Transmitting Boundaries and Seismic Response

Abstract

A parametric study of the seismic response of a chosen site was conducted to demonstrate the obvious inconsistencies in computed ground response as a result of different assumptions made on the transmitting boundary for the site. The cause of these inconsistencies in computed response is the departure from the ideal assumption that the soil deposit below the transmitting boundary is a linear elastic and homogeneous half‐space. It is shown that under low‐intensity shaking, multiple reflections from the discontinuities below the boundary can be significant, and under strong shaking, nonlinear soil response can alter the wave form, as well as reduce the intensity of the motion. Either situation makes correlation between the input motion at the boundary and the control outcrop motion uncertain. For seismic response analyses of sites underlain by very thick soil deposits, it is suggested that consideration also be given to the shear strength profile at the site when deciding on the location of a transmitting boundary.