Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 267

although correlations are possible. The field tests are also not considered here. The details of

the test procedures are available in the relevant I.S. codes or any book on laboratory testing,

such as Lambe (1951).

8.8.1 Direct Shear Test
The direct shear device, also called the ‘shear box apparatus’, essentially consists of a brass
box, split horizontally at mid-height of the soil specimen, as shown schematically in Fig. 8.8.
The soil is gripped in perforated metal grilles, behind which porous discs can be placed if
required to allow the specimen to drain. For undrained tests, metal plates and solid metal
grilles may be used. The usual plan size of the specimen is 60 mm square ; but a larger size
such as 300 mm square or even more, is employed for testing larger size granular material
such as gravel. The minimum thickness or height of the specimen is 20 mm.
After the sample to be tested is placed in the apparatus or shear box, a normal load
which is vertical is applied to the top of the sample by means of a loading yoke and weights.
Since the shear plane is predetermined as the horizontal plane, this becomes the normal stress
on the failure plane, which is kept constant throughout the test. A shearing force is applied to
the upper-half of the box, which is zero initially and is increased until the specimen fails.
Two types of application of shear are possible—one in which the shear stress is control-
led and the other in which the shear strain is controlled. The principles of these two types of
devices are illustrated schematically in Fig. 8.8 (b) and (c), respectively. In the stress-control-
led type, the shear stress, which is the controlled variable, may be applied at a constant rate or
more commonly in equal increments by means of calibrated weights hung from a hanger at-
tached to a wire passing over a pulley. Each increment of shearing force is applied and held
constant, until the shearing deformation ceases. The shear displacement is measured with the
aid of a dial gauge attached to the side of the box. In the strain-controlled type, the shear
displacement is applied at a constant rate by means of a screw operated manually or by motor.
With this type of test the shearing force necessary to overcome the resistance within the soil is
automatically developed. This shearing force is measured with the aid of a proving ring—a
steel ring that has been carefully machined, balanced and calibrated. The deflection of the
annular ring is measured with the aid of a dial gauge set inside the ring, the causative force
being got for any displacement by means of the calibration chart supplied by the manufac-
turer. The shear displacement is measured again with the aid of another dial gauge attached
to the side of the box.

Dial gauge to measure

compression or expansion

of sample

Normal load

(constant)

Dial gauge to measure

shear displacement Loadingplate/plunger

Porous stone

Metal grille

Shearing force

(variable)

Plane of shear

Metal grille

Porous stone

Soil sample

(a) Schematic Diagram