Geotechnical Engineering

DHARM

238 GEOTECHNICAL ENGINEERING

0.020

0.015

0.010

0.005

Coefficient of consolidation mm /s 0

2

Effective stress,skN/m^3

100 200 300 400 500 600

Fig. 7.29 Variation of coefficient of consolidation with effective stress
The value of Cv is useful in the determination of the time required for a finite percent-
age of consolidation to occur. Usually 90 to 95% consolidation time may be treated to be that
required for ultimate settlement.

It is interesting to note that a consolidation test provides an indirect way of obtaining
the coefficient of permeability of a clay by applying Eq. 7.20, after evaluating the coefficient of
consolidation by one of the available fitting methods.

*7.8 Secondary Consolidation

The time-settlement curve for a cohesive soil has three distinct parts as illustrated in Fig. 7.30.

When the hydrostatic excess pressure is fully dissipated, no more consolidation should
be expected. However, in practice, the decrease in void ratio continues, though very slowly, for
a long time after this stage, called ‘Primary Consolidation’. The effect or the phenomenon of
continued consolidation after the complete dissipation of excess pore water pressure is termed
‘Secondary Consolidation’ and the resulting compression is called ‘Secondary Compression’.
During this stage, plastic readjustment of clay platelets takes place and other effects as well as
colloidal-chemical processes and surface phenomena such as induced electrokinetic potentials
occur. These are, by their very nature, very slow.

H

II

I

Settlement

Time t

Elastic compression and

compression of pore air

Primary consolidation

Secondary consolidation

I

II

III

:

:

:

Fig. 7.30 Time-settlement curve for a cohesive soil