Compressibility & Consolidation | GATE CE 2020 | Geotechnical Engg. | Gradeup

COMPRESSIBILITYAND CONSOLIDATION

* The property of soil by virtue of which its volume changes under compressive stress is known as

compressibility. It is expressed as decrease in volume per unit increase of pressure.

* When compressive load acts on a saturated soil mass, the change in volume takes place due to

explosion of water from pores and rearrangement of soil skeleton. This gradual process of

compression resulting into reduction of volume is known as consolidation.

* The total compression is the sum of:

(i) Elastic or immediate compression

(ii) Primary consolidation

(iii) Secondary compression

The settlement of soil which occurs immediately on application of compressive force under

undrained condition is called immediate consolidation. This is calculated by assuming the soil mass

to behave as an elastic material.

The application of compressive load on soil mass creates hydrostatic pressure in saturated soils

resulting into gradual expulsion of water through the voids. The volume change due to this process is

known as primary consolidation.

Primary consolidation ends when pore water pressure approaches zero and secondary compression

starts by compressing the clay layers. This phenomenon is very slow. No satisfactory theory is so far

developed to estimate this settlement.

Coefficient of volume compressibility The change in volume of soil per unit initial volume due to a

given increase in the pressure is called the coefficient of volume compressibility

mv =

where av = change in volume

eo = initial void ratio

Coefficient of consolidation It is used to indicate the combined effects of permeability and

compressibility of a soil on the rate of volume change. It is given mathematically as

Cv =

where k = Coefficient of permeability

eo = Initial void ratio

gw = Unit weight of water

Degree of consolidation It is the ratio of settlement of clay at a particular time to the final settlement

when the process of consolidation is complete. It depends upon:

1. Thickness of clay layer

2. Coefficient of permeability

3. Coefficient of consolidation

4. Magnitude and distribution of consolidating pressure.

Time Factor It is a dimensionless constant and is defined by the equation

Tv =

where Cv = coefficient of consolidation

t = time

d = drainage path

Note The time factor Tv

is given by

Tv =

 when U £ 60%

= – 0.9332 log –0.0851 when U > 60°

where U is the degree of consolidation.

Compaction It is the process by which the soil particles are forced to move together due to the

expulsion of air from the voids.

At the end of compaction and consolidation

1. Soil grains are packed closer

2. Shear strength increases

3. Compressibility and permeability decreases.

Table 

 between compaction and consolidation

Water content-density curve Figure 12.5 shows water content and density curve for cohesive and

moderately cohesive soils. Figure 12.6 shows such curve for cohesionless sandy.

Fig. 12.5 For cohesive soil

Fig. 12.6 For cohesionless soil

In case of sandy soil initially, there is decrease in density due to bulking of sand. Then the density

reaches the maximum when it is fully saturated. The trend reverses when sufficient quantity of water

is available. Thus, the maximum density is reached when the soil is either dry or saturated.