Geotechnical Engineering

DHARM

280 GEOTECHNICAL ENGINEERING

If we add these contributions considering both the top and bottom faces and equate to

the torque T at failure, we get Eq. 8.45, and if only one face is considered. we get Eq. 8.46.

Regarding the shearing stress distribution on the soil cylinder, it is assumed uniform on

the cylindrical surface but it is triangular over the shear end faces, varying from zero at the

axis of the vane device, to maximum at the edge, as shown in Fig. 8.20.

D

H

Fig. 8.20 Shearing distribution on the sides and faces

of soil cylinder in the vane shear test

The vane shear test is particularly suited for soft clays and sensitive clays for which

suitable cylindrical specimens cannot be easily prepared.

*8 .9 Pore Pressure Parameters

Pore water pressures play an important role in determining the strength of soil. The change in
pore water pressure due to change in applied stress is characterised by dimensionless coeffi-
cients, called ‘Pore pressure coefficients’ or ‘Pore pressure parameters’ A and B. These param-
eters have been proposed by Prof. A.W. Skempton (Skempton, 1954) and are now universally
accepted.

In an undrained triaxial compression test, pore water pressures develop in the first
stage of application of cell pressure or confining pressure, as also in the second stage of appli-
cation of additional axial stress or deviator stress.

The ratio of the pore water pressure developed to the applied confining pressure is
called the B-parameter:

B =

∆

∆σ

∆

∆σ

uuc

c

= c

3

...(Eq. 8.47)

Since no drainage is permitted, the decrease in volume of soil skeleton is equal to that in
the volume of pore water. Using this and the principles of theory of elasticity, it can be shown
that

B =^1

1 +n C

C

v

c

.

...(Eq. 8.48)

where Cv and Cc represent the volume compressibilities (change in volume per unit volume
per unit pressure increase) of pore water and soil respectively and n is the porosity.