Soil Mechanics and Foundation Engineering

12.1 INTRODUCTION

* Karl Terzagi is known as the father of soil mechanics. He defined soil mechanics as the application

of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other

unconsolidated accumulations of solid particles produced by the mechanical and chemical

disintegration of rocks regardless of whether or not they contain any admixture of organic

constituents.

Now the term soil mechanics is generally means the discipline of engineering which deals with the

properties and behaviour of soil as a structural material.

* Residual soil means the soil resulting from disintegration of rock and staying at the place of their

formation.

* Transported soil is the soil that is carried away from its place of formation by natural forces.

Classification of transported soil

1. Talus: The soil transported by gravity.

2. Aeoline deposits (loess): Soil deposits formed by wind.

3. Alluvial deposits: Soil transported by streams and rivers.

4. Glacial Soils (or till): Soil transported by glaciers.

5. Lacustrine deposits: Soil deposited in lake bed.

6. Marine deposit: Soil deposits formed by seawater.

Soils of India

1. Red soil It is formed by decomposition of granite, gneiss, quartzite and feldspar rocks rich in iron

and magnesium bearing minerals. It does not contain soluble and calcareous materials.

2. Moorum It is a powdered rock which includes all kinds of disintegrated rock.

3. Desert soil It is mainly sand covering an irregular rocky floor. In most of the area, the sand is piled

up into dunes.

4. Alluvial soil Alluvial soil is formed due to sediment deposits by stream and rivers. Indo-Ganges

plain is the biggest alluvial soil deposit. The older alluvial is generally darker in colour. They

contain impure calcium carbonate. They are loamy soils possessing high degree of fertility.

5. Lateritic soil It is a product of tropical alteration suffered by alkali rocks and sedimentary rocks.

The soil is rich in iron and aluminium content. When fresh it can be easily cut but after weathering

it turns into a hard stone.

6. Saline and alkaline soils These soils contain high salt concentration. They are often found in

areas of poor drainage with high evaporation.

7. Black cotton soil The name is due to its colour and its immense fertility for growing cotton. It

covers about 20% of total area of India. It is an expansive soil. The thickness of this soil deposit

varies from 0.3 m to 15 m. It usually contains high alumina, lime and magnesia.

8. Clay It is an aggregate of mineral particles of very fine particles (microscopic). It may be organic

or inorganic.

9. Loam It is a mixture of sand, silt and clay.

10. Shale This is a material in the stage of transformation from clay to slate.

11. Kaolin It is a pure form of white clay. It is known as china clay. It is used in ceramic industry.

12. Peaty and marshy soils This type of soil is formed in the waterlogged area. They are rich in

organic materials.

Structure of Soils

* Structure of soil may be defined as the physical constitution of a soil material as expressed by the

size, shape and the arrangement of solid particles.

* Size of soil particles:

1. The soil particles coarser than 0.075 mm are visible to the naked eye or may be examined by

means of a hand lense.

2. It is possible to distinguish the grains lying between 0.075 mm to 2m [m = micron = 1 × 10

–6 m =

0.001 mm].

3. Grains of size 2m to 0.1 m can be observed under a microscope but their shapes cannot be made

out. The shape of these particles may be determined by means of an electron microscope.

* Specific surface is defined as the total area of the surface of grains expressed in square centimetres

per gram or cubic centimetre. A sphere has least specific surface whereas a plate exhibits the

maximum.

* Soil mass structure: Bulk soil is random assembly of soil particles with air and water occupying the

voids amongst the particles. Thus, soil mass is a three-phase system consisting of solid particles,

water and air as shown in Fig.

Basic Terms

* Volume related:

1. Void ratio e = . This value may range from 0.5 to 1.5.

2. Porosity n = × 100 = . Its range is between 0 and 100.

3. Degree of saturation, S = 

=

S = 0, dry soil

S = 100, saturated soil

0 £ S £ 100, partially saturated soil.

* Weight related terms

Water content, w = .

It can be zero to as high as 400%

2. Unit weight of water gw =

3. Unit weight of soil solids gs =

4. Bulk weight of soil g =

5. Dry unit weight of soil gd =

6. Saturated unit weight of soil gsat =

7. Submerged unit weight of soil gsub = gsat – gw

* Density related terms

1. Bulk density, r =

2. Dry density, rd =

3. Density of solids, rs =

4. Saturated density, rsat =

5. Submerged density, rsub = = rsat – rw

* Specific gravity related terms

1. Specific gravity: The specific gravity of a substance is the ratio of its mass in air to the mass of

an equal volume of water at 4°C.

Specific gravity of soil solids, G = .

Generally, G varies between 2 and 3.

2. Mass specific gravity.

3. Absolute specific gravity. The absolute volume of soil is the volume of soil minus the volume of

permeable and impermeable voids. Hence, absolute specific gravity

Ga =

It is also known as ‘grain specific gravity’ or ‘specific gravity of solids’.

4. Density Index: It is the ratio of the difference between the voids ratio of the soil in its loosest

(emax

) and its natural void ratio (e) to the difference between the void ratio in the loosest and

denset state (emin

)

ID =

Important Relations

The following relationships among soil parameters can be easily derived.

Determination of Specific Gravity

Constant volume or pycnometer method is commonly used for determining specific gravity of soil.

Pycnometer is a bottle of volume 500 cc. Distilled water and soil sample of about 200 cc are

required for this test. Figure 12.2 shows diagrammatically the materials to be weighed.

Determination of Water Content

Over dry method Find the mass of clean container with lid, M1

. Fill the container with about 300

grams of sample and find the mass, M2

. Oven dry the sample and fill the container and find the mass,

M3

. Then

Water content w =

Note Oven temperature should be 110°C. Clay takes 10 to 15 hours to dry. Sandy soil takes 4 hours to

dry.

2. Pycnometer method In this method pycnometer of 1 litre capacity with conical cap is used.

(a) Find mass of clean pycnometer, M1

(b) Find mas of pycnometer + wet sample, M2

(c) Find mass of pycnometer + sample + water, M3

(d) Find mass of pycnometer + water, M