Civil Engineering Reference
In-Depth Information
of the top of the wall and the settlement of the foundation are both 10 mm and
these would be acceptable displacements in many designs. The mean shear strains in
the ground near the wall and the volumetric strains below the foundation are 0.1%.
In practice there will be local strains greater than these, especially near the edge of
the foundation, and the strains will decay to zero far from the structures. This means
that in the ground soil stiffness will vary continuously with position and with loading
throughout most of the range illustrated in Fig. 13.5.
13.5 Measurement of soil stiffness in laboratory tests
The best method for investigating soil stiffness and evaluating stiffness parameters is
to conduct stress path triaxial tests in the laboratory using one of the hydraulic triaxial
cells described in Sec. 7.9. This apparatus permits tests to be carried out in which the
initial state and the loading path can be controlled. The principal problem arises in
the measurement of the small and very small strains required to investigate the whole
of the characteristic stiffness strain curves shown in Fig. 13.5. To examine the whole
of the stiffness-strain curve it is necessary to measure strains less than 0.001%; if the
length of the sample is about 100 mm you will need to measure displacements smaller
than 0.001 mm or 1
m.
The problem is not so much with the resolution and accuracy of the dial gauges,
displacement transducers and volume gauges used to measure axial and volumetric
strains in triaxial tests, as with the errors that occur due to compliance, or move-
ment, in the apparatus. (Do not forget the distinction between accuracy and resolution
discussed in Chapter 7.) Figure 13.7 illustrates a conventional triaxial test; the axial
displacement
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is measured using a displacement transducer or dial gauge mounted
on the loading ram and the volume change is measured from the volume of pore water
entering or leaving the sample through the drainage leads. Errors arise due to (1) axial
Figure 13.7
Measurement of strains in triaxial tests.
