Novel evaluation of bender element transmitter response in transparent soil

Bender elements (BEs) are commonly used by geotechnical laboratories worldwide to measure the shear-wave velocity of soils. However, the actual behaviour of BEs inside the soil specimen has never been directly measured. There is experimental, numerical and analytical evidence that the shape and frequency content of the actual transmitter movement are different from the shape and frequency content of the input voltage signal, although this disparity has never been validated for BEs placed inside a soil specimen. In order to systematically investigate the disparity between the input signal and the actual transmitter response and to advance the understanding of the frequency response of the transmitter inside the soil, an experimental programme is designed to measure, for the first time, the responses of BEs placed inside soil specimens. To capture the transmitter response when placed inside the soil, a transparent granular soil is used to allow penetration of a laser beam from a laser vibrometer. The results show that, contrary to common assumptions, the transmitter response inside the soil specimen is different from the input excitation signal. The time delay between input excitation and transmitter response is found to be a function of frequency and the surrounding medium. Moreover, the vibration modes of BEs are studied by analysing the transmitter response to different input signals. These observations are used to understand the actual behaviour of the BEs placed inside the soil.