subsurface data have been presented in the form of a profile along the centreline or baseline, and on cross sections where applicable

6. Check if the following information is included in the site investigation report Test hole logs Field test data Laboratory test data Photographs (if pertinent) Plan and subsurface profile

7. Ensure whether the site investigation adequately characterize the soil and rock masses

8. For upgrading projects where the alignment has been shifted, check if additional subsurface explorations have been conducted along the new route

Field boring log: 1. Location and depth of boring or excavation 2. Exploration identification number 3. Description of the project 4. Soil and bedrock symbols and descriptions 5. Sample types and depths 6. If cone penetration tests were made, include plots of cone resistance with depth 7. Groundwater levels measured 8. In situ test records Subsurface profile: 1. Check if field explorations are located on a plan view 2. Ensure if explorations are plotted and correctly numbered on the profile at their true

elevation and location

3. Verify if cross-sections have been developed to show subsurface conditions disclosed by a series of borings or pits across the centreline

Laboratory test data: 1. Soil classification tests such as moisture content, gradation, Atterberg limits,

performed on selected representative samples to support visual soil identification

2. Tests to confirm design values 3. Index tests of construction materials 4. Test results of the sub-grade 5. Test results of special problems



Table 10.1: Site Investigation Checklist Station-to-station descriptions for: Y N 1. Types of slope materials along the alignment 2. The stability of these slopes in natural conditions 3. Slides, slumps, and faults noted along the alignment 4. Existing surface and subsurface drainage 5. Evidence of springs and excessively wet areas Station-to-station recommendations for: 1. General soil cut and fill 2. Specific surface and subsurface drainage conditions 3. Excavation limits of unsuitable materials 4. Erosion protection measures for back slopes, side slopes, and ditches, including

riprap recommendations or special slope treatment.

5. Special usage of excavated soils 6. Locations of spoil areas Potential sources of construction materials: 1. Have soil samples representative of all materials encountered during pit investigation

been submitted and tested?

2. Are laboratory quality test results included in the report? 3. For soil borrow sources, have possible difficulties been noted, such as above

optimum moisture content for clay-silt soils, waste due to high PI, boulders, etc.?

4. For aggregate sources, do the laboratory tests (such as Los Angeles abrasion, sodium sulphate, degradation, absorption, reactive aggregate, etc.) indicate if suitable materials can be obtained from the deposit using normal processing?

5. If the laboratory tests indicate that suitable material cannot be obtained from borrow pits as they exist naturally according to the specification, were the different sources rejected or are detailed recommendations provided for further processing or controlled production?

6. Where high moisture content clay-silt soils must be used, are recommendations provided on the need for aeration to allow the materials to dry out sufficiently to meet compaction requirements?

7. Do the proven material site quantities satisfy the estimated project quantity needs? 8. Are there any environmental impacts associated with using the sources? 9. Have pit reclamation requirements been covered adequately? 10. Ensure if a material site sketch (plan and profile) has been provided for inclusion in

the report, which contains:

Proposed locations of quarries and borrow pits Material site numbers Expected amount or volume Previously mined sites North arrow and scale Test hole or test pit logs, locations, numbers and date Water table elevation and seasonal fluctuation Depth of unsuitable overburden, which will have to be stripped Suggested overburden disposal area Existing or suggested access roads

Chapter 11 Site Investigation Manual – 2013 References


11 REFERENCES Central Statistical Agency and Ethiopian Development Research Institute (2006). Atlas of the Ethiopian Rural Economy. Addis Ababa, Ethiopia, Committee of State Road Authorities (CSRA) (1985). Guidelines for road construction materials - TRH 14, Department of Transport, Pretoria, Republic of South Africa. Committee of State Road Authorities (CSRA) (1993). The investigation, design, construction and maintenance of road cuttings - TRH 18. Department of Transport, Pretoria, Republic of South Africa. Emery S J (1985). Prediction of Moisture Content for use in Pavement Design. Doctoral Thesis, University of the Witwatersrand, Johannesburg, South Africa. Ethiopian Geological Survey (2006). Simplified Geological Map of Ethiopia, Addis Ababa, Ethiopia. Ethiopian Road Authority (2002). Site Investigation Manual. Addis Ababa, Ethiopia. Federal Negarit Gazeta (2004) Labour Proclamation No. 377/2003. Federal Negarit Gazeta of the Federal Democratic Republic of Ethiopia, Proclamation No. 377/2003, Page 2453, 10th Year No. 12, 26th February 2004, Addis Ababa. Nata Tadesse, Shishay Tadios & Mekdes Tesfaye (2010). The Water Balance of May Nugus Catchment, Tigray, N Ethiopia. Agricultural Engineering International: CIGR Journal, Manuscript 1306, Volume XII. South African Institute of Civil Engineers, Geotechnical Division (2010). Site Investigation Code of Practice, 1st Edition. South African Institute of Civil Engineers, Mid-Rand, South Africa. Thornthwaite, C W (1948). An approach towards a rational classification for climate. Geological Review, Volume 38, No 1. Transport Research Laboratory (TRL) Ltd (1999). Guidelines on the Selection and Use of Construction Materials. Department for International Development (DFID), Report No. PR/INT/203/00, R6898. London, UK. Transport Research Laboratory (TRL) Ltd (2004). Dynamic Cone Penetrometer (DCP) tests and analysis, Technical Information Note. Department for International Development (DFID), Report No. PR/INT/277/04, R8157. London, UK. US Department of Transportation, Federal Highway Administration (2002). Evaluation of soil and rock properties, Geotechnical Engineering Circular No. 5. Report No. FHWA-IF-02-034, Washington DC, USA US Department of Transportation, Federal Highway Administration (2006), Geotechnical aspects of pavements. Publication No. FHWA NHI-05-037, Washington DC, USA

Chapter 11 References Site Investigation Manual – 2013


US Department of Transportation, Federal Highway Administration (2001). Manual on Subsurface Investigations, National Highway Institute. Publication No. FHWA NHI-01-031. Washington DC, USA. US Department of Transportation, Federal Highway Administration (2006). Soil and Foundations. Publication No. FHWA NHI-06-088. Washington DC, USA. Weinert, H H (1980). The natural Road Construction Materials of Southern Africa. Academia, Pretoria, South Africa.

Appendix A Site Investigation Manual – 2013 DCP Test

Ethiopian Roads Authority Page A-1

Appenidix A THE DYNAMIC CONE PENETROMETER (DCP) TEST

A1. Introduction

The Dynamic Cone Penetrometer (DCP) test is used as a rapid means of assessing the sequence, thickness and in-situ bearing capacity of the unbound layers and underlying sub-grade that comprise the pavement structure. Probably, the greatest benefit of the DCP device lies in its ability to provide a continuous record of relative soil strengths with depth. By plotting a graph of penetration index (PI) versus depth below the testing surface, a user can observe a profile showing layer depths, thicknesses, and strength conditions. This can be particularly helpful in cases where the original as-built design for a project were lost, never created, or found to be inaccurate. The DCP's other strength lies in its small and relatively lightweight design. The DCP is ideal for testing through core holes in existing pavements.

Data from a DCP test is processed to produce a penetration index (PI) which is simply the distance the cone penetrates with each drop of the hammer. The PI is expressed in terms of mm per blow, and can be plotted on a layer strength diagram, or directly correlated with common pavement design parameters such as CBR.

DCP testing can be done during preliminary soil investigations for route selection or design to quickly map areas with weak material and define uniform sections. For example in an area where potentially collapsible soils are expected, running an initial test, and then flooding the location with water and running another test, can allow to delineate these soils produce based on a noticeable increase in the PI (less shear strength might indicate a potentially collapsible or moisture sensitive soil).

One of the major applications of DCP testing has been in the structural evaluation of existing pavements. During construction or later in the maintenance programme, The DCP test is an ideal tool for monitoring all aspects of construction of a pavement sub-grade and base or inspect their remaining strength. In new roads, it can be used to verify the level and uniformity of compaction over a project. During rehabilitation and reconstruction, it is useful to define problem areas that develop due to unavoidable soil conditions brought on external factors.

A2. The DCP device

The most common DCP device consists of two 16 mm diameter rods, with the lower rod containing an anvil, a replaceable 60o cone having a maximum diameter of 20mm, and depth markings every 1 mm. The upper rod contains an 8 kg hammer dropping through a height of 575 mm, an end plug for connection to the lower rod, and a top grab handle (Figure A-1). An optional depth residing device can be attached to eliminate the need to measure penetration depth at ground level.

A3. Data collection

Normally, three people are needed to complete the test. One person stands on the stool and holds the apparatus by the handle while the second person lifts the drop weight. The third observes the readings and records them on the appropriate form. If only two persons are available, one can drop the hammer and the other records the depth of penetration. It is

Appendix A DCP Test Site Investigation Manual – 2013

Page A-2 Ethiopian Roads Authority

extremely important to gain maximum height for each drop but care must be taken not to strike the weight against the handle.

Figure A-1: Schematics of the DCP device.

During testing the operators should not put their hands near the Anvil to ensure that their fingers are not trapped underneath the Hammer when it is dropped.

In order to avoid any potential damage to the underground utilities, it is essential to ensure that there are no utilities beneath the test location before the test starts.

The readings are taken with each blow of the weight. If the penetration rate is less than 20 mm/blow, the frequency of readings may be decreased to the following:

One for every two blows with readings from 10-20 mm One for every five blows with readings from 5-9 mm One for every ten blows with readings from 2-4 mm. Penetration depth less than 1 mm for 20 blows is considered as refusal.



The ultimate depth of investigation is determined by the purpose and stage of investigation. Normally, readings are taken up to 1.0m below the contact with the sub-grade. No test should be less than 1.0m from ground surface.

Upon reaching the desired depth or refusal, the instrument is withdrawn with a jack. The forked part of the modified jack is placed under the anvil during extraction. An alternative method would be to strike the drop weight against the bottom of the handle, reversing the entry procedure. But, this is usually time consuming and adds additional stress to the threaded components, reducing instrument life.

The DCP is capable of penetrating through asphalt and base course materials. But, tests in these materials cause additional wear on the instrument. Hence, an area on the asphalt large enough to accommodate the base of the instrument is removed by coring and the base course materials excavated to the sub-base or sub-grade

DCP testing results are expressed in terms of the penetration index (PI), which is defined is the downward vertical movement of the DCP cone produced by one drop of the sliding hammer (mm per blow). Stiffer or stronger soils require a higher number of blows or drops of the hammer to achieve a given penetration.

All the pertinent location data, the number of blows and depth readings, and PI values are recorded on the DCP Test form shown in Table A-1.

Test results are typically processed using a spreadsheet. Data for the first two columns (blow number and depth of penetration) in Table A-1 are directly recorded in the field. The third column is an average of the present and previous depth readings. By averaging the readings, the strength of a soil layer between DCP readings is represented by a uniform PI located at the midpoint of the layer. The fourth column is the PI, which is calculated by dividing the difference in the present and previous DCP depth readings by the number of hammer blows between these readings.

A4. Data analyses

Once the results are processed, a graph of penetration index (column 4) versus penetration below the surface (column 3) in Table A-1 can be prepared. This graph will clearly show a profile of layers with different strengths. Alternatively, a plot can be prepared by using the number of blows (column 1) along the x-axis and the penetration reading (column 2 or 3) along the y-axis. Depending on the pavement structure, this plot is divided into "best fit" straight lines. The ratio between the change in penetration and the change in the number of blows for each straight line is then computed and expressed as mm/blow or PI as shown in Figure A-2.

To determine strength and understand material characteristics, the PI value can be correlated with field CBR values. A number of correlations between DCP and field CBR values have been developed in the past few decades and used in many places with varied successes. Some of these correlations are given in Figure A-3.



Table A-1 DCP data recording form (sample only)

Project Name: Client:

Project No.: Consultant:

Chainage (Km): Easting:

Location: Layers removed

Position from center line: Right or Left Northing:

Offset (m): Surface type:

Lane number: Surface condition:

Direction: Surface thickness:

Zero error: Test date:

Tested by: Approved by:

Number of blows Depth (mm) Mid-range depth (mm) PI (mm/blow)

0 0.0 -- 0

1 1.8 0.9 1.8

2 3.9 2.9 2.1

3 5.1 4.5 1.2

5 6.5 5.8 0.7

6 7.2 6.9 0.7

Figure A-2: An illustration of the DCP test result interpretation



A5. Frequency of testing

Sampling frequency will depend on the objective of the testing. Table A-2 gives recommended minimum distances between the DCP tests.

Table A-2 Recommended DCP test spacing

Objective Minimum test spacing

Routine testing for the rehabilitation 500 m or less

Upgrading of gravel roads to sealed roads 500 m or less

Areas of distress and spot improvement 100 m or at each distressed location

Pavement condition survey (new road) 50 m or increase frequency as needed

Delineating uniform sections during design Minimum 3 per homogenous section



Figure A-3. Correlations between DCP results and field CBR values

A6. Repeatability

The DCP has a relatively high degree of repeatability, with a coefficient of variation (CV) in the order of 40%. Should the rod leave its vertical alignment, no attempt should be made to correct this, as contact between the bottom rod and the sides of the hole leads to erroneous results. It is recommended that if the rod is deflected for various reasons, a second test in the same vicinity should be completed.

A7. Sources of error

When used on base course material, the DCP may produce high and sometimes misleading results. This is because the type, size and compaction of the granular particles affect the



penetration. Generally, while the DCP can be driven through asphalt and base course, it is recommended that the results from these materials should be supported by data from other methods of investigation. It should also be noted that the results can easily become unrealistic if the DCP encountered a rock or debris during a test (one or two points with near zero penetration index).

Appendix B Site Investigation Manual – 2013 Systems of Rock & Discontinuity Description

Ethiopian Roads Authority Page B-1

Appenidix B SYSTEMS OF ROCK MATERIALS AND DISCONTINUITY DESCRIPTION

Table B-1 Rock material description Descriptions should follow the "color, grain size, texture, weathering, strength, type" form. Example: Dark bluish grey, fine-grained, crystalline, slightly weathered, moderately strong basalt.

Color Grain size Texture/ Fabric Primary

shade Secondary Type Particle size

Retained on sieve size

Equivalent soil grade

Light/ Dark

Pinkish Pink Very coarse 60 mm Coarse gravels,

cobbles, boulders Crystalline Reddish Red Coarse 2 - 60 mm No. 8 Gravel Yellowish Yellow Medium 60 µ - 2 mm No. 200 Sand Brownish Brown Fine 2 - 60µ Silt

Granular Olive Olive Very fine < 2 µ Clay Greenish Green

Note: grains > 60µ are visible to the naked eye. Bluish Blue

Glassy White Greyish Grey

Black

Rock strength

Term Grade Unconfined

compressive strength (MPa)

Field description

Extremely strong rock R6 > 250 Specimen can only be chipped with a geological hammer.

Very strong rock R5 100 - 250 Specimen requires many blows of a geological hammer

to fracture it.

Strong rock R4 50 - 100 Specimen requires more than one blow of geological hammer to fracture it.

Medium strong rock R3 25 - 50

Cannot be scraped or peeled with a pocket knife, specimen can be fractured with single firm blow of geological hammer.

Weak rock R2 5 - 25 Can be peeled by a pocket knife with difficulty, shallow indentations made by firm blow with point of geological hammer.

Very weak rock R1 1 - 5 Crumbles under firm blows with point of geological hammer, can be peeled by a pocket knife.

Extremely weak rock R0 0.25 - 1 Indented by thumbnail.

Hard clay S6 > 0.50 Indented with difficulty by thumbnail. Very stiff clay S5 0.25 - 0.50 Readily indented by thumbnail.

Stiffclay S4 0.10 - 0.25 Readily indented by thumb but penetrated only with great effort.

Firm clay S3 0.05 - 0.10 Can be penetrated several meters by thumb with moderate effort.

Soft clay S2 0.025 - 0.05 Easily penetrated several meters by thumb. Very soft clay S1 < 0.025 Easily penetrated several meters by fist.

Appendix B Systems of Rock & Discontinuity Description Site Investigation Manual – 2013

Page B-2 Ethiopian Roads Authority

Table B-2 Discontinuity description in a rock mass Discontinuity description should include type, number of sets, location, orientation (dip/dip direction), fracture spacing, separation of fracture surfaces, infilling, persistence (continuous length) and surface roughness and shape. Example: "Columnar jointed with vertical columns and one set of horizontal joints, spacing of vertical joints is very wide, spacing of horizontal joints is wide, joints lengths are 3 to 5 m vertically and 0.5 to 1 m horizontally; joint aperture is open and the fracture infilling is a very soft clay. The vertical columnar joints are smooth, while the horizontal joints are very rough".

Type Spacing Orientation Persistence

Joint Fault Cleavage plane Bedding plane Schistocity plane Weakness zone Fissure Tension crack Foliation

Extremely wide > 6 m

Dip, dip direction and trend of lineation expressed as degrees

Very low <1 m

Very wide 2 - 6 m Low 1 - 3m

Wide 600 - 2 m Medium 3 - 10m

Moderate 200 - 600 mm High 10 - 20 m Close 60 - 200 mm

Very close 20 - 60 mm Very high > 20m Extremely close < 20 mm

Block Size Aperture

Term Block size Equivalent

discontinuity spacing in blocky rock

Size Term Feature

Very large > 8 m3 Very wide to

extremely wide < 0.1 mm Very tight

Closed 0.1 - 0.25 mm Tight

Large 0.2 - 8 m3 Wide 0.25 - 0.5 mm Partly open 0.5 - 0.25 mm Open

Gapped Medium 0.008 - 0.2 m3 Moderate 2.5 - 10 mm Moderately wide

Small 0.0002 - 0.008 m3 Close > 10 mm Wide 1 - 10 cm Very wide

Open Very small < 0.0002 m3 Less than close

10 - 100 cm Extremely wide > 1 m Cavernous

Form Roughness JRC

Planar Polished Shiny smooth and slippery in all directions 0 - 2

Slickensided Polished in one direction and showing evidence of significant movement 2 - 4

Undulating Smooth Smooth to the touch 4 - 10

Slightly rough Asperities on the fracture surfaces are visible and can be distinctly felt 10 - 12

Stepped Medium rough Asperities are clearly visible and fracture

surface feels abrasive 12 - 16

Rough Large angular asperities can be seen and distinctly felt 16 - 20

Irregular Very rough Highly irregular jagged surfaces > 20

Defined ridges Supplemental - used with above terms JRC = Joint roughness coefficient Small steps Supplemental used with above terms

Appendix C Site Investigation Manual – 2013 Summary of Geotechnical Needs and Testing Considerations

Ethiopian Roads Authority Page C-1

Appenidix C SUMMARY OF GEOTECHNICAL NEEDS AND TESTING CONSIDERATIONS

Table C-1 Summary of geotechnical information needs and testing considerations for pavement design related issues Geotechnical

issue Engineering needs Required information Field tests Laboratory tests

Cut slopes

Internal stability External stability Bottom heave Softening/progressive

failure Dewatering Lateral earth pressure Pore pressures behind

wall Retaining walls Down-drag on wall

Subsurface soil profile (soil, ground water, rock) Shear strength of soil Shrink/swell properties Horizontal earth pressure coefficients Interface shear strength (soil and reinforcement) Hydraulic conductivity Geologic mapping including orientation Characteristics of rock discontinuities

Test cut to evaluate stand-time

Geotechnical instrumentation

SPT, CPT and DCP tests Vane shear Pullout tests (anchors,

nails) Geophysical testing

Triaxial tests Direct shear Grain size distribution Atterberg limits Hydraulic conductivity Moisture content Unit weight Slake durability Rock uniaxial compression test Intact rock modulus Point load strength test

Fill slopes

Internal stability External stability Settlement Horizontal deformation Lateral earth pressures Bearing capacity Pore pressures behind

wall Borrow source

evaluation Reinforcement

Subsurface profile (soil, ground water, rock) Horizontal earth pressure coefficients Interface shear strengths foundation soil/wall fill shear

strengths Compressibility parameters (including consolidation

shrink/swell potential, and elastic modulus) Chemical composition of fill/ foundation soils Hydraulic conductivity Time-rate consolidation parameters

SPT, CPT, DCP tests Dilatometer Vane shear Geotechnical monitoring

instruments Test fill Nuclear density Pullout tests for

supported fills Geophysical testing

1-D Oedometer Triaxial tests Direct shear tests Grain size distribution Atterberg limits pH, chemical tests Moisture content Organic content Moisture-density relationships Hydraulic conductivity Unit weight Shrink/swell

Shallow foundations

Bearing capacity Settlement (magnitude

& rate) Shrink/swell of

foundation soils (natural soils or embankment fill)

Chemical compatibility

Subsurface profile (soil, groundwater, rock) Shear strength parameters Compressibility parameters (including consolidation,

shrink/swell potential, and elastic modulus) Stress history (present and past vertical effective

stresses) Depth of seasonal moisture change

Vane shear test SPT (granular soils),

CPT, dilatometer Rock coring (RQD) Nuclear density Plate load testing Geophysical testing

1-D Oedometer tests Direct shear tests Triaxial tests Grain size distribution Atterberg limits Moisture content Unit weight

