Effect of frequency in bender element tests for a fine volcanic ash

The small strain shear modulus, Gmax is a fundamental parameter for seismic analysis and design. Bender elements (BE) are routinely used to estimate the shear wave velocity of geomaterials, from which the Gmax may then be evaluated. Given their increasing popularity, in the last decade the Japanese Geotechnical Society (JGS) and American Society for Testing and Materials (ASTM) proposed standards to harmonize and reduce the subjectivity previously noted in the interpretation of BE waveforms. However, the standard methods recommended by both JGS and ASTM, per se, should be considered as a good set of guidelines, since hitherto, no one technique may be universally applied to all materials. This study aimed at understanding the effect of frequency on the shear wave signals of very fine volcanic ash soils (fines content > 90%) with very high water content (>150%). Remoulded volcanic ash with nano-sized clay minerals like allophane were sampled from two sites affected by the 2016 Kumamoto Earthquake in Japan. BE tests were carried out on saturated soil specimens at different void ratios and frequencies. For comparison purposes, similar tests were also conducted on Toyoura sand. Owing to the damping nature of the ash, the output signals have mostly a lower frequency than the input waves. Moreover, the interference of “near field effects” was evident at low input frequencies, which affects the proper determination of the arrival time. By considering the relationship between input and output frequencies and the transfer functions of the BE-soil systems, the optimum input frequencies for this type of soil was estimated, together with the delimitating travel length to wavelength ratio.