Comparing Frequency and Time Domain Interpretations of Bender Element Shear Wave Velocities

Abstract

Bender element testing is the most common means to measure the shear wave velocity across soil specimens in the laboratory. There are two classes of shear wave velocity interpretations: time domain and frequency domain, and different methods of interpretation tend to give different answers. Frequency domain approaches provide a means to account for the systematic effect of frequency on the interpreted velocity. Various proposed frequency domain methods have been published, but have so far obtained values that disagreed with more common time domain interpretations. This paper demonstrates that the errors in bender element interpretations of the shear wave velocity are predominantly systematic. The source of these systematic errors is traced to the frequency content of the selected trigger signal waveform and the interpretation method. The systematic errors are demonstrated using experimental data from bender element testing on a triaxial specimen of loose saturated Fraser River Sand. The results from a suite of bender element triggers and interpretation methods found that the shear wave velocity had a range of 30.9 m/s. It is demonstrated that this range is a systematic effect of the bender element trigger signal and interpretation method, not random error. The consequence of the systematic error is that there is no scientifically justifiable reason to take the average of a suite of trigger signals and interpretation methods. The average would not be an unbiased estimate of the mean velocity due to the unaccounted for effects of trigger signal type, frequency, and interpretation method.