A Hybrid Bender Element–Ultrasonic System for Measurement of Wave Velocity in Soils

Abstract

In recent years, the determination of the small strain modulus of soils from laboratory measurement of the elastic wave velocity has become more common. Piezoelectric elements are used in these tests, with two types of configurations: fixed to the back face of a platen as an ultrasonic transducer, or as a bimorph that slightly protrudes out of the platen. A survey of the literature shows that bender/extender elements are more widely used than ultrasonic transducers in soil tests. However, bender/extender elements are invasive in nature. Besides introducing some degree of specimen disturbance, they need to be sufficiently robust for insertion into the soil specimen, and they also need to be properly waterproofed to avoid short-circuiting. Ultrasonic transducers can solve the above-mentioned problems. However, the detection of shear waves can be challenging in soils, as reported in the literature. The objectives of this paper are to examine the difficulties of shear wave detection by ultrasonic transducers and to propose a hybrid bender element–ultrasonic system that partially alleviates the problems associated with bender/extender elements while still providing reliable shear wave velocity measurement. The hybrid system used was calibrated with standard materials, and tests were conducted on sand and kaolin specimens. Relative to bender/extender elements and ultrasonic transducers, the hybrid system showed better performance in determining wave velocities in soil tests, especially for shear waves.