S-wave impedance measurements of the uppermost material in surface ground layers: Vertical load excitation on a circular disk

Abstract

S-wave impedance is one of the most effective parameters used to study the ground motion amplification of soil deposits. We propose a new approach to measure the S-wave impedance of the uppermost material in surface ground layers. First, a circular disk is set on the ground surface, and it is vertically loaded by sinusoidal wave excitation. When the time series of the loading velocity is synchronized with the reaction force, the ratio of the reaction force to the loading velocity is proportional to the S-wave impedance. We then estimate the proportionality coefficient from numerical experiments and check its accuracy. The measurement error is estimated to be within 1% for the homogeneous half-space case. We also discuss the applicability of this new approach and its limitations on the basis of numerical experiments for inhomogeneous media: a two-layered medium and a one-dimensional (1-D) random medium. The proposed approach is effective for both cases if we select the appropriate circular disk size.