TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment

TRRL LABORATORY REPORT 740

A GUIDE TO SITE INVESTIGATION PROCEDURES FOR TUNNELS

by

M J Dumbleton, BSc, PhD, FGS

and

G West, FGS

Any views expressed in this report are not necessarily those of the Department of the Environment

Tunnels Division Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1976

ISSN 0305-1293

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on ! st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Objectives and stages of the site investigation

2.1 Objectives

2.2 Planning and stages of the site investigation

3. Site investigation Stage I: Preliminary appreciation of site and ground conditions

3.1 Study of available information

3.2 Geological and engineering enquiries

3.2.1 Consultation with the Institute of Geological Sciences and other geologists

3.2.2 Discussion with Water Authorities and hydrologists

3.2.3 Ground information from existing excavations and other works

3.2.4 Location of underground services and structures

3.2.5 Location of mines, shafts and pits

3.3 Air photographs and surface reconnaissance

3.3.1 Use of air photographs

3.3.2 Surface reconnaissance

3.4 Preliminary appreciation report

3.4.1 Legal authority or statutory approval

4. Site investigation Stage II: Ground investigation before construction

4.1 Preliminary ground investigation

4.1.1 Preliminary ground investigation report

4.2 Main ground investigation

4.2.1 General

4.2.2 Site investigation at portals

4.2.3 Contract documents

4.2.4 Direction and supervision on site

4.2.5 Boreholes

4.2.6 Trial pits

4.2.7 Large diameter boreholes, shafts and headings

4.2.8 Sampling, description, logging and testing

4.2.9 In situ testing

4.2.10 The interpretation of test results

4.2.11 Ground water

4.3 Other investigations before construction

4.3.1 Underground works and services

4.3.2 Investigation of mines and shafts

4.3.3 Geophysical investigations

4.3.4 Other investigations and trials

4.4 Main ground investigation report

4.4.1 The factual report

4.4.2 Interpretation of the site investigation

4.4.3 Information to be given to the contractor for construction

'4.4.4 Deposition of copies of reports

4.5 Design statement and construction brief

Page

1

1

2

2

2

3

3

6

6

6

6

6

6

7

7

7

7

8

8

8

8

8

8

10

10

10

10

11

11

12

13

13

13

14

14

14

15

15

15

15

15

16

16

16

5. Site investigation Stage III: Ground investigation during construction

5.1 Observations on ground conditions

5.2 Probing ahead

5.3 Other investigations and trials during construction

5.4 Review and amendment of plans and sections

6. Post-construction reports and records of ground conditions and construction experience

7. Conclusion

8. Acknowledgements

9. References

10. Appendix: Site investigation in mining areas

10.1 Study of available information

10.2 Consultation with the National Coal Board

17

17

18

18

18

19

19

20

20

23

23

23

© CROWN COPYRIGHT 1976

Extracts f rom the text may be reproduced, except for commercial

purposes, provided the source is acknowledged

GUIDE TO SITE INVESTIGATION PROCEDURE FOR TUNNELS

ABSTRACT

The report is a general guide to site investigation for bored and cut-and-cover

tunnels and associated shafts and portals, constructed for sewers, aqueducts,

transport and other services. It outlines the objectives of a site investigation

for a tunnel and reviews the steps required in carrying out and reporting the

investigation, the methods that may be used, and the considerations that

must be borne in mind.

Emphasis is placed on the importance of good core recovery and the

full description of strata, the thorough examination of ground water

conditions, and the value of in situ tests and trials. The value of a staged

approach to the investigation, and the importance of full reporting of each

stage of the work with full communication of results and conclusions to all

interested parties, are also stressed.

1. INTRODUCTION

This Report is a general guide to site investigation procedures for bored and cut-and-cover tunnels and associated

shafts and portals, constructed for sewers, aqueducts, transport and other services. The Report also applies

generally to immersed-tube tunnels, for which, however, additional factors may also require consideration (eg

settlement in soft deposits, effect of scour and changing channel patterns, berthing and navigational conditions

and restrictions during construction).

For more detailed treatment of each aspect of the subject reference will be required to other publications,

which will be referred to in the appropriate places. British Standard Code of Practice CP 20011 should be used

as a guide to general site investigation practice. Much of the guidance on planning, directing and reporting site

investigations for roadworks given in TRRL Report LR 6252 is also applicable to site investigations for tunnels.

That Report should be read alongside the present Report, and should be referred to for detailed references that

will not generally be repeated here. Some references specific to site investigation for tunnels may also be

consulted 3,4,5.

For particular projects any specific requirements of the Promotor should also be noted. For instance,

for road works carried out for the Department of the Environment, the Techni6al Memorandum on site

investigation for highway tunnels should be referred to and Technical Memorandum No. Hl l /70

"Site Investigation Procedure" for associated approach roads. Technical Memorandum (Highways) H3/76,

"Model Contract Document for Site Investigation", introduces the "Model Contract Document for Site

Investigation Contracts for Highways" (Engineering Intelligence Division, DOE, Feb. 1976); although this

refers to road projects, it can serve as a basis for contracts for tunnels. These Memoranda are obtainable from

Highways Manual Branch, Department of the Environment, 2 Marsham Streetl London SW1P 3 EB. Queries

relating to other DOE requirements for road tunnels shouldbe addressed to: Tunnels Engineering Branch,

Department of the Environment, St Christopher House, Southwark Street, London SE1 0TE.

It is essential that an engineer with geotechnical knowledge or an engineering geologist, with experience

of the requirements of the design and construction of tunnels, should be associated with the project, and very

desirable that he should maintain continuity through all stages from the :preliminary appreciation to final

completion, which may cover a period of several years. It may also be necessary to commission specialist

professional advice on particular aspects of the investigation, such as interpretation of geological structure or

stratigraphy, analysis of ground water conditions or use of geophysical methods of investigation.

2. OBJECTIVES AND STAGES OF THE SITE INVESTIGATION

2.1 Objectives

The objectives of the investigation are to provide information for the assessment of the technical and

economic merits of alternative schemes, the selection of the most suitable alternative, the preparation of an

adequate and economic design, and to allow difficulties that may arise during construction to be foreseen and provided for.

Tile cost of a tunnel can vary widely depending upon ground conditions and, in urban areas, on

constraints such as the types of structures and foundations in the vicinity, and the principal objective at the

outset should be to establish whether a tunnel scheme is a technically and economically feasible solution to

the engineering problem in the prevailing ground conditions. It is important to bear in mind that if a tunnel is

proposed for part of a scheme such as a road it is likely to be a dominant cost element in the scheme, and in

many instances will account for a substantial proportion of the total cost. Because of this it is essential to locate

and consider the most economical sites for the tunnel even if they are not on the ideal aligmnaent; the saving in

cost by doing this may well outweigh the extra cost of a less direct alignment. A favourable line and level for

the tunnel must be selected, and also favourable positions for shafts or portals (see 4.2.2).

To allow these objectives to be achieved, the site investigation must establish in three dimensions the

geological structure, the succession and character of the strata present on the site, the ground water conditions

and the presence of any special hazards. This involves examining existing information, making field investigations,

taking samples and carrying out in situ tests as needed, and includes a programme of laboratory tests to obtain

the required information on the properties of the site materials. The investigation must also locate any existing underground works and services.

2.2 Planning and stages of the .site investigation

An effective site investigation is best achieved by carrying out the work in the broad stages shown in

Table 1, each item of which is dealt with in more detail later in the text. Each successive step aims to narrow

down the choice towards the best line and level for the tunnel route, to indicate the next steps required in the

investigation, to fill gaps in the existing knowledge of the site, or to confirm or correct earlier predictions. In

practice each stage may not be sharply divided from that following it, and more (or occasionally fewer) stages

may be involved. A rigidly prescribed programme should not be followed; the philosophy of the planning and execution of the site investigation should be:

l) to decide what information to look for, from a sound appreciation of the geotechnical needs

of the project coupled with an understanding of the general geology and character of the area

and a growing more detailed knowledge gained from the progress of the investigation.

2) to design the site investigation to provide this information using the most suitable methods,

and to be alert to variations that may be required in the programme so as to be able to modify

it promptly when necessary.

Glossop's comments on this matter 6 (see 7) are very relevant, and Muir Wood also gives a set of rules as a guide

to the conduct of site investigations for tunnels 3.

Each stage of the investigation should be followed by a report, along the lines indicated in Table 2,

summarizing the work done and the conclusions arrived at and making proposals for the next stage. In

addition to providing a permanent record, the preparation of a report is often the best way of effecting a

systematic appreciation of the subject and of formulating future requirements.

3. SITE I N V E S T I G A T I O N STAGE I: PRELIMINARY APPRECIATION OF SITE AND GROUND CONDITIONS

The aim of the preliminary appreciation of site and ground conditions is to examine the existing and more

readily available information in order to assess feasibility, to select possible routes, to make preliminary

estimates of cost, and to plan more detailed investigations. The zone of interest initially examined should

include all locations where the construction of the works might prove practicable.

The methods used for preliminary site appreciation may also be useful at later stages of the site

investigation and during construction.

3.1 Study of available information

Preliminary information on ground conditions at the site of a projected tunnel is obtainable from

geological maps and memoirs issued by the Institute of Geological Sciences (IGS) 7,8. From these and other

sources, sufficient information can generally be obtained to plot the approximate geological structure on a

longitudinal section of a tunnel alignment. In addition to published information the IGS maintains records of

strata encountered in boreholes, shafts etc and these can be inspected by arrangement at the IGS libraries in

London, Leeds, Exeter, Edinburgh and Belfast. Universities which have departments of geology or mining may

have specialised knowledge of ground conditions in their areas and are therefore another possible source of

information. It must be remembered that existing maps will not have been prepared with the objectives and

needs of the tunnelling project in mind, and the type and accuracy of the information presented may not be

adequate and may need confirming or supplementing by further work. Information on underground services

and mine workings should also be sought (see 3.2.4 and 3.2.5).

A comprehensive guide to the most important sources of preliminary site information has been given in

TRRL Report LR 403 (revised 1976) 8. References given in that Report will not generally be repeated here.

TABLE 1

Stages of site investigation - and activities that may be included in them

STAGE I:

STAGE II:

STAGE II1:

PRELIMINARY APPRECIATION OF SITE AND GROUND CONDITIONS

Examination of existing and more readily available information to assess feasibility at first

sight, to select possible routes, to make preliminary estimates of cost, and to plan more

detailed investigations.

a) Available information

- including geological and other maps and reports

b) Geological and engineering enquiries

c) Air photographs and surface reconnaissance

d) Interpretation and recommendations for next stage of investigation

GROUND INVESTIGATION BEFORE CONSTRUCTION

a) Preliminary ground investigation

- an amount of work, where required, sufficient to confirm the feasibility and to

establish the approximate cost of the project, to narrow route options, and to aid

in the planning of the main ground investigation; it may include selected boreholes

or open excavations, perhaps a geophysical investigation, and selected tests

b) Main ground investigation

- to obtain the information required for the final alignment, design and construction

of the tunnel. It will usually include a programme of in situ and laboratory tests

c) Other investigations before construction

- eg geophysical surveys, trial adits, trial shafts, grouting trials, dewatering trials, rock

bolt trials, monitoring of experimental sections, recording the condition of

buildings

d) Interpretation and recommendations for ground investigation during construction

GROUND INVESTIGATION DURING CONSTRUCTION

Observation, and investigation where necessary, continued during the construction phase to.

confirm and supplement the earlier investigations.

a) Observations on ground conditions during construction

- these should always be made

b) Probing ahead in tunnels

c) Other investigations during construction

- eg extra boreholes, observation of ground movement and settlement, grouting

trials, rock bolt trials, monitoring of experimental sections

d) Review and amendment of plans and sections

- this is a continuous process throughout the work

4

TABLE 2

Site investigation reports

lI:

III:

PRELIMINARY APPRECIATION REPORT

The report should:

a) summarise work done and list sources of information located

b) give preliminary appreciation of site and ground conditions in relation to the project

c) make recommendations for the next stage of the investigation.

Part of this work may be incorporated into a Feasibility Report or application for Parliamentary

or statutory approval, which may also include results from the Preliminary Ground Investigations.

GROUND INVESTIGATION REPORTS (PRE-CONSTRUCTION')

Each phase may have its own report, but where timing permits the information should all be

correlated in the report of the Main Ground Investigation. Recommendations should be

included for observations and investigations to be made during construction.

DESIGN STATEMENT AND CONSTRUCTION BRIEF

A separate document stating the assumed ground conditions on which the Engineer has based the

design, and indicating how ground conditions may affect construction procedures.

POST-CONSTRUCTION REPORTS AND RECORDS

a) Geotechnical Report

including - as-found records of geology and ground conditions

- investigations during construction, including probing ahead and monitoring of performance

b) Fuller information kept for general construction records

Further items complementing a) and b) but outside the scope of this report:

c) Record of construction experience, incidents and expedients

d) As-built records of the structure.

5

3.2 Geological and engineering enquiries

3.2.1 Consultation with the Institute of Geological Sciences and other geologists The feasibility of any proposed tunnel, in terms of geological conditions and the engineering problems they pose,

must be ascertained at the very outset of any investigation. The engineer and the geologist must work together

so that the geological features on the proposed route that have engineering importance can be identified. These

include faults, buried valleys, igneous intrusions, mineralization, aquifers, jointing and cambering (see 4.2.1).

Although unlikely in the UK, the possibility of seismic and tectonic disturbance, particularly in relation to

reactivated faults, will need to be considered where sensitive structures are involved, for example tunnels

associated with nuclear power stations.

The IGS should be consulted for all schemes, except perhaps very straightforward ones where the

conditions are well known. The IGS may also be able to provide continuing advice on geological conditions

through their local geologist on a fee basis throughout the project. This is to be recommended where possible

on large-scale projects where extensive and detailed local geological knowledge is essential.

3 .2 .2 Discussion with Water Authorities and hydrologists ]t may be necessary to discuss with the

Water Authority the possible effect of tunnelling on aquifers and any implications this may have on the site

investigation. The relation between tunnel capacity, stream flows and the incidence of flooding may need to be

considered, and monitoring programmes may need to be instituted.

3 .2 .3 Ground information from existing excavations and other works Excavations on or near

the site may have been made by Statutory Undertakers laying gas, water and electricity mains, or by local

authorities constructing sewers. Enquiries made of these bodies may produce useful information about ground

conditions. Nearby pits, quarries, mines, and cuttings and tunnels for roads and railways (including underground

railways) can also reveal soil and rock types and their structure and stability characteristics. (Rock exposures

will allow detailed study of joint patterns 7, which may be delayed until a later stage in the investigations.)

Similarly, there may be in the vicinity embankments, buildings or other structures having a settlement history

because of the presence of compressible or unstable soils. Experience of tunnelling in similar ground in areas

remote from the site may also be considered. It is therefore possible by inspection, search of records and enquiry

to gain, in many cases, considerable knowledge of the ground conditions of a site before any formal site

investigation is initiated. However, any conclusion reached should take account of the different behaviour of

the same material near the surface and at depth, particularly when subject to high water pressures.

3 .2 .4 Location of underground services and structures Copies of any plans and records showing

the location of existing underground works such as public utilities and services, pipes, cables, tunnels, wells,

cellars and foundations including piles should be obtained. This is particularly important for cut-and-cover tunnels,

but is also necessary for bored tunnels in order to assess the possible effects of any settlement on the existing

works, and for the siting of shafts and portals. It may be possible to locate underground services by geophysical

means 9. This work is not usually undertaken in great detail at this stage, but it may be critical in the choice of

the location of urban tunnels, especially shallow ones.

3.2.5 Location of mines, shafts and pits Maps, plans and other information on the location of mines

and open or infilled pits should be obtained (see Appendix). Mining record offices (see LR 403 (revised 1976) 8)

and Mining Consultants and Surveyors should be consulted, and local enquiries should be made, eg of the County

Archivist. Details of mining structures and methods of detecting shafts have been discussed 10, and Norman 11

describes the use of air photographs for detecting mining subsidence, which may have affected the ground

through which tunnelling is to be carried out. Continuing subsidence may affect the completed tunnel and may cause movements on existing faults.

The location of filled clay pits and other areas of fill is important because the fill, which may be unstable

or water-bearing, may be penetrated by shallow tunnelling. Industrial or domestic fills can also present special

hazards. Old Ordnance Survey maps may be useful here, but ancient unrecorded workings may occur wherever mineral resources occur at shallow depth.

3.3 Air photographs and surface reconnaissance

3.3.1 Use of air photographs Air photographs of the alignment will frequently be available (or may

be specially commissioned) and may be of assistance in studying the site and in locating places where special

attention may be necessary, including faults, mining subsidence, filled pits, instability, cambering and valley

bulging. They should be examined along with the study of geological and other maps and with surface reconnaissance on foot. Expert services are available for this work.

Sources of air photographs, methods of examining them and applications illustrated by stereo-pairs are

given in TRRL Report LR 36912, and Norman 11 has recently reviewed the detection of subsidence by air photographs, including that caused by mining.

Enquiries about existing air-photograph cover of an area should be addressed to: Central Register of Air

Photography, Department of the Environment, Whitehall, London SWl 3BL, or Scottish Development

Department, New St Andrew's House, St James Centre, Edinburgh EH 1 3SZ. Air survey companies may also

be approached direct, and may be able to provide the photographs required at short notice.

3.3.2 Surface reconnaissance At an early stage a thorough examination should be made of the strip of

ground above possible tunnel alignments. It is important to map evidence of the outcrop of strata and of the

position and dip of boundaries and faults, as these will help to improve the accuracy of the longitudinal section.

The surface and land form will also frequently reveal signs of sub-surface conditions that need special

investigation. Such signs include springs, swallow holes, changes in slope, broken or undulating ground on hill-

sides, subsidence and unexpected depressions. Distinctive changes in the appearance of crops and other

vegetation may indicate filled pits and quarries or other changes in subsoil conditions, and are often well seen

on air photographs. Local enquiries will frequently yield further useful information.

At the time of the surface reconnaissance note may be made of any buildings or other structures that

might be adversely affected by settlement or vibrations due to tunnelling operations. In due course a survey of

their physical condition, including photographs where appropriate, may be required before tunnelling

commences, for reference in the event of any movements noted during tunnelling operations.

3.4 Preliminary appreciation report

The main information obtained from the preliminary sources should be summarised in a report (see

Table 2). This should contain a provisional geological map of the site and provisional longitudinal section, with

notes on the likely range of characteristics of each bed, likely ground water conditions,and likely causes of

variation in relation to tectonic and other features. The probability of any special hazards should also be

considered. Block diagrams can be used to show the characteristics of the most important engineering

situations. The report should aim to establish if a tunnel is likely to be technically feasible at acceptable cost,

and if so to establish the alignment options and the likely controlling ground conditions and design options.

It should also indicate the lines on which the Ground Investigation should be planned, and point out where

special investigations or advice may be required.

3.4.1 Legal authority or statutory approval The preliminary appreciation, and any necessary

preliminary ground investigation (see 4.1), should be carried out sufficiently early to provide promptly the

information required for the preparation of any necessary application for legal authority or statutory approval

for the project.

4. SITE INVESTIGATION STAGE I1: GROUND INVESTIGATION BEFORE CONSTRUCTION

The methods available for ground investigation before construction include boreholes, trial pits, exploratory

shafts, adits and pilot tunnels, along with associated laboratory and in situ tests and trials, and geophysical

methods of exploration. At the same time the sources of information and methods of investigation described

in Section 3 should continue to be employed where appropriate.

4.1 Preliminary ground investigation

A preliminary ground investigation is usually necessary for a projected tunnel, although in some cases

enough information will be available from the preliminary appreciation to allow it to be dispensed with. The

preliminary ground investigation involves a limited amount of investigation using methods such as boreholes or

open excavation, possibly a geophysical exploration (see 4.3.3), and selected tests. It also allows selection or

confirmation of the preferred route and the approximate estimation of costs, and is needed to determine the

feasibility of the engineering work involved and to assess the nature and scope of the main site investigation,

for which it facilitates the preparation of a sound Specification and Bill of Quantities. It helps decisions to be

made on the frequency, location and types of boring, sampling and testing necessary. It should, in conjunction

with information gained from other sources, reveal any parts of the site which require particular attention in

the main site investigation, and help to locate where large diameter boreholes, shafts or headings may be

required to permit access for visual examination or for special tests related to design or construction (see 4.2.7).

4 .1.1 Preliminary ground investigation report The information obtained from the preliminary site

investigation should be presented in a report (see 4.4 for detailed recommendations on reporting). The report

should select within fairly close limits a provisional alignment for the proposed tunnel from the options

derived from the previous work (where more than one exists), or suggest a superior one. The report should

also give detailed recommendations for the procedures to be followed for the main site investigation, and

indicate where special methods or advice may be required.

4.2 Main ground investigation

4.2.1 General The main ground investigation should be planned in the light of the knowledge of site

conditions already gained and of the provisional line and depth of the tunnel and the position of shafts and

portals. The investigation has to establish the geological structure and succession and the character of the

8

strata present on the site. Description of the strata and both laboratory and in situ testing to quantify their

characteristics is one of the principal aims of the work.

Where problems are known or suspected to exist the appropriate measures to determine their character

and magnitude should be allowed for in the contract. In particular the extent, depth, properties and nature

of the following should be established:

a) interface between soil and rock

b) ground water conditions

c) unstable or caving ground

d) rock faults or shatter zones and fissured or heavily jointed rock

e) spacing and character ofjointing and bedding surfaces

f) weathered rock

g) soft soils, silt and peat

h) buried valleys and channels

i) bands of hard ground or boulders; igneous intrusions

j) underground works and services

k) old mine workings

1) any other adverse ground conditions or special hazards within the zone of interest of the

tunnel works

m) evidence where available of a degree of uniformity and limited range of ground properties

suggesting the practicability of using a tailor-made method of tunnelling.

To do this it may be necessary to vary the programme of work as the investigation proceeds.

The main site investigation should complete as far as possible the attainment of the objectives, given in

Section 2, of obtaining the information required for the location, design and construction of the tunnel, shafts

and portals, although it may be decided to obtain some of the information during the construction stage (see 5).

Information is required for making technical and economic decisions on the following:

a) fixing tunnel line and level

b) fixing shaft and portal positions

c) selecting method of excavation and construction

d) predicting stability and selecting method and timing of ground support

e) predicting and controlling water conditions

f) predicting overbreak in relation to possible methods of excavation

g) assessing the need for special expedients, eg compressed air, ground treatment.

The site investigation should be designed to obtain not only the information required for the design of the

tunnel, but also that required by the Contractor for planning and carrying out its construction.

9

4.2.2 Site investigation at portals Difficult ground conditions are often encountered at portals,

often before the Contractor's site management and workforce has gained experience of the character and

behaviour of the ground. The rock in a hill or valley side location is often affected by weathering, stress

relief or displacement. Cambering, valley bulging or unstable surface deposits may be present. Unloading by

excavation to prepare the portal face may result in instability, fracture or ground movements. Rock may be

heavily jointed or may contain open or filled fissures, or a major joint direction oriented parallel to a slope

surface may cause slabbing. In addition, irregular ground water conditions may be encountered.

Selection of portal positions and their investigation should therefore be done particularly carefully, and

a trial excavation or adit may be called for, but the area involved is usually limited and accessible. Problems

and remedial measures at portals are described by Craig and Brockman 13. Similar conditions can affect shafts

in hillside or valley locations.

4.2.3 Contract documents The Department of the Environment's Model Contract Document for Site

Investigation Contracts for Highways (see 1.2), suitably modified to meet the special needs of tunnels, may be

used as a general basis for the contract documents. In the Bill of Quantities items are required to cover both

continuous undisturbed core sampling during soft ground boring and continuous coring in rock, and where

the ground is such that difficulties of adequate core recovery are likely to occur alternative prices should be

provided for coring at larger diameters, and with double, and with triple tube core barrels. Because good core

recovery within the zone of interest of the tunnel is so important 100 per cent recovery is desirable, and a

minimum of 90 per cent recovery should be required unless it can be shown conclusively that this is

impracticable (see 4.2.8).

The Contractor may be required to provide facilities for authorised persons to examine exposures in

accessible excavations, to observe the recovery and extrusion of samples, and to examine samples, both on site

and in the laboratory.

If required, the Tunnels Engineering Branch DOE will advise on the appointment of suitable Site

Investigation Contractors for DOE contracts for road tunnels.

4.2.4 Direction and supervision on site Full-time site direction and supervision by an experienced

and competent civil engineer or engineering geologist is essential. This person should preferably be on the staff

of the Engineer, but if this is not possible he may be provided by the Site Investigation Contractor, by

agreement. Whatever the arrangement made, the site engineer must be thoroughly briefed on the objectives of

the investigation and on the expected and possible ground conditions so that he can appreciate where variations

in the programme are required. He may be given the authority to vary or extend the plan of work at his

discretion during the investigation within limits laid down by the Engineer. It may, however, be more satisfactory

to make these decisions away from the site, but this depends on the arrangements made for analysis and

appraisal of the results.

All site investigation, sampling and testing operations must be closely supervised and frequently inspected.

4.2.5 Boreholes Boreholes will usually be sited at each shaft position, and at key positions for the

interpretat!on of the geological structure, eg in the neighbourhood of faults. Other boreholes will be placed to

interpolate between these points, the separation depending on the anticipated complexity of the ground.

Further boreholes may need to be added later to provide missing information. It is important to take boreholes

10

to a generous depth, because changes in design often result in lowering of the level Of a tunnel, and because

the zone of influence of the tunnel may be extended by the nature of the ground at a greater depth.

Key boreholes should be taken to a considerably greater depth to check on the geological succession and

on the character of deeper strata, which otherwise might have an unanticipated effect on the tunnel. This

information can also be valuable for extrapolating to predict the stratigraphy and ground properties at tunnel level between boreholes.

At the sites of proposed access shafts to the tunnel a borehole should be sited on axis, and in some cases

boreholes around the shaft position may be needed to establish the geological structure. In the case of a bored

tunnel the rest of the boreholes should be sited on either side of the proposed line if possible, clear of the

ground that will be tunnelled through so as to avoid the possibility of their interfering with the subsequent

tunnel construction. However for cut-and-cover tunnels it may be preferable to site the boreholes within the

strip of ground to be excavated. It must be remembered that a three-dimensional understanding of the site is

required, and this may call for work off the alignment.

Boreholes need not be bored vertically. They are sometimes inclined or horizontal, the option depending

as much on topography as on geology. In the case of tunnels through jointed strata the boreholes may be

inclined towards the direction of the tunnel line so as to intersect the joints at a more satisfactory angle.

Careful drilling of inclined holes can yield valuable information on joint spacing, roughness, filling and direction,

and can also be used to intersect and examine faults, veins, dykes and old mine workings, etc.

Care should be taken to confirm the accuracy of borehole positions both on site and as marked on the

drawings, and to bear in mind the possibility of unintentional deviation from the vertical or other intended

orientation due to drilling or geological factors, especially in deep boreholes and those put down from floating

craft. For critical work borehole survey techniques should be used at intervals, where appropriate, to check the

inclination and azimuth of the boreholes.

Full records of boring methods and progress must be reported, as they can be very valuable for the interpretation of the results 14.

All boreholes and other penetrations must be backfilled both on account of hazards to the project

arising from possible shifts in alignment or errors in borehole positions and on account of the potential hazard

to future works in the area. This work is essential and must be done in such a manner as to prevent seepage of

water or air up or down the hole, unless the ground is of such a high permeability that it would be pointless to

do so. Grouting is usually the most suitable method.

4 .2 .6 Trial pits Trial pits can be very effective and economical for the investigation of surface layers, for

example at shaft positions and for shallow tunnels, and are particularly useful in variable or structured deposits

(Glossop 6 gives a good example from the investigation for Hinkley Point nuclear power station). They provide

many of the advantages of large diameter boreholes, shafts and headings (see 4.2.7), allowing direct examination

of the deposits, sampling and in situ testing. Adequate safety measures must be adopted if trial pits are entered by personnel 15,16,17

4.2.7 Large diameter boreholes, shafts and headings For deeper investigation at crucial positions,

exploration of the ground by large diameter boreholes, shafts or headings may be required, allowing access by

investigating personnel. Exposed surfaces in them should be adequately supported but provided with means

11

for inspecting the ground, and adequate safety measure must be observed 15,16,17.

These methods allow large scale in situ observation and testing of the ground to be carried out and large

undisturbed samples to be taken, and can provide vital information such as assessment of the stand-up time of

potentially unstable ground that may be impossible to obtain from normal boreholes. They will also enable

prospective tenderers for the construction of the works, tunnelling machine manufacturers, and others to

inspect the ground directly. A case history study of exploration shafts is given by Knights 18.

These methods should be considered where they are likely to provide information that could affect

overall costs significantly. However they are costly, and if ground is very variable or there is no prospect of a

tailor-made tunnelling system being employed they may not be justified.

4 . 2 .8 Sampling, description, logging and testing It is essential to have a complete and detailed

description of the ground, its lithology and structure as well as laboratory test results on selected samples.

For this purpose continuous undisturbed sampling should be undertaken whenever possible at shaft positions,

and throughout the zone of interest of the tunnel. The extent of the zone examined in greatest detail will

depend on the possible effects on the tunnel of the worst ground conditions that are likely to occur beyond

the limits of the examined zone. Most information is required within a zone extending two diameters above,

below and on either side of the tunnel; continuous undisturbed samples should normally be taken through

this zone, and in any case at least from half a diameter above to half a diameter below the proposed tunnel.

It must be remembered that the depth of the tunnel may be altered in the course of the preparation or

execution of the scheme, and that knowledge of the strata above or below the zone influencing the tunnel

directly can be valuable for extrapolating to predict ground conditions at tunnel level between boreholes.

Careful records of core recovery are essential, and every case of non-recovery or poor core quality

should be investigated thoroughly because it may indicate a condition of critical importance to the work.

A rate of recovery of at least 90 per cent should be required if at all practicable. Consideration might be given

to incentives to encourage good rotary core recovery, such as payment of a bonus to the drillers for good

recovery in difficult ground conditions. Payment for footage should be discouraged as being inimical to the

best standards of boring and coring. If cores of about 70 mm diameter drilled with a double tube, swivel

core-barrel and water flush give inadequate recovery in poor ground it may be necessary to use larger

diameter cores or a triple tube core barrel incorporating an inner liner that can be used for core handling and

storage. The value of the latter method has recently been referred to by Meigh 19. Cores should not be

allowed to dry out or freeze, and it is desirable to sheath some types of material in plastic or foil to keep them

in good condition.

Logging of cores must be done by suitably qualified staff, and the work should be coordinated with the

selection of samples suitable for testing and recorded alongside full details of boring methods and progress.

Guidance on logging is available 1,2,14. The degree of fracturing is of major importance in rocks; core quality

can be a guide to this, and should be recorded, for instance by using the RQD method 20'21 , along with other

information on fracture spacing, orientation and character.

Representative soft-ground cores should be split open longitudinally during logging, for detailed

examination and description of their structure. Cores often deteriorate rapidly after extrusion; and it may be

desirable to provide the prospective contractors and others concerned with the works with the opportunity to

examine the core at the time of extrusion. The exposed surface of cores (and also of accessible excavations)

may be photographed, preferably in colour, to provide a permanent visual record (Fig.l)~ Here it will be added

12

that photography, particularly in colour, can be used to advantage at all stages of site investigation to record

the character of samples and of exposures, for instance in existing cuts, in pits, shafts and headings, and in

working faces 22. The photographs can be invaluable to both design and contractor's engineers.

Where the nature of the ground makes it impossible to obtain undisturbed samples from boreholes,

disturbed samples should be taken for description and testing, and accessible exploratory investigation may be

required.

The usual classification tests should be carried out for soils (moisture content; liquid limit, plastic limit

and grading as appropriate). Other tests carried out should relate to the possible behaviour of the soil or rock

in relation to the proposed methods of tunnel construction, and to the design, stability and safety of the works

(see 4.2.1). For example, the requirements for pipe jacking have recently been stated 23. Large numbers of

laboratory tests may not be needed on samples of uniform strata, and may be of little value where stratum

changes are so rapid that detailed prediction of the ground character on the line of the tunnel is impossible.

Good soil and rock descriptions are always valuable, and often more useful than numerous soil tests.

4 .2 .9 ' In situ' testing In situ tests will often be needed to determine in situ ground properties when

undisturbed samples cannot be recovered, or when tests on samples cannot give the information required.

For example, in non-cohesive soils, in situ penetration tests can give a guide to possible ground settlement,

and in situ permeability tests a guide to the behaviour of ground water and to possible methods of ground

treatment.

4 .2 .10 The interpretation of test results The results of tests on small samples of soil are often not representative of the behaviour of the soil in the mass. For example, deposits of alluvial clays may contain fine

bands or partings of silt and sand so that the material as a whole would have a much higher permeability than

tests on small samples of the clay indicate 24'25. Again, strength tests on a fissured clay can give higher values

than apply to the mass if the samples tested contain no fissures, although sample disturbance may lead to lower

strengths being obtained 25'26'27. It may not be possible to overcome these problems except by testing larger

samples than are normally available from boreholes. Therefore whenever exploratory shafts and headings are

made, consideration should be given to taking large undisturbed samples or conducting in situ tests. The

possible effects of anisotropy also need to be borne in mind.

In very variable soils and rocks it is difficult to obtain samples and compile borehole logs that will be of

value in judging the behaviour and characteristics o f the ground. These formations often make interpolation

between boreholes difficult or impossible. If this interpolation can be made in more than one way, attention

should be drawn to the several possibilities and the likelihood of the more adverse possibilities should be

discussed. Uncertainty may of course indicate the need for further investigation.

It is essential to be able to relate the observed properties of the soil or rock to the engineering

characteristics of the ground they represent. If this cannot be done, trials such as those mentioned in 4.3.4

and 5.3 became of increasing importance.

4.2.1 1 Ground water The presence or absence of ground water will be one of the most critical factors

in tunnel construction, and every effort should be made during the site investigation to detect the presence

of water and to establish the prevailing flow characteristics. Ground water levels found during boring and shaft

sinking should be recorded together with the times at which they are measured. At each water strike it may be

necessary to suspend boring or sinking until all the measurements required have been taken, although depending

13

on circumstances and upon whether piezometers are to be installed it may be adequate to note depth of

striking water and to record the overnight levels. The nature of the ground-water condition should be clearly

established, eg whether perched water table, free water table, artesian or sub-artesian water table.

The depth of water table and its seasonal variation can be obtained using standpipe piezometers,

monitored over at least one year and preferably up to and beyond the commencement of tunnel construction.

The installation of standpipe piezometers of both upstanding and flush types is described in the Model Contract

Document for Site Investigation Contracts for Highways (see 1 above). Any exceptional rainfall conditions

need to be noted, and considered when interpreting ground water observations.

It may be necessary to measure ground water flow characteristics by carrying out pumping or other tests

in boreholes, and to examine the feasibility of dewatering or ground water lowering. The effect of these

processes on ground settlement and on neighbouring structures, especially when organic or other highly compressible soils are present, will need to be considered.

Appropriate sampling and chemical testing of ground water should be carried out. Corrosive or sulphate-

bearing soils and ground water may attack metal or concrete shaft or tunnel linings or drainage installations.

The problem may be particularly severe in ground underlying old industrial areas such as chemical or gas Works.

Oxidation of natural or fill materials on exposure to air can also produce aggressive or dangerous products.

Chemical tests may also be able to indicate the aquifer from which the ground water is derived, as in the case of

artesian water passing up through fissures into higher strata. /

4.3 Other investigations before construction

4.3.1 Underground works and services The existence of underground works and services in the zone

of interest should have been established during the preliminary appreciation (see 3.2.4). At the main ground

investigation stage their more exact locations should.be established if they are likely to interfere with or be

directly affected by the construction work or by any subsequent effects such as settlement or vibration. I f little

documentary evidence is available, it may be necessary to search for buried pipes and cables by geophysical

methods (see TRRL Supplementary Report 69UC for details of techniques and equipment 9) or by trenching,

and to search for and explore abandoned underground works by means of shafts and headings.

4 .3 .2 Inves t iga t ion o f mines and shaf t s Detailed borehole investigation may be needed to attempt to

locate abandoned workings. They should be carefully sited on the basis of available information on the possible

position of the workings, but they frequently fail in their purpose.

I f workings are penetrated , a borehole camera can be used to examine their condition. Boreholes may also

be needed to examine the nature and thickness of mine-shaft fillings. Workings may be entered for direct

exploration, either through existing openings or by means of shafts and headings made for the purpose. The

latter may also be used to determine the condition of workings and their filling prior to the construction stage

so that the method to be adopted for tunnelling through them can be formulated. Appropriate safety

precautions must be taken to detect the presence of mine gases 15,16,17 (which may also be present in unworked

Coal Measures deposits, in industrial or domestic fill material, and in ground contaminated by industrial wastes).

The exploration must also provide the information needed to enable the Engineer to decide whether the

workings should be exposed and backfilled, the method of stowage or grouting that should be adopted, or

measures needed to prevent escape of water or gases from old workings or broken strata. All workings located

14

should be shown on the plans and sections. In the case of coal workings, shafts and adits the National Coal

Board must be consulted about methods of investigation and treatment (see 10.2).

4 .3 .3 Geophysical investigations Geophysical methods can be of value in certain circumstances. For

a subaqueous tunnel, a marine seismic survey (eg sparker survey) can be of assistance in charting the river bed.

It may also be able to help in determining the bedrock surface, discovering if any buried channels are present,

deciding on the location of boreholes and interpolating the strata between them. Reflection profiling systems

having greater power of penetration generally give lower accuracy and local detail, and specific objectives must

be made clear if the desired results are to be achieved. Methods of geophysical investigation for use over land

are more difficult to apply effectively, and should be used with caution. The planning and effectiveness of

geophysical investigations are discussed in LR 6252 and LR 68028; SR 17129, Burton30 and Higginbottom 31

review the methods available and their capabilities. It is always essential to have adequate borehole control, and

a geophysical survey will nearly always be subsidiary to a broader investigation.

4 .3 .4 Other investigations and trials Other investigations that may be required include trials of con-

struction measures such as rock bolting and ground injection, and the recording of ground surface levels and

the condition of buildings over or near the line of the tunnel (see 3.3.2). They may also involve the monitoring

of experimental sections of tunnel construction to establish design criteria (see also 5).

4.4 Main ground investigation report

The following recommendations apply both to the report of the Main Ground Investigation and more

generally. Each part of the ground investigation may have its own report, but where timing permits the

information should all be correlated in the report of the Main Ground Investigation.

Site Investigation Reports should be prepared so that the factual information with the drawings of

inferred geological structure, and the interpretation and advice, are bound in separate parts. They should be

discussed in draft with the Engineer before finalization.

4.4.1 The factual report The factual report must include borehole log sheets, test results and plans and

sections showing the position of the boreholes, shafts and headings together with the strata and ground water

levels encountered in them, and any other data the Engineer has requested. The sections should indicate the

depths at which in situ tests have been made and samples have been taken; the test results should be shown

alongside where possible. A symbolic or colour code should be used to distinguish the different types of soils

and rocks, for which full descriptions, including geological formation name (taking geological advice if

necessary), should be given. The inferred geological structure should be shown, and the degree of confidence

that is considered can be placed in the various interpolations should be indicated.

The ratio of vertical to horizontal scale used on sections should not be excessively large. The use of a

large vertical scale exaggeration can lead to incorrect interpretation of the geology, and for this reason it may

be necessary to represent critical parts of the alignment on sections drawn with the same horizontal and

vertical scale. For other purposes vertical scale exaggerations varying from 10:1 to 4:1 are satisfactory, eg

horizontal scale of 1 : 1250 with vertical scale of 1 : 125, and horizontal scale of 1:500 with vertical scale of

1:125.

4.4.2 Interpretation of the site investigation The interpretation of the results of the site

investigation in engineering terms is the responsibility of the Engineer, who should propose solutions for any

15

problems that have been shown to exist on the site. When a particularly difficult problem is encountered that

is outside the experience of the Engineer the engagement of an expert on the subject concerned should be considered.

The contractor who carries out the site investigation should be required to provide an interpretation of

the probable geological structure, and may also be asked to comment on aspects of the engineering behaviour

of the strata along the tunnel alignment in addition to presenting the factual data. The contractor for the site

investigation should not be asked to comment on aspects of the tunnel design and on construction procedures

unless he has considerable experience of the particular problems of tunnel construction. The engineering

behaviour of the ground is often highly dependent upon the method of tunnelling and hence this and other

interpretations may well be better undertaken by the Engineer and his staff.

Whatever the means employed to obtain information and advice about a site the Engineer must remain

responsible for the interpretation used in his design.

The interpretative and advisory report should also indicate what observations, investigations and trials

should be carried out during construction (see 5).

4.4.3 Information to be given to the contractor for construction All rele.vant information on the

site and ground conditions should be disclosed to the contractor for construction. The limitations of inter-

polations and interpretations should be discussed, but no general disclaimer should be made. The factual

information and the drawings of inferred geology should normally be included in the contract documents and

made available to tenderers. Access should also be offered to cores, samples, photographs, and accessible

exposures. The interpretative and advisory parts of reports are not generally made available to contractors but

the Engineer's Design Statement and Construction Brief(see 4.5)stating the data, both factual and inferential,

upon which he based the design and any instructions or recommendations for construction procedures, should

be included in the contract documents and made available to tenderers.

The importance of communications between the Engineer and the Contractor is discussed in a report of

the Institution of Structural Engineers32; although this relates to deep basements it is also very relevant to tunnelling works.

During discussions at the tendering stage or after letting of the contract the Contractor may suggest that

further items of site investigation would be desirable, and these may be agreed to either at the Promotor's or the Contractor's expense.

4 . 4 .4 Deposition of copies of reports Copies of site investigation reports for all types of tunnelling

scheme, including in due course the Post-Construction Report (see 6), would be welcomed by the Transport

and Road Research Laboratory for information. Technical Memorandum No. H 11/70 and that on site

investigation for tunnels require copies of the reports for DOE highway schemes to be supplied to Engineering

Intelligence Division DOE, the Transport and Road Research Laboratory, and the Institute of Geological Sciences.

4.5 Design statement and construction brief

The Engineer should prepare a statement setting down the observed or inferred ground conditions and

properties on which the design of the tunnel is based (this is required for DOE road projects by Technical

Memorandum H 11/70, paragraph 19), and indicating the way in which ground and ground water conditions

16

may affect construction methods and procedures. The classification of the ground into zones for which the

likely ground and water conditions and construction and support methods are indicated can be very useful to

the Engineer and to the Contractor.

The design statement and construction brief should be made available to tenderers for the construction

contract.

5. SITE INVESTIGATION STAGE II1: GROUND INVESTIGATION DURING CONSTRUCTION

It is never possible beforehand to determine complete information on the strata that will be encountered by a

tunnel or fully to anticipate their behaviour, and for this reason provision must be made for observation and any

other necessary investigation during construction.

The 'observational method' reviewed by Peck 33 involves the modification of the design as necessary in

the light of ground conditions and structural behaviour during construction; the 'New Austrian Tunnelling

Method '34 for selecting tunnel lining systems is a good example of the use of this procedure. This involves the

monitoring of sections of construction, although if experimental trials can be carried out before the main

construction the financial consequences may be much more favourable (see 4.3.4).

Information may be collected during the construction phase by engineers, geologists or inspectors on

the Engineer's staff, and the Contractor for Construction may be instructed to provide certain information.

Under some circumstances the Site Investigation Contractor's staff who made the original investigation and

interpretation may be required to make further observations or investigations during construction, under the

direction of the Engineer, on a basis of payment that could be agreed at the time that the site investigation

contract is negotiated. The advice or services of other specialists may also be required.

5.1 Observations on ground conditions

It is essential that the predictions made on the ground structure and materials should be confirmed or

corrected as direct information becomes available during the sinking of shafts and the construction of the tunnel.

Inspectors may be instructed to make regular records of the geology of the face, although this work is usually

undertaken by engineers or geologists, and regular samples should be taken for measuring moisture content,

plasticity, strength, or other properties relevant to the work. Photographic records of the tunnel face can also

be made 22. Frequency and orientation of joints and rates of water inflow should be recorded. Evidence of

stratigraphical level should also be noted and any fossils retained; the expert advice of a stratigrapher or

palaeontologist may be required for their interpretation. For cricital work the services of a geologist or

engineering geologist may be required for detailed observation, recording and interpretation of ground

conditions in the tunnel. Methods of recording are discussed by Proctor 35.

The information collected can provide correlation against records of performance, and may be valuable

later in the drive to maintain optimum working methods based on the earlier experience and to anticipate

conditions ahead of the face. The records should be agreed promptly between the Engineer and the Contractor.

17

5.2 Probing ahead

Further information on ground conditions may be obtained during construction by probing ahead of

the tunnel face. This can give advance warning of hazards such as water-bearing fissures, buried channels or old

mine workings, and can provide advance information on changes in ground conditions. It can also be used to

carry out ground treatment ahead of the drive.

Probing ahead involves drilling short distances ahead of the tunnel face, say at each change of shift, or

longer distances, say at weekly intervals. The distance and direction probed and the pattern of the array used

should be related to the tunnel dimensions, the rate of advance, and the nature and size of the anticipated

hazards. Geophysical methods used either by themselves or in combination with drilling may be used in the

future 29, but considerable research and development remains to be done before this stage is reached. It may be

possible to obtain cores, but it is more usual to use non-coring methods which are faster but give less information.

A responsible observer should be present during probe drilling to record the geological and ground water

conditions encountered (nature and changes in the cuttings, cavities, colour and loss or gain of water return, rate

of advance etc), and to report any significant changes so that prompt action can be taken if necessary.

Probing ahead is usually adopted in potentially hazardous conditions to provide day-to-day warning of

conditions ahead of the face, but it also provides a means of refining and adding to the continuing site investigation.

5.3 Other investigations and trials during construction

If unexpected ground conditions are met with in the tunnel it may be necessary to sink a borehole ahead

of the face or at some other part of the tunnel line to investigate the strata or ground water conditions. Outcrops

or evidence of faults at the surface may be examined to see if a re-interpretation of the geological structure is

required. Geophysical methods may also be employed, but the cautions given in 4.3.3 should be kept in mind.

Some investigations may have been delayed until the construction stage because of necessity or

convenience. These may include observations on ground movements and subsidence, continued observations on

ground water levels and the effect of pumping from the tunnel or ground dewatering measures, injection trials

for ground treatment, rock bolt and other ground support trials, and monitoring designed to test behaviour

against expectation and, in the case of experimental construction, to establish design criteria before completion

of construction. It may not be possible to locate or fully investigate known or suspected mine-workings and

shafts during the main site investigation, and detailed exploration may need to be continued into the con-

struction stage. Surface excavations to expose the shafts may be required.

5.4 Review and amendment of plans and sections

The new information obtained by observation, probing ahead and other investigations during construction

must be accurately recorded, and used to revise the plan and longitudinal section. This should be done by the

staff of the Engineer, incorporating both information obtained by themselves and that specifically requested

from the contractor for the project or commissioned from site investigation specialists.

Any implications of the new information for the adopted design or construction method or the timetable

must be noted and action taken where appropriate. The contractor for construction should be kept informed

of significant alterations or extensions of the original interpretation and their implications.

18

6. POST-CONSTRUCTION REPORTS AND RECORDS OF G R O U N D CONDITIONS AND CONSTRUCTION EXPERIENCE

Plans and sections should be amended in the light of observations and investigations made during construction,

to form the permanent as-found record of geology and ground conditions. A record should also be made of the

investigations carried out during construction and the results obtained. Together these will form the post-

construction geotechnical report.

A permanent record should also be made of construction experience in the ground conditions

encountered and with the equipment and methods employed, of hazards and difficulties occurring during the

work, of measures adopted to overcome them, and of any increase in costs attributable to unexpected ground

conditions.

These records, along with the as-built records of the structure (see Table 2, part III), may be valuable

during future maintenance, repair or modifications to the tunnel. They may also be valuable when constructing

other underground works in the area or in similar ground conditions. The publication in an appropriate journal

of a summary of unusual or instructive experience gained during site investigation, design and construction of

the tunnel is also very desirable.

7. CONCLUSION

"If you do not know what you shouM be looking for in a site &vestigation, you are not likely to find much o f

value. What you look for should be suggested by the natural environment, and by the nature o f the constructional

problem to be solved. Thus, a detailed programme of investigation cannot be decided on day one and adhered to,

and the engineer who in the long run is responsible for the solution o f the constructional problem should not

expect to order a site investigation and then dismiss the matter from his mind until a report is placed upon his

desk." R Glossop

Eighth Rankine Lecture

The report has outlined the objectives of a site investigation for a tunnel and the various considerations that must

be borne in mind, indicated the main stages in which the investigation is usually best conducted and the methods

that may be employed in each, and suggested the stages at which reports should be prepared. References have

been given to sources of more detailed information on the numerous topics mentioned.

The main types of information required from the site investigation will be listed in order of their

importance in most projects:

1) The detailed succession of strata with full descriptions, obtained from continuous sampling in the

zone of interest of the tunnel, along with the structural features of the site including faults and

cavities.

2) Detailed information on ground water conditions - the biggest single factor contributing to

difficulties at the construction stage - related to the strata present and to the ground structural

features.

19

3) Results of any necessary in situ trials and in situ tests.

4) Laboratory test results.

In addition the following points will be emphasized:

5) Each investigation must be designed for the ground conditions and engineering requirements of

the particular, project, to serve the needs of both design and construction.

6) An adequate and economical site investigation is best achieved by a staged approach, each stage

reducing uncertainties about the site and indicating the next steps required in the investigation

to fill in gaps in existing knowledge or to confirm or correct earlier predictions.

7) The Work must be managed flexibly so that the investigations can be varied if necessary in the light of the growing understanding of the site.

The more frequent publication of reports on the site investigation aspects of individual tunnelling projects

would be a very useful complement to the more usual type of paper which lays more emphasis on the

construction aspects of the work. Site investigation case history studies of a number of tunnelling proiects are

being made by the Laboratory 36. They will provide a further insight into the art and science of site investigation for tunnels under a range of practical circumstances, the information needs of the various parties concerned, and the areas where improvements in techniques are required.

8. ACKNOWLEDGEMENTS

This report was prepared in the Tunnels Division of the Structures Department of the Transport and Road

Research Laboratory. The authors wish to thank the following for useful comments on the draft for the

report: Mr A M Clarke (National Coal Board), Mr D J Coats (Messrs Babtie, Shaw and Morton), Dr L M Lake

(Messrs Mott, Hay and Anderson) and Mr A M Muir Wood (Sir William Halcrow and Partners). Mr J B Boden

of the Laboratory also made many valuable suggestions. Fig.1 is published by courtesy of the Tyne and Wear

Passenger Transport Executive; consultant engineers Mott, Hay and Anderson.

1 .

2 .

.

4 .

.

9. REFERENCES

BRITISH STANDARDS INSTITUTION. British Standard Code of Practice CP2001 (1957) Site

Investigations. London 1957 (British Standards Institution). (Under revision).

DUMBLETON, M J and G WEST. Guidance on planning, directing and reporting site investigations.

Department o f the Environment, TRRL Report LR625. Crowthorne, 1974 (Transport and Road Research Laboratory).

MUIR WOOD, A M. Tunnels for roads and motorways. Q.J1 Engng Geol., 1972, 5 (1 and 2), 111-126.

VAN JAARSVELD, A P. Site exploration in tunnelling projects Council for Scientific and Industrial

Research, CSIR Report MEG 1008. Pretoria 1971 (National Mechanical Engineering Research Institute).

ASH, J L, B E RUSSELL and R R ROMMEL. Improved subsurface investigation for highway tunnel

design and construction. Vol 1. Subsurface Investigation System Planning. Federal Highway Admini-

stration, Report FHWA-RD-74-29. Springfield 1974 (National Technical Information Service).

20

.

7.

.

9.

10.

11.

12.

13.

14.

15.

16.

GLOSSOP, R. The rise of geotechnology and its influence on engineering practice. Eighth Rankine

lecture. Geotechnique 1968, 18 (2), 105-150.

SABINE, P A. Geological information for underground works. Tunnels and Tunnelling, 1975, 7 (1),

19-25 and discussion 7 (2), 39-41.

DUMBLETON, M J and G WEST. Preliminary sources of information for site investigation in Britain

(revised edition). Department of the Environment, TRRL Report LR 403 (revised edition).

Crowthorne, 1976 (Transport and Road Research Laboratory).

KEIR, W G. Techniques and equipment for detecting underground services. Department of the Environment, TRRL Supplementary Report 69 UC. Crowthorne, 1974 (Transport and Road Research

Laboratory).

OVE ARUP and PARTNERS. Reclaimation of Derelict Land 1. Procedure for locating abandoned mine

shafts. Report for Department of the Environment No. DGR 482/13, 1976.

NORMAN, J W and I WATSON. Detection of subsidence conditions by photogeology. Engng Geol., 1975, 9, 359-381.

DUMBLETON, M J and G WEST. Air photograph interpretation for road engineers. Ministry of Transport, RRL Report LR 369. Crowthorne, 1970 (Road Research Laboratory).

CRAIG, C L and L R BROCKMAN. Survey of tunnel portal construction at US Army Corps of

Engineering projects. Symposium on underground rock chambers, Phoenix, Arizona, Jan 13-14, 1 9 7 1 .

ASCE.

GEOLOGICAL SOCIETY ENGINERING GROUP WORKING PARTY. The logging of rock cores for

engineering purposes. Q.J1 Engng Geol. 1970 3 (1), 1-24.

BRITISH STANDARDS INSTITUTION. British Standard Code of Practice CP2011 (1969). Safety

precautions in the construction of large diameter boreholes for piling and other purposes. London,

1969 (British Standards Institution).

INSTITUTION OF CIVIL ENGINEERS. Safety in wells and boreholes. Revised edition, London 1972

(The Institution of Civil Engineers).

17. BRITISH STANDARDS INSTITUTION. British Standard Code of Practice CP2002 (in preparation).

18.

19.

20.

21.

KNIGHTS M C. Exploratory shafts for in-town surveys. Ground Engng 1974, 7 (1), 43--45.

MEIGH, A C. The Triassic rocks, with particular reference to predicted and observed performance of

some major foundations. Sixteenth Rankine Lecture. G~otechnique 1976, 26 (3), 391--452.

DEERE, D U, A J HENDRON, F D PATTON, and E J CORDING. Design of surface and near-surface

construction in rock. Proceedings of the 8th Symposium Rock Mechanics, Minnesota 1967. 237-302.

PRIEST, S D and J A HUDSON. Discontinuity Spacings in Rock. Int. J. Rock Mech Min ScL 1976, 13, 135-148.

21

22. WEST, G. A note on a use of stereophotography in tunnelling. Department o f the Environment TRRL

Supplementary Report 211 UC'. Crowthorne, 1976 (Transport and Road Research Laboratory).

23. PIPE JACKING ASSOCIATION. Jacking concrete pipes. Design and Specification Bulletin No 1,

London, 1975 (Pipe Jacking Association).

24. LEWIS W A, R T MURRAY and I F SYMONS. Settlement and Stability of embankments constructed

on soft alluvial soils. Proc Instn Cir. Engrs, Part 2, 1975 59 (Dec.), 571-593.

25. ROWE, P W. The relevance of soil fabric to site investigation practice. Twelfth Rankine lecture.

G~otechnique 1972 22 (2), 195-300.

26. WARD, W H, A MARSLAND and S G SAMUELS. Properties of the London Clay at the Ashford

Common shaft: in-situ and undrained strength tests. GOotechnique 1965 15 (4), 321-344.

27. SAMUELS, S G. Some properties of the Gault Clay from the Ely-Ouse Essex water tunnel.

Gkotechnique 1975 25 (2), 239-264.

28. WEST G and M J DUMBLETON. An assessment of geophysics in site investigation for roads in

Britain. Department of the Environment TRRL Report LR680. Crowthorne 1975. (Transport and

Road Research Laboratory).

29. THE BUILDING RESEARCH ESTABLISHMENT/TRANSPORT AND ROAD RESEARCH

LABORATORY WORKING PARTY ON PROBING AHEAD FOR TUNNELS. Probing ahead for

tunnels: a review of present methods and recommendations for research. Department o f the

Environment, TRRL Supplementary Report 171 UC. Crowthorne, 1975 (Transport and Road Research

Laboratory).

30. BURTON, A N. The use of geophysical methods in engineering geology, Part 1. Seismic techniques.

Ground Engng 1976, 9 (1), 32-36 .

31. HIGGINBOTTOM, I E. The use of geophysical methods in Engineering geology, Part 2. Resistivity,

magnetic and gravity methods. Ground Engng 1976, 9 (2), 34-38.

32. THE INSTITUTION OF STRUCTURAL ENGINEERS. The design and construction of deep basements.

(Communications, p .53-54) . London, 1975 (The Institution of Structural Engineers).

33. PECK R B. Advantages and limitations of the observational method in applied soil mechanics. Ninth

Rankine lecture. G~otechnique. 1969 19 (2), 171-187.

34. VON RABCEWICZ, L and J GOLSER. Application of the NATM to the underground works at Tarbela,

Part 1 and Part 2. Water Power 1974 (Sept.), 314-321 ; (Oct.), 330-335.

35. PROCTOR R J. Mapping geological conditions in tunnels. Bull. Assn. EngngGeologists 1971,8 (1), 1-43.

36. DUMBLETON, M J and A F TOOMBS. Site investigation aspects of the Sydenham Road sewer

tunnel. Department o f the Environment, TRRL Supplementary Report 235UC. Crowthorne, 1976

(Transport and Road Research Laboratory).

22

10. APPENDIX

SITE INVESTIGATION IN MINING AREAS

If old mine workings are known or suspected to occur on the tunnel line careful consideration must be given to

measures for locating their position and determining their physical condition (see also 3.2.5 and 4.3.2). Mineral

extraction below tunnel level may have disturbed the overlying rocks which could adversely affect construction.

Flooded workings or loose fills above tunnel level, the presence of mine gases, and subsidence after construction

of the tunnel, could also present hazards.

Any future extraction plans should also be ascertained so that the possibility of future mining subsidence

affecting the completed tunnel can be considered.

10.1 Study of available information

Geological Survey Maps and Memoirs show where coal and various other minerals may occur. A useful map

which shows areas of coal and iron working in Great Britain is published by the Ordnance Survey (Coal and Iron,

1:625000), and is available only from the Map Library, Department of the Environment, Whitehall, London

SWl 3BL. Early Ordnance Survey maps reveal the positions of some shafts and adits but many others were sunk,

abandoned and filled in or covered up long before such maps were made. Signs of working in the form of spoil

heaps and shafts are frequently visible on the surface where mining has been carried out. The study of air photo-

graphs and the use of geophysical methods may be of assistance in locating abandoned workings. A knowledge of

the geology and of the age of possible workings will indicate the depth below which workings are unlikely to

occur. Ancient workings may be much shallower than is usual in modern practice, and are often unrecorded.

Local enquiries can frequently produce a great deal of information which is otherwise unobtainable, and Mining

Consultants may have access to valuable plans and other information for mining areas. TRRL Report LR 403

(revised edition) 7 gives details on records of mines and mineral deposits and their custodians, including those of

coal, oil shale and other mineral deposits. The deposition of plans of abandoned mines became compulsory in

1872; records of mines abandoned earlier are very incomplete.

10.2 Consultation with the National Coal Board

The records of Abandoned Mines kept by the National Coal Board indicate the position and extent of

known workings for coal and their depths as measured at shafts, but are not complete. A limited amount of

boring, as part of the site investigation, may suffice to fill the gaps in available knowledge about the geological

structure.

The freehold interest in coal and coal mines, including shafts, is generally vested in the Board, which is

also responsible for securing shafts and outlets of abandoned mines vested in them so that no person may enter

an outlet or fall down a shaft. These provisions apply regardless of whether the Board owns the surrounding

surface. It is therefore necessary to consult the Board before any site investigation is carried out that may

penetrate either worked or intact coal seams. The Board is entitled to impose conditions on the manner in which

boring or the inspection of old workings may be carried out.

The Board should also be asked to give advice on the possibility of gas in the proposed tunnel.

23

TYNE AND WEAR METRO

SITE INVEST IGAT ION

STRATA

11.00- 11.25m Firm becoming

stiff, strongly laminated brown silty CLAY with light brown silt on lam- inations 11.25 - 11.35m

Alternations of firm laminated brown silty CLAY and light brown silty fine SAND 11.35 - 11.50m

Firm becoming stiff brown sandy silty CLAY with occasional fine gravel, faint lamin- ations and patches of light brown fine sand.

HAND PENETROMETER READINGS

BOREHOLE No :

SAMPLE DEPTH (m) :

BOREHOLE LOCAT ION:

GRID CO--ORDINATES :

G.L.(m) •

' l l Ill l i b i Depth (m)

Reading

11.00 11.1 0 11.20 11.30 11.40

1950 2050 2000 1100 1800

ENGINEERING B E H A V I O U R A L CLASSIFICATION . . . . . . . . . . . . . . . . . CLASS A

SOIL TYPE : Laminated clay (buried valley) wi th thin layers

of si l ty sand

Note: The numerical value of the hand penetrometer reading divided by 2 may be used as a VERY ROUGH guide to shear strength in Ibf / ft 2

Fig. 1 SPLIT CORE PHOTOGRAPH A N D RECORD SHEET (IN THE ORIG INAL THE PHOTOGRAPH IS IN COLOUR)

(1814) Dd20944"7 1,5OO 11/76 HPLtdSo'ton G1915 PRINTED IN ENGLAND

ABSTRACT

A guide to site investigation procedures for tunnels: M J DUMBLETON, BSc, PhD, FGS and G WEST, FGS: Department of the Environment, TRRL Laboratory Report 740: Crowthorne 1976 (Transport and Road Research Laboratory). The report is a general guide to site inves- tigation for bored and cut-and-cover tunnels and associated shafts and portals, constructed for sewers, aqueducts, transport and other services. It outlines the objectives of a site investi- gation for a tunnel and reviews the steps required in carrying out and reporting the investiga- tion, the methods that may be used, and the considerations that must be borne in mind.

Emphasis is placed on the importance of good core recovery and the full description of strata, the thorough examination of ground water conditions, and the value of in s i tu tests and trials. The value of a staged approach to the investigation, and the importance of full reporting of each stage of the work with full communication of results and conclusions to all interested parties, are also stressed.

ISSN 0305-1293

ABSTRACT

A guide to site investigation procedures for tunnels: M J DUMBLETON, BSc, PhD, FGS and G WEST, FGS: Department of the Environment, TRRL Laboratory Report 740: Crowthorne 1976 (Transport and Road Research Laboratory). The report is a general guide to site inves- tigation for bored and cut-and-cover tunnels and associated shafts and portals, constructed for sewers, aqueducts, transport and other services. It outlines the objectives of a site investi- gation for a tunnel and reviews the steps required in carrying out and reporting the investiga- tion, the methods that may be used, and the considerations that must be borne in mind.

Emphasis is placed on the importance of good core recovery and the full description of strata, the thorough examination of ground water conditions, and the value of in s i tu tests and trials. The value of a staged approach to the investigation, and the importance of full reporting of each stage of the work with full communication of results and conclusions to all interested parties, are also stressed.

ISSN 0305-1293