Lecture 10 & 11 122

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

In this lecture;

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A- Soil Characteristics

B- Soil Classification

C- Embankments

D- Soil Compaction

Soil Engineering for Highway Design - Overview

The information listed in this lecture is mainly taken from; Standard Specifications for Engineering Materials and Methods of Sampling and Testing (AASHTO, 2013), Iraqi General Specifications for Roads and Bridge (SCRB, 2007), Traffic and Highway Engineering (Garber and Hoel, 2009) and Highways (OFlaherty, 2007).

A- Soil Characteristics

The basic characteristics of a soil are usually described in terms of its origin, formation, grain size, and shape. The principal engineering properties of any soil are mainly related to the basic characteristics of that soil.

- Origin and Formation of Soils (weathering)

- Surface Texture (fine and coarse)

Basic engineering properties of soils

Highway engineers must be familiar with those basic engineering properties of soils that influence their behaviour when subjected to external loads. Highway engineers must always keep in mind that the behaviour of any soil depends on the conditions of that soil at the time it is being tested.

- Phase Relations (includes; porosity, void ratio, moisture content, degree of saturation, soil density)

- Atterberg limits (includes, PL, SL and Liquid limit.

- Permeability (DArcy law v = k i)

- Shear strength (S = C + tan )

Lecture 10 & 11 123

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

B- Soil Classification Soil can be classified according to their probable engineering characteristics. It helps as a means of identifying suitable subbase materials and predicting the probable behaviour of a soil when used as subgrade material.

The most commonly used classification system for highway purposes is the American Association of State Highway and Transportation Officials (AASHTO) Classification System. The Unified Soil Classification System (USCS) also is used to a lesser extent.

AASHTO Soil Classification System

The system has been described by AASHTO as a means for determining the relative quality of soils for use in embankments, subgrades, subbases, and bases. In AASHTO classification, soils are classified into seven groups, A-1 through A-7, with several subgroups, as shown in the table below. The classification of a given soil is based on its particle size distribution, LL, and PI. Soils are evaluated within each group by using an empirical formula to determine the group index (GI) of the soils, given as

GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10)

where GI = group index F = percent of soil particles passing 0.075 mm (No. 200) sieve in whole number based on material passing 75 mm (3 in.) sieve LL = liquid limit expressed in whole number PI = plasticity index expressed in whole number The GI is determined to the nearest whole number. A value of zero should be recorded when a negative value is obtained for the GI. Also, in determining the GI for A-2-6 and A-2-7 subgroups, the LL part of the above equation is not used - that is, only the second term of the equation is used.

Lecture 10 & 11 124

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

Lecture 10 & 11 125

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

Under the AASHTO system, granular soils fall into classes A-1 to A-3. A-1 soils consist of well-graded granular materials, A-2 soils contain significant amounts of silts and clays, and A-3 soils are clean but poorly graded sands.A-4 to A-7 are silty to clayey soils. Classifying soils under the AASHTO system will consist of first determining the particle size distribution and Atterberg limits of the soil and then reading the Table from left to right to find the correct group. The correct group is the first one from the left that fits the particle size distribution and Atterberg limits and should be expressed in terms of group designation and the GI. Examples are A-2-6(4) and A-6(10).

C- Embankments Foundation (formation and construction)

Highway embankments are formed

The process of constructing one layer at a time facilitates obtaining uniform strength and moisture content in the embankment. Compaction of thick layers may result in variable strengths within the embankment, which could lead to differential settlement between adjacent areas.

by spreading thin layers of uniform thickness of the material and compacting each layer at or near the optimum moisture content.

Most states specify a thickness of 6 to 12 inches for each layer.

Lecture 10 & 11 126

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

The table below gives commonly used relative density values for different embankment heights. The relative density is given as a percentage of the maximum dry density obtained from the standard AASHTO (T99) test.

Lecture 10 & 11 127

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad

The construction control

D- Soil Compaction

of an embankment entails frequent and regular checks of the dry density and the moisture content of materials being compacted. The bulk density is obtained directly from measurements obtained in the field, and the dry density is then calculated from the bulk density and the moisture content. The laboratory moisture-density curve is then used to determine whether the dry density obtained in the field is in accordance with the laboratory results for the compactive effort used.

Compaction equipment used in the field can be divided into two main categories. The first category includes the equipment used for spreading the material to the desired layer or lift thickness, and the second category includes the equipment used to compact each layer of material.

Lecture 10 & 11 128

Highway Eng. Embankment and Compaction 14 15

Dr. Firas Asad