Geoscience Reference
In-Depth Information
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bender element
shear waves
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extender element
compression waves
Fig. 6.3. Piezoelectric bender and extender elements for generation
of shear waves and compression waves
Pluviated samples have a vertical axis of symmetry leading to cross-anisotropic stiffness
which matches the symmetry of the conventional triaxial apparatus for vertical samples.
Exploration of stiffness in such a way as to leave the fabric undisturbed requires very
small perturbations: laboratory geophysical techniques are an obvious choice.
Bender elements (Figure 6.3) (Pennington et al., 1997; Lings and Greening, 2001) have
become particularly popular as a laboratory geophysical technique for their ability to
give a 'zero strain' or 'constant fabric' stiffness—we can probe the stiffness characteris-
ticsofthesoilwithoutdisturbingthearrangementofthesoilparticles.Atwo-layerpiece
of piezoelectric material is electrically wired in such a way that, when a voltage pulse is
applied,onelayerwantstoextend,theothertocontract.Thedeviceisthusforcedtobend.
Embedded in the boundary of a soil sample, this pulsed bending generates a shear wave
whichpassesthroughthesoilsampletobereceivedbyasimilarelementattheotherside
of the sample. The velocity at which the wave travels across the sample can be directly
interpreted in terms of shear stiffness. We have also developed a 'bender-extender' ele-
ment (Figure 6.3) (Lings and Greening, 2001) which, when excited in such a way that
both layers extend together, tends to generate more of a compression wave through the
soil. This can be detected in essentially the same way as the shear wave. In dry soil, the
compression wave can reveal relevant stiffness information. We have used bender ele-
mentsinend-to-endmeasurementsintriaxialapparatusandalsoformeasurementacross
a diameter of the sample, installingthem through theflexible membrane surrounding the
sample.
The wave velocity is proportional to the square root of the appropriate elastic stiffness.
The determination of the shear wave velocity requires the determination of the time it
takesforthewavetotravelfromtransmittertoreceiver.Therearemanyreasonswhythis
isnotstraightforward.Thedynamicmotionisheavilydampedasitpassesthroughthesoil
sothatthereceivedsignalhasamuchloweramplitudethanthesentsignal.Thereceived
signals never bear much resemblance to the sent signals because of reflections from the
boundary and because of the dispersion of the wave through the soil—the dependence
of the shear wave velocity on the frequency of the perturbation—and most input signals
do not contain only one single frequency of excitation. The bender may try to excite
