Nonlinear Seismic Response of Soft Clay Sites

A nonlinear stress-strain model, utilizing Ramberg-Osgood idealization of backbone curve and allowing for its progressive degradation due to cyclic loading, is used to conduct site response studies at two soft clay sites. The sites selected are: (1) Southern Pacific Building, San Francisco, and (2) Station No. 119 Site, Tokyo, Japan. The subsurface conditions and the material properties used in analyses are presented. The computer program DCHARM used for nonlinear analysis allows for progressive degradation of the backbone curve. For comparison the results obtained from program SHