Modeling shear rigidity of stratified bedrock in site response analysis

Where a distinct soil-rock interface exists, the bedrock medium is commonly treated as elastic half-space and the bedrock surface as the lower boundary of the soil-column model for site response analyses (or the lower boundary of the finite element model for soil-structure interaction analyses). While shear wave velocity in bedrock varies with depth, there has been no consensus amongst scientists and practitioners over the value of “effective depth” into bedrock at which the “half-space” shear wave velocity value should be taken for modeling purposes. This paper reports an interesting and important observation that the effective depth into bedrock is sensitive to the shear wave velocity profile of the overlying soil sediments. A simple and heuristic method, namely Resonant Period Equivalence (RPE) Method, is proposed herein for representing a stratified elastic bedrock of inhomogeneous properties by an equivalent homogeneous elastic half-space medium, which is characterized by a single equivalent shear wave velocity (V_R) value. The proposed calculation method has been verified by extensive comparative analyses involving the use of programs SHAKE and NERA and employing the complete shear wave velocity models of both the soil sediments and the underlying stratified bedrock.