Civil Engineering Reference
In-Depth Information
During a test the total stresses could be changed, or held constant, and the resulting
strains measured; such a test is called stress controlled. Alternatively, the strains could
be changed, or held constant, and the resulting stresses measured; such a test is called
strain controlled. In a particular test one set of stresses (i.e. axial or vertical) could be
stress controlled and another set (i.e. radial or horizontal) could be strain controlled
or vice versa.
Loads may be applied to soil samples by rigid plates or by fluid pressures acting on
flexible membranes. In the first case the displacements and strains are uniform but the
stresses may vary across the plate; in the second case the stresses will be uniform but
the strains may vary. Rigid plates may be smooth, in which case shear stresses should
be zero and so the faces of the sample are principle planes or they may be rough, in
which case there will be both shear and normal stresses to be measured.
To control drainage and measure pore pressures the sample must be isolated within
an impermeable membrane and the pore water connected through drainage leads to
a pressure transducer and volume gauge, as shown in Fig. 7.3. (This shows details
of drainage connections in a typical triaxial test apparatus but the general principles
apply also to other soil testing apparatus.) There is a second drainage lead to the sample
with a flushing valve. This is to allow water to be flushed through the drainage leads
and the bottom drain for de-airing; this is an important requirement of soil testing.
If both valves are closed the sample is undrained and if the drainage valve is open the
sample is drained; the flushing valve is normally closed and it is only opened when the
drainage leads are being flushed. The back pressure
u
0
may be atmospheric or at some
elevated pressure. Sometimes special tests are carried out in which the pore pressures
are changed independently of the total stresses.
The general requirements of soil tests described above are often conflicting and a
number of different soil tests have been developed for different specific purposes. The
principal tests in routine use in practice are the oedometer test, the direct shear test
and the triaxial test, which will now be described. If you read the literature of soil
mechanics and become sufficiently interested to specialize in this area you will come
across many other special tests; all you have to do is work out what are the boundary
conditions and the abilities and limitations of the tests.
Figure 7.3
Control of drainage and measurement of pore pressure in soil tests.
