Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 269

sn

s (^3) s
1
s 1 s 3
t
t
Failure plane
(predetermined)
t
(, )st 1 f 1
Minor principal plane
Op
Failure plane
(Origin of planes)
Major principal
plane
c
f
(a) Conditions of stress s
in the shear box
(b) Mohr’s circle for direct shear test
Fig. 8.9 Mohr’s circle representation of stress conditions in direct shear test
The direct shear test is a relatively simple test. Quick drainage, i.e., quick dissipation of
pore pressures is possible since the thickness of the specimen is small. However, the test suf-
fers from the following inherent disadvantages, which limit its application.

1. The stress conditions are complex primarily because of the non-uniform distribution
   of normal and shear stresses on the plane.


2. There is virtually no control of the drainage of the soil specimen as the water content
   of a saturated soil changes rapidly with stress.


3. The area of the sliding surface at failure will be less than the original area of the soil
   specimen and strictly speaking, this should be accounted for.


4. The ridges of the metal gratings embedded on the top and bottom of the specimen,
   causes distortion of the specimen to some degree.


5. The effect of lateral restraint by the side walls of the shear box is likely to affect the
   results.


6. The failure plane is predetermined and this may not be the weakest plane. In fact,
   this is the most important limitation of the direct shear test.

8.8.2 Triaxial Compression Test
The triaxial compression test, introduced by Casagrande and Terzaghi in 1936, is by far the
most popular and extensively used shearing strength test, both for field application and for
purposes of research. As the name itself suggests, the soil specimen is subjected to three
compressive stresses in mutually perpendicular directions, one of the three stresses being
increased until the specimen fails in shear. Usually a cylindrical specimen with a height equal
to twice its diameter is used. The desired three-dimensional stress system is achieved by an
initial application of all-round fluid pressure or confining pressure through water. While this
confining pressure is kept constant throughout the test, axial or vertical loading is increased
gradually and at a uniform rate. The axial stress thus constitutes the major principal stress
and the confining pressure acts in the other two principal directions, the intermediate and
minor principal stresses being equal to the confining pressure. The principle is shown in
Fig. 8.10.
The apparatus, consists of a lucite or perspex cylindrical cell, called ‘triaxial cell’, with
appropriate arrangements for an inlet of cell fluid and application of pressure by means of a
compressor, outlet of pore water from the specimen if it is desired to permit drainage which
otherwise may serve as pore pressure connection and axial loading through a piston and load-
ing cap, as shown in Fig. 8.11.