Geotechnical Engineering

DHARM

268 GEOTECHNICAL ENGINEERING

Shear

box

Normal load Dial gauge to

measure compression

or extension of sample

Dial gauge to measure

shear displacement

Forced

plane

of shear

Shear

force

Calibrated dead weights

(b) Stress-control type

Shear

box

Normal load Dial gauge to

measure compression

or extension of sample

Dial gauge for

shear displacement

Shear force

Proving ring

for shear force

Crank for constant rate
of displacement
(c) Strain-control type
Fig. 8.8 Direct shear device
In both cases, a dial gauge attached to the plunger, through which the normal load is
applied, will enable one to determine the changes in the thickness of the soil sample which will
help in the computation of volume changes of the sample, if any. The strain-controlled type is
very widely used. The strain is taken as the ratio of the shear displacement to the thickness of
the sample. The proving ring readings may be taken at fixed displacements or even at fixed
intervals of time as the rate of strain is made constant by an electric motor. A sudden drop in
the proving ring reading or a levelling-off in sucessive readings, indicates shear failure of the
soil specimen.
The shear strain may be plotted against the shear stress; it may be plotted versus the
ratio of the shearing stress on normal stress; and it may also be plotted versus volume change.
Each plot may yield information useful in one way or the other. The stresses may be obtained
from the forces by dividing them by the area of cross-section of the sample.
The stress-conditions on the failure plane and the corresponding Mohr’s circle for direct
shear test are shown in Fig. 8.9 (a) and (b) respectively.
The failure plane is predetermined as the horizontal plane here. Several specimens are
tested under different normal loads and the results plotted to obtain failure envelopes.