Civil Engineering Reference
In-Depth Information
7.9 Hydraulic triaxial cells - stress path tests
Later we will discover that many features of soil strength and stiffness are governed
by the initial state of the soil, its history of loading and unloading and the changes of
axial and radial stress during loading or unloading. Consequently, in order to examine
soil behaviour properly we will need to be able to control the axial and radial stresses,
and perhaps the pore pressures, independently. In the conventional triaxial apparatus
shown in Fig. 7.7 the axial stress is applied by strain-controlled loading and it is difficult
to vary the axial stress in a controlled way.
Tests in which the paths of the effective stresses (i.e. the graph of
σ
r
or
the graph of
q
against
p
) are varied, are called stress path tests and are carried out
in hydraulic triaxial cells, illustrated in Fig. 7.9. Details of the sample, platens and
drainage arrangements are the same as those for the conventional triaxial cell shown
in Fig. 7.7, the principal difference being in the application of the axial stress. Another
difference to notice is that the loading ram should be connected to the top platen so
that extension tests can be carried out where
σ
a
against
σ
a
<σ
r
and the force in the ram
F
a
is
σ
r
are always positive because uncemented soils cannot
sustain tensile stresses and, in any case, the platens are not generally attached to the
sample.)
A simple hydraulic triaxial cell can be made by adding a hydraulic cylinder to the
loading ram, as illustrated in Fig. 7.9(a). Alternatively, special hydraulic triaxial cells
are widely used in which a frictionless hydraulic ram is incorporated into the base of the
cell, as illustrated in Fig. 7.9(b). In both cases the axial forces
F
a
should be measured
independently using a load cell because it is inaccurate to calculate the value from
measurements of the pressures in the hydraulic rams. Conventional strain-controlled
triaxial tests can be carried out in both cells, in the first case by locking the hydraulic
cylinder and using the motor drive in the loading frame as in a conventional test or,
in the second case, by pumping fluid into the hydraulic ram at a constant rate from a
screw ram.
In many modern hydraulic triaxial cells all the instruments are electronic and read-
ings are made on a logger controlled by a PC and the pressures in the axial ram, in the
cell and in the pore pressure leads are applied through electronic pressure converters.
σ
a
and
negative. (Note that
Figure 7.9
Hydraulic triaxial apparatus.
