PROPERTIES OF SOILS

1. Shape of Grains

Particles coarser than 0.075 mm can be observed by naked eye to get some qualitative idea about the

behaviour of soil. Coarser fraction comprising angular grains have higher bearing capacity. They can

be compacted to a dense mass by vibration.

2. Size of Grains

The properties of cohesionless soil depend upon the grain size distribution to a great extent whereas

the properties of cohesive soil depend upon the grain size distribution to some extent.

Based on the size of grain the soil is classified as shown in Table below.

Determining the percentage of particles of different sizes in a soil is known as ‘grain size distribution’

or ‘mechanical analysis’ of soil. Mechanical analysis may be divided into:

1. Sieve analysis

2. Sedimentation analysis/wet mechanical analysis.

Sieve Analysis It is carried out by sieving about 500 gm of dry soil sample through a set of standard

gives arranged one over the other is ascending order of their sizes. The percentage of sample

retained on each sieve is determined by weight and the percentage finer than the sieve size found.

The result is plotted in the form of a graph on a semi-log paper with the percentage finer on

arithmetic scale and the particle size on logarithmic scale. A smooth curve is drawn through the

points to see grain size distribution as shown in Fig. 

Fig. Grain size distribution curve

C1C1 – Well graded, since it ranges over a large range of particle sizes.

C2C2 – Gap graded, since some of the particle sizes missing.

C3C3 – Poorly graded or uniformly graded, since it is confined to a narrow range of particles.

* The sizes corresponding to 30% finer is designated as D30

, 60% finer as D60 and so on.

* Size D10

is known as effective diameter.

* Uniformity coefficient, Cu =

Cu < 5, uniform size

Cu = 5 to 10, medium graded soil

Cu > 15, well graded soil.

* Coefficient of curvature

CC =

The soil is said to be well graded if CC lies between 1 and 3.

Sedimentation Analysis The particle size distribution of soil fraction less than 75m is determined by

sedimentation analysis, which is based on Stokes’ law. According to this law, fine particles settle

in liquid at different rates according to their size. Coarser size particles settle quickly.

Stokes’ law states

V =

where V = Velocity of settling particle in still water

D = Equivalent diameter of particle

g = Gravitational acceleration

G = Specific gravity of particle

h = Viscocity of water

Limitations of Stokes’ law

1. It is true for spherical particles only.

2. It cannot be applied to particles of size smaller than 0.002 mm (0.2 m).

3. The upper limit of particle size to which this law holds good is 0.2 mm3

. The limitation is because

the liquid develops a turbulent motion at the boundaries of the particles.

4. All particles may not have same specific gravity.

5. Side walls of the container also affect the fall of particle.

* It may be noted that if a soil suspension contains less than 50 gm of solids per litre, the influence

of the particles on each other is not appreciable.

* Note that if organic matter and calcium compounds are present they bind aggregates of particles.

Hence, they should be removed by pretreatment. Pretreatment consists of:

1. Prepare soil hydrogen peroxide mixture at about 60°C and stir gently to remove organic

matter.

2. Then boil the mixture to decompose hydrogen peroxide.

3. After cooling the mixture treat it with about 0.2 N hydrochloric acid.

4. When the reaction with calcium compound is complete, filter the mixture and wash with

distilled water until acid free.

5. Then dry it to constant weight.

* Another method of pretreatment: Use dispersing agent hexametaphosphate (trade name Colgon).

The dispersing agent solution is prepared by dissolving 38 gm of Colgon and 12 gm of sodium

carbonate in distilled water to make 1 litre solution. Soil soaked with the dispersing agent solution

is kept for test.

* The hydrometer method of test is preferred for finding grain size distribution of finer soils.

* Effect of temperature on velocity of fall:

1. Density of water varies with temperature. However, it is not much hence, its effect on the

velocity of sedimentation may be neglected.

2. Variation in viscocity should be considered.

3. Viscocity of water decreases with temperature.

* Viscocity is measured in the unit ‘poise’.

1 poise = 10

–4 kN sec/m2

.

* Hydrometers are normally calibrated at 27°C. Hence, temperature corrections are required to

hydrometer readings if soil sample is at different temperature while testing.

* The corrections are required for:

1. correction for meniscus (always +ve)

2. correction for dispersing agent (always –ve)

Density Index of Cohesionless Soils

* In the loosest form spherical shaped particles can have void ratio = 0.90.

* If such grains are packed, void ratio can be about 0.35.

* In the soil, grains are not uniform, hence smaller grains fill the space between the bigger ones and

the void ratio can get reduced to as low as 0.25 in the densest state.

* Sandy soils can be classified as

1. Very loose, if density index is 0–15

2. Loose, if density index is 15–50

3. Medium dense, if density index is 50–70

4. Dense, if density index is 70–85

5. Very dense, if density index is 85–100

Consistency of Cohesive Soil

* Consistency is a term used to indicate the degree of firmness of cohesive soils. This is expressed

qualitatively as soft, stiff, very stiff and hard.

* A gradual increase water content changes a fine grained soil from solid state to semi-solid state,

plastic state and liquid state.

* Curve showing transition stages from the liquid to solid state is as shown in Fig. 12.4.

* The water contents corresponding to transition from one state to the next state are known as the

liquid limit (WL

), plastic limit (WP

), and shrinkage limit.

Liquid Limit (WL

) It is the minimum water content at which a pat of soil cut by a groove of standard

dimensions, flow together a distance of 13 mm under the impact of 25 blows in a standard liquid limit

apparatus. It is the water content at which the soil shows shearing resistance as the water content is

reduced.

Plastic Limit (WP

) It is the limit of water content that represents the boundary between plastic and

semi-solid states of soil. It is determined as the minimum water content at which the soil can be

rolled into a thread approximately 3 mm in diameter without breaking.

Shrinkage Limit (WS

) It is the lowest water content at which a soil can still be completely saturated.

It represents the boundary between semi-solid and solid states of soil.

In the laboratory, shrinkage limit is determined by completely drying out a lump of soil and measuring

its volume and mass.

WS =

where Wi = Initial wet weight of soil

Wd = Final dry weight of soil

Vi = Initial volume of soil

Vd = Dry volume of soil

Plasticity index It is the range of water content in which the soil behaves like plastic material.

Ip = Liquid limit – Plastic limit.

Consistency index It is defined as the ratio of liquid limit minus the natural water content to the

plasticity index.

I

c =

where w is natural water content

I

c = 0 means soil is at its liquid limit

I

c = 1 means soil is at its plastic limit

I

c

is negative, soil behaves just like liquid

I

c

> 1, the soil is in a semi-solid state, i.e., it is stiff.

Liquidity index

IL =

Shrinkage ratio It is given by

SR =

where V1 = Volume of soil mass at water content, w1

V2 = Volume of soil (mass at water content, w2

)

Vd = Volume of dry soil mass

Thus, it may be defined as the ratio of given volume change, expressed as a percentage of the dry

volume, to the corresponding change in the water content above the shrinkage limit.