Geotechnical Engineering

DHARM

212 GEOTECHNICAL ENGINEERING

1.2

1.0

0.8

0.6

0.4

Void ratio e

10 20 30 40 50 100 200 300 400 500 1000

Virgin compression curve

Rebound curve

PressureskN/m (log scale)^2
Fig. 7.10 Pressure-void ratio relationship for a typical clay
(Pressure to logarithmic scale)
In the semi-logarithmic plot, it can be seen that the virgin compression curve in this
case approximates a straight line from about 200 kN/m^2 pressure. The equation of this straight
line portion may be written in the following form:

e = e 0 – Cc log 10

0

σ

σ

...(Eq. 7.3)

where e corresponds to σ and e 0 corresponds to σ 0. The value arbitrarily chosen for σ 0 is 100
kN/m^2 , usually (1 kg/cm^2 ), although the straight line has to be produced backward to reach
this pressure.

The numerical value of the slope of this straight line, Cc, which is obviously negative in
view of the decreasing void ratio for increasing pressure, is called the ‘Compression index’:

Cc =

()

log

ee− 0

10

0

σ

σ

...(Eq. 7.4)

The rebound curve obtained during unloading may be similarly expressed with Ce des-
ignating what is called the ‘Expansion index’:

e = e 0 – Ce log 10

0

σ

σ

...(Eq. 7.5)

If, after complete removal of all loads, the sample is reloaded with the same series of
loads as in the initial cycle, a different curve, called the ‘recompression curve’ is obtained. It is
shown in Figs. 7.11 and 7.12, with the pressure to arithmetic scale and to logarithmic scale
respectively. Some of the volume change due to external loading is permanent. The difference
in void ratios attained at any pressure between the virgin curve and the recompression curve
is predominant at lower pressures and gets decreased gradually with increasing pressure. The
two curves are almost the same at the pressure from which the original rebound was made to
occur during unloading. The recompression curve is less steep than the virgin curve.