Impact of Damping Boundaries on the Quality of the Output Signal in Bender Element Experiments
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Bender elements are a popular experimental device for the measurement of the shear modulus of geomaterials. While their relatively low price is probably the main attractor, bender elements have many other advantages. They are easy to use, can be easily installed in most geotechnical devices (e.g. triaxial apparatus, oedometers, and Rowe cells), and yield shear modulus readings that compare well with those obtained from resonant column tests. Typically, bender element tests involve inducing a shear wave at one end of a sample (the input signal) and reading its arrival at the other end (the output signal). However, the wave propagation induced by bender elements is complex, hindering the interpretation of the output signal and inducing considerable uncertainty in the shear modulus readings. Indeed, besides the desirable shear wave, the vibration of the transmitter also generates laterally propagating compression waves, which reflect from the lateral envelope back into the sample and pollute the output signal. This study analyses the effect of lateral boundaries especially conceived to dampen the incoming compression waves on the quality of the output signal. In this context, damping moulds are designed based on computational simulations of the transient dynamics of the wave propagation, to promote an output signal that presents a clearly identifiable arrival of the shear wave, without it being concealed by compression wave pollution. Prototypes of a few promising designs are produced using 3D printing and tested in the laboratory using a benchmark material (Toyoura sand) and a range of input frequencies. The results are compared with those obtained in a conventional setup with no damping moulds.