Civil Engineering Reference
In-Depth Information
Figure 7.5
Shear tests.
rate of displacement. The vertical and horizontal displacements
h
are measured
using displacement transducers or dial gauges. Drains are provided at the top and
bottom and the pore pressures
u
t
and
u
b
are zero. Tests on clays could be undrained
if they were carried out quickly, so there was negligible drainage during the test, but
as the pore pressures in the sample are not measured effective stresses are unknown.
It is fairly obvious looking at Fig. 7.5(a) that the states of stress and strain within the
sample are likely to be highly non-uniform, particularly near the ends of the box.
The design of the simple shear apparatus avoids non-uniform strains by allowing the
sides to rotate. The most common type, known as the NGI (Norwegian Geotechnical
Institute) simple shear apparatus, is illustrated in Fig. 7.5(b). The sample is cylindrical
and is sealed inside a rubber sleeve like a triaxial sample (see Sec. 7.8). The rubber
sleeve has a spiral wire reinforcement which prevents radial strains but permits shear
strains as shown. Applications of the normal and shear stresses and measurements of
strains are generally similar to those used for direct shear tests. The drain at the bottom
is connected to drainage apparatus like that shown in Fig. 7.3, so that tests may be
drained or undrained with measurements of pore pressure.
Notice that if the shear stresses and horizontal displacements in the shear tests in
Fig. 7.5 are zero, the conditions are just the same as those in the one-dimensional
compression tests in Fig. 7.4.
A major problem with direct and simple shear tests arises with interpretation of
the test results. In the apparatus illustrated in Fig. 7.5 only the shear stresses
δ
n
and
δ
τ
n
and
σ
τ
h
and
σ
h
in Fig. 7.6(a) are unknown. This means that we can only plot one point T on the
Mohr diagram shown in Fig. 7.6(b). There are many Mohr circles that pass through
the point T; two possibilities are shown. In some special simple shear test apparatus
the stresses
n
on horizontal planes are measured and the stresses on the vertical planes
σ
h
on the vertical planes are measured, and in this case the Mohr
circle is properly defined, but for the conventional tests in Fig. 7.5 it is not certain that
the stresses measured,
τ
h
and
τ
n
and
σ
n
, are those on the most critical planes.
7.8 Conventional triaxial compression tests
The triaxial test is by far the most common and versatile test for soils. The conventional
apparatus and the standard test procedures were described in detail by Bishop and
