Geotechnical Engineering

DHARM

236 GEOTECHNICAL ENGINEERING

this can be read off from the laboratory plot. The point corresponding to 100% primary consoli-

dation may be easily extrapolated on this plot.

The coefficient of consolidation, cv, may be obtained from

cv =

TH

t

90

2

90

...(Eq. 7.31)

where t 90 is read off from Fig. 7.27(a)

T 90 is 0.848 from Terzaghi’s theory

H is the drainage path, which may be taken as half the thickness of the sample for

double drainage conditions, or as (Th 0 + Thf)/4 in terms of the sample thickness

(Fig. 7.26).

The primary compression is that from Th 0 to Th 100 /d 0 to d 100 in terms of sample thick-

ness/dial gauge reading; the total compression is that from Thi to Thf /di to df. The ratio of

primary compression to total compression is called the “Primary Compression ratio”.

Thus, the total compression in a loading increment of a laboratory test has three parts.

The part from Thi to Th 0 /di to d 0 is instantaneous elastic compression; that from Th 0 to Th 100 /

d 0 to d 100 is primary compression; and that from Th 100 to Thf /d 100 to df is secondary compres-

sion. The secondary compression may be as much as 20% or more in a number of case.

7.7.2 The Logarithm of Time Fitting Method

This method was devised by A. Casagrande and R.E. Fadum (1939). The point corresponding

to 100 per cent consolidation curve is plotted on a semi-logarithmic scale, with time factor on a

logarithmic scale and degree of consolidation on arithmetic scale, the intersection of the tan-

gent and asymptote is at the ordinate of 100% consolidation. A comparison of the theoretical

and laboratory plots in this regard is shown in Figs. 7.28(a) and (b).

0.1 1 10 100 10002000

Th di/i

Th d 0 / 0 D

D

t =1/4

t=1

Th 100 /d 100

Time, t minutes(log scale)

Sample thickness/Dial gauge reading

(a) Sample thickness/Dial gauge reading

versus logarithm of time (Laboratory curve)