Measurements of Vp and Vs in dry, unsaturated and saturated sand specimens with piezoelectric transducers
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Evaluation of the compression and shear wave velocities (Vp and VS) of sand specimens using piezoelectric transducers was performed in this study. The selected piezoelectric transducers were piezoelectric discs (PDs) and bender elements (BEs). These transducers were used to measure Vp and VS, respectively. The seismic measurements with PDs and BEs involved the development of an instrumented triaxial chamber (ITC) in which the transducers were placed. The same types of PDs and BEs were also installed in the top caps and base pedestals of a combined resonant column and torsional shear (RCTS) device. The soil tested was washed mortar sand, and all the specimens were formed using the undercompaction method. The sand specimens were seismically tested in dry, unsaturated and saturated conditions in the ITC. The ITC was designed to permit the evaluation of wave velocities for soils at different stages during the saturation process. Incorporation of piezoelectric transducers in the RCTS device allowed three different measurement techniques to be performed in the same soil specimen: (1) torsional resonance measurements, (2) direct measurement (slow cyclic) of the stress-strain loops in shear, and (3) direct-arrival-time measurements of wave propagating in the specimen to determine VS and Vp. In addition, seismic measurements using BEs were performed simultaneously with torsional shear tests in the RCTS device in a synchronized manner. Synchronization of the BEs measurements with the slow-cyclic torsional shear test allowed the determination of VS at the different points on the hysteresis loops applied to the specimens with the RCTS device. Among the findings of this study: (1) small-strain VS values measured using BEs were found to be about 3 to 7% greater than those measured with RC tests, (2) Vp measurements using PDs are a good way of studying the saturation process in soils when the degree of saturation is above 97%, and (3) synchronized measurements with BEs during slow-cyclic loading in the RCTS device represents a new experimental effort in the development of improved nonlinear soil models to describe cyclic stress-strain relationships in soils.