Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 281

For a saturated soil Cc is very much greater than Cv, and B is very nearly unity; for a dry
soil Cv, the value for pore air is much greater than Cc and B is practically negligible or zero.
The variation of B with degree of saturation, found experimentally, is shown in Fig. 8.21.
1.0

0.8

0.6

0.4

0.2

0

70 75 80 85 90 95 100

Degree of saturation, S%

Pore pressure coefficient, B

Fig. 8.21 Variation of B-factor with degree of saturation
The value of B is also known to vary somewhat with stress-change. Pore water pres-
sures develop during the application of the deviator stress also in a triaxial compression test;
the pore pressure coefficient or parameter A is defined from A as follows :

A =

∆

∆σ ∆σ

ud

() 13 −

...(Eq. 8.49)

where ∆ud = port pressure developed due to an increase of deviator stress

(∆σ 1 – ∆σ 3 ), and
A is the product of A and B.
The A-factor or parameter is not a constant. It varies with the soil, its stress history and
the applied deviator stress. Its value can be specified at failure or maximum deviator stress or
at any other desired stage of the test. A-factor varies also with the initial density index in the
case of sands and with over-consolidation ratio in the case of clays. Its variation will over-
consolidation ratio, as given by Bishop and Henkel (1962), is shown in Fig. 8.22.

The general expression for the pore water pressure developed and changes in applied
stresses is as follows:

∆u = B {∆σ 3 + A(∆σ 1 – ∆σ 3 )} ...(Eq.8.50)

+ 1.0

0.5

0

Pore pressurecoefficient, A– 0.5

f

124 81632

Over consolidation ratio

(After Bishop

and Henkel, 1962)

Fig. 8.22 Variation of A-factor at failure with over-consolidation ratio