DHARMSHEARING STRENGTH OF SOILS 271
(ii) The specimen is enveloped by a rubber membrane to isolate it from the water with
which the cell is to be filled later; it is sealed with the pedestal and top cap by rubber
‘‘O’’ rings.
(iii) The cell is filled with water and pressure is applied to the water, which in turn is
transmitted to the soil specimen all-round and at top. This pressure is called ‘cell
pressure’, ‘chamber pressure’ or ‘confining pressure’.
(iv) Additional axial stress is applied while keeping the cell pressure constant. This
introduces shearing stresses on all planes except the horizontal and vertical planes,
on which the major, minor and intermediate principal stresses act, the last two
being equal to the cell pressure on account of axial symmetry.
(v) The additional axial stress is continuously increased until failure of the specimen
occurs. (What constitutes failure is often a question of definition and may be differ-
ent for different kinds of soils. This aspect would be discussed later on).
A number of observations may be made during a triaxial compression test regarding the
physical changes occurring in the soil specimen:
(a) As the cell pressure is applied, pore water pressure develops in the specimen, which
can be measured with the help of a pore pressure measuring apparatus, such as Bishop’s pore
pressure device (Bishop, 1960), connected to the pore pressure line, after closing the valve of
the drainage line.
(b) If the pore pressure is to be dissipated, the pore water line is closed, the drainage
line opened and connected to a burette. The volume decrease of the specimen due to consolida-
tion is indicated by the water drained into the burette.
(c) The axial strain associated with the application of additional axial stress can be
measured by means of a dial gauge, set to record the downward movement of the loading
piston.
(d) Upon application of the additional axial stress, some pore pressure develops. It may
be measured with the pore pressure device, after the drainage line is closed. On the other
hand, if it is desired that any pore pressure developed be allowed to be dissipated, the pore
water line is closed and the drainage line opened as stated previously.
(e) The cell pressure is measured and kept constant during the course of the test.
(f) The additional axial stress applied is also measured with the aid of a proving ring
and dial gauge.
Thus the entire triaxial test may be visualised in two important stages:
(i) The specimen is placed in the triaxial cell and cell pressure is applied during the
first stage.
(ii) The additional axial stress is applied and is continuously increased to cause a shear
failure, the potential failure plane being that with maximum obliquity during the
second stage.
Area Correction for the Determination of Additional Axial Stress or Deviatoric Stress
The additional axial load applied at any stage of the test can be determined from the proving
ring reading. During the application of the load, the specimen undergoes axial compression
and horizontal expansion to some extent. Little error is expected to creep in if the volume is
supposed to remain constant, although the area of cross-section varies as axial strain increases.
The assumption is perfectly valid if the test is conducted under undrained conditions, but, for
drained conditions, the exact relationship is somewhat different.