Geotechnical Engineering

DHARM

234 GEOTECHNICAL ENGINEERING

Unless otherwise stated, it is the average consolidation ratio for a stratum that is re-

ferred to.

7.7 EVALUATION OF COEFFICIENT OF CONSOLIDATION FROM

OEDOMETER TEST DATA

The coefficient of consolidation, Cv, in any stress range of interest, may be evaluated from its
definition given by Eq. 7.20, by experimentally determining the parameters k, av and e 0 for the
stress range under consideration. k may be got from a permeability test conducted on the
oedometer sample itself, after complete consolidation under the particular stress increment.
av and e 0 may be obtained from the oedometer test data, by plotting the e – σ curve. However,
Eq. 7.20 is rarely used for the determination of cv. Instead, cv is evaluated from the consolida-
tion test data by the use of characteristics of the theoretical relationship between the time
factor T, and the degree of consolidation, U. These methods are known as ‘fitting methods’, as
one tries to fit in the characteristics of the theoretical curve with the experimental or labora-
tory curve. In this context, it is pertinent to note the striking similarity between curve I of Fig.
7.24 and the typical time-compression curve for clays given in Fig. 7.2.

The more generally used fitting methods are the following:

(a) The square root of time fitting method

(b) The logarithm of time fitting method

These two methods will be presented in the following sub-sections.

7.7.1 The Square Root of Time Fitting Method

This method has been devised by D.W. Taylor (1948). The coefficient of consolidation is the soil

property that controls the time-rate or speed of consolidation under a load-increment. The

relation between the sample thickness and elapsed time since the application of the load-

increment is obtainable from an oedometer test and is somewhat as shown in Fig. 7.26 for a

typical load-increment.

Th 0 Elastic compression

Compression due

to consolidation Total compression

or reduction in

thickness

Thickness of sampleThf

Time t

Tht

Fig. 7.26 Time versus reduction in sample thickness for a load-increment

This figure depicts change in sample thickness with time essentially due to consolida-

tion; only the elastic compression which occurs almost instantaneously on application of load

increment is shown. The effect of prolonged compression that occurs after 100% dissipation of