Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 209

∴ ∆e = [(1 + e)/H].∆H ...(Eq. 7.1)

2.4

2.0

1.6

1.2

0.8

Void ratio e

PressureskN/m^2

0 100 200 300 400 500

Fig. 7.4 Pressure-void ratio relationship

Working backwards from the known value of the final void ratio, the void ratio corre-

sponding to each pressure may be computed. A typical pressure-void ratio curve is shown in

Fig. 7.4.

The slope of this curve at any point is defined as the coefficient of compressibility, av.

Mathematically speaking,

av = −

∆

∆

e

σ

...(Eq. 7.2)
The negative sign indicates that as the pressure increases, the void ratio decreases.
(Alternatively, the curve may be approximated to a straight line between this point and an-
other later point of pressure and its slope may be taken as av). It is difficult to use av in a
mathematical analysis, because of the constantly changing slope of the curve. This leads us to
the fact that compressibility is a function of the effective stress as shown in Fig. 7.5.

Coefficient of compressibility a

v

Effective stresss

Fig. 7.5 Compressibility—a function of effective stress

If the void-ratio is plotted versus the logarithm of the pressure, the data will plot ap-

proximately as a straight line (or as a series of straight lines, as described later), as shown in

Fig. 7.6. In this form the test data are more adaptable to analytical use.