Civil Engineering Reference
In-Depth Information
continue to produce reasonable output signals. The consequence of this is that use of
electronic instrumentation in soil testing does not necessarily improve the accuracy of
the results compared with old-fashioned instruments and may even reduce the accuracy
considerably unless the instruments are frequently checked and recalibrated. The moral
of all this is that you should always be suspicious of the accuracy of all laboratory tests.
7.11 Summary
1. Laboratory tests are carried out for description and classification of soils, to inves-
tigate their basic mechanical properties and to determine values for the stiffness
and strength parameters.
2. The principal tests for description and classification are grading by sieving or
sedimentation and the Atterberg limit tests which determine the liquid and plastic
limits.
3. The principal loading tests are one-dimensional compression (oedometer) tests,
shear tests and triaxial tests. These may be drained or undrained and they may be
stress controlled or strain controlled.
4.
Special loading or unloading stress path tests are carried out in hydraulic triaxial
cells. In these tests the axial and radial stresses or strains and the pore pressure
can be varied independently to follow the desired stress path.
Worked examples
Example 7.1: Interpretation of a constant head permeameter test
A constant head
permeameter has a diameter of 100 mm and the standpipe tapping points are 150 mm
apart. Results of a test on a relatively coarse-grained soil are given in Table 7.1.
Table 7.1
Results of constant head permeability test - Example 7.1
Volume of water collected
in 1 min (cm
3
)
Difference in standpipe
levels (mm)
270
75
220
60
160
45
110
30
The seepage velocity
V
is given by Eq. (7.5) and the hydraulic gradient
i
is given by
Eq. (7.6). For the first observation,
75
150
=
=
i
0.5
(0.01)
3
=
Q
270
×
10
−
4
m/s
V
t
=
60
=
5.7
×
0.1
2
A
(
π
/4)
×
×
Figure 7.11 shows values of
V
plotted against
i
. These fall close to a straight line
through the origin, which demonstrates that the basic form of Darcy's law (Eq. 7.4)