Appendix C Summary of Geotechnical Needs and Testing Considerations Site Investigation Manual – 2013

Page C-2 Ethiopian Roads Authority

Table C-1 Summary of geotechnical information needs and testing considerations for pavement design related issues Geotechnical

issue Engineering needs Required information Field tests Laboratory tests

of soil and concrete Frost heave Scour Extreme loading

Unit weights Geologic mapping

Organic content Collapse/swell potential tests Rock uniaxial compression test Intact rock modulus

Driven pile foundations

Pile end-bearing Pile skin friction Settlement Down-drag on pile Lateral earth pressures Driveability Presence of boulders/

very hard layers Scour Vibration/heave damage

to nearby structures Extreme loading

Subsurface profile Shear strength parameters Horizontal earth pressure coefficients Interface friction parameters (soil and pile) Compressibility parameters Chemical composition of soil/rock Unit weights presence of shrink/swell soils (limits skin

friction) Geologic mapping including orientation and

characteristics of rock discontinuities

• SPT, CPT, dilatometer • Pile load test • Vane shear test • Piezometers • Rock coring (RQD) • Geophysical testing

• Triaxial tests • Interface friction tests • Grain size distribution • 1-D Oedometer tests • pH, resistivity tests • Atterberg Limits • Organic content • Moisture content • Unit weight • Collapse/swell potential tests • Slake durability • Rock uniaxial compression test • Intact rock modulus • Point load strength test

Drilled shaft foundations

• Shaft end bearing • Shaft skin friction • Constructability • Down-drag on shaft • Quality of rock socket • Lateral earth pressures • Settlement (magnitude

& rate) • Groundwater seepage/

dewatering • Presence of boulders or

very hard layers • Scour • Extreme loading

• Subsurface profile (soil, ground water, rock) • Shear strength parameters • Interface shear strength friction parameters (soil and

shaft) • Compressibility parameters • Horizontal earth pressure coefficients • Chemical composition of soil/rock • Unit weights • Permeability of water-bearing soils • Presence of artesian conditions • Presence of shrink/swell soils (limits skin friction) • Geologic mapping including orientation and

characteristics of rock discontinuities • Degradation of soft rock

• Technique shaft • Shaft load test • Vane shear test • SPT, dilatometer, CPT • Piezometers • Rock coring (RQD) • Geophysical testing

• 1-D Oedometer • Triaxial tests • Grain size distribution interface

friction tests • pH, resistivity tests • Permeability tests • Atterberg Limits • Moisture content • Unit weight • Organic content • Collapse/swell potential tests • Rock uniaxial compression test • Intact rock modulus • Point load strength test

Slake durability

Appendix D Site Investigation Manual – 2013 Common Soil Laboratory Tests

Ethiopian Roads Authority Page D-1

Appenidix D COMMON SOIL LABORATORY TESTS

Table D-1 Soil laboratory tests commonly used in pavement design Engineering Parameter Description Use in pavement design AASHTO /ASTM

designation

Moisture content The moisture content expresses the amount of water present in a quantity of soil.

Calculation of soil total unit weight, void ratio, and other volumetric properties,

Correlations with soil behavior, other soil properties.

AASHTO T 265 or ASTM D 2216 (conventional oven)

ASTM D 4643 (microwave)

ASTM D2922 (Nuclear gauge)

Density Density is the total weight divided by total volume for a soil sample.

Calculation of in-situ stresses, Correlations with soil behavior, other soil

properties, Compaction control.

ASTM D2922 (Nuclear gauge)

ASTM D1556 (Sand cone)

Specific gravity The specific gravity of soil solids is the ratio of the weight of a given volume of soil solids at a given temperature to the weight of an equal volume of distilled water at that temperature.

Calculation of soil unit weight, void ratio, and other volumetric properties,

Analysis of hydrometer test for particle distribution of fine-grained soils.

AASHTO T 100 or ASTM D 854

Compaction characteristics

Compaction characteristics are expressed as the equivalent dry unit weight versus moisture content relationship for a soil at a given compaction energy level. Of particular interest are the maximum equivalent dry unit weight and corresponding optimum moisture content at a given compaction energy level.

In conjunction with other tests (e.g., resilient modulus), determines influence of soil density on engineering properties,

Field QC/QA for compaction of natural sub-grade, placed sub-base and base layers, and embankment fills.

AASHTO T 99 or ASTM D 698 (Standard Proctor)

AASHTO T 180 or ASTM D 1557 (Modified Proctor)

Gradation The grain size distribution is the percentage of soil finer than a given size versus grain size.

Soil classification, Correlations with other engineering properties.


Plasticity Plasticity describes the response of a soil to changes in moisture content quantified by Atterberg limits.

Soil classification, Correlations with other engineering properties.

AASHTO T89 or ASTM D 4318 (liquid limit)

AASHTO T90 or ASTM D 4318 (plastic limit)

AASHTO T 92 or ASTM D 427 (shrinkage limit)

Swelling potential

Swelling is a large change in soil volume induced by changes in moisture content.

Swelling sub-grade soils can have a seriously detrimental effect on pavement performance.


Appendix D Common Soil Laboratory Tests Site Investigation Manual – 2013

Page D-2 Ethiopian Roads Authority

Table D-1 Soil laboratory tests commonly used in pavement design Engineering Parameter Description Use in pavement design AASHTO /ASTM

designation Swelling soils must be identified so that they can be either removed, stabilized, or accounted for.

Collapse potential

Collapsible soils exhibit large decreases in strength at moisture contents approaching saturation, resulting in a collapse of the soil skeleton and large decreases in soil volume.

Collapsible sub-grade soils can have a seriously detrimental effect on pavement performance. ASTM D 5333

CBR The California Bearing Ratio or CBR is an indirect measure of soil strength based on resistance to penetration.

Direct input to some empirical pavement design methods,

Correlations with resilient modulus and other engineering properties

AASTHO T 193 or ASTM D 1883

Resilient modulus

The resilient modulus (Mr) is the elastic unloading modulus after many cycles of cyclic loading. In-situ resilient modulus values can be estimated from back-calculation of falling weight deflectometer (FWD) test results or correlations with DCP values

Characterization of sub-grade stiffness for flexible and rigid pavements,

Determination of structural layer coefficients in flexible pavements.

AASHTO 1986/1993

Appendix D Site Investigation Manual – 2013 Common Soil Laboratory Tests

Ethiopian Roads Authority Page D-3

Table D-2 The AASHTO or ASTM designation of common soil laboratory tests

Test Category Name of Test Test Designation

AASHTO ASTM

Visual identification

Practice for Description and Identification of Soils (Visual or Manual Procedure) - D 2488

Index properties

Test Method for Determination of Water (Moisture) Content of Soil by Direct Heating T 265 D 2216

Test Method for Specific Gravity of Soils T 100 D854 D5550

Method for Particle-Size Analysis of Soils T 88 D 422 Test Method for Classification of Soils for Engineering Purposes M 145 D 2487

D 3282 Test Method for Amount of Material in Soils Finer than the No. 200 (0.075 mm) Sieve - D 1140

Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils

T 89 T 90 D 4318

Compaction

Test Method for Laboratory Compaction of Soil Using Standard Effort (2.5 kg Hammer for 300 mm height) T 99 D 698

Test Method for Laboratory Compaction of Soil Using Modified Effort (4.5kg Hammer for 450 mm height) T 180 D 1557

Strength

Test Method for Unconfined Compressive Strength of Cohesive Soil T 208 D 2166

Test Method for Unconsolidated, Undrained Compressive Strength of Cohesive Soils in Triaxial Compression T 296 D 2850

Test Method for Consolidated, Undrained Compressive Strength of Cohesive Soils in Triaxial Compression T 297 D 4767

Method for Direct Shear Test of Soils under Consolidated Drained Conditions T 236 D 3080

Test Method for CBR (California Bearing Ratio) of Laboratory-Compacted Soils - D 1883

Test Method for Resilient Modulus of Soils T 294 -

Consolidation, Swelling, Collapse properties

Test Method for One-Dimensional Consolidation Properties of Soils T 216 D 2435

Test Methods for One-Dimensional Swell or Settlement Potential of Cohesive Soils T 258 D 4546

Test Method for Measurement of Collapse Potential of Soils - D 5333

Permeability Test Method for Permeability of Granular Soils (Constant Head) T 215 D 2434

Corrosivity (Electro-chemical)

Test Method for pH of Soils - D 4972 Test Method for Sulfate Content T 290 D 4230 Test Method for Chloride Content T 291 D 512

Organic content Test Methods for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils T 194 D 2974