Geotechnical Engineering

DHARM

208 GEOTECHNICAL ENGINEERING

Pressures 1

Pressures 2

Pressures 3

Compression

Time t

Fig. 7.3 Time-compression curve for successive increments of stress

Since compression is due to decrease in void spaces of the soil, it is commonly indicated

as a change in the void ratio. Therefore, the final stress-strain relationships, are presented in

the form of a graph between the pressure and void ratio, with a point on the curve for the final

condition of each pressure increment.

Accurate determination of the void ratio is essential and may be made as follows:

e =

V

Vs

− 1

Vs =

W

G

s

.γw

V = A. H

Here, A= area of cross-section of the sample;

H= height of the sample at any stage of the test;

Ws= weight of solids or dry soil, obtained by drying and weighing the sample at the end

of the test;

G= specific gravity of solids, found separately for the soil sample.

At any stage of the test, the height of the sample may be obtained by deducting the

reduction in thickness, got from dial gauge readings, from the initial thickness which will be

the same as the internal height of the consolidometer.

Alternatively, the void ratio at any stage may be computed as follows:

The void ratio at the end of test may be obtained as e = w. G, where w is the water

content at the end of the test.

V = A.H = Vs(1 + e)

If ∆H is the change in H and the corresponding change in void ratio is ∆e, A.∆H = Vs.∆e

Dividing one by the other,

∆∆H

H

e

e

=

()1+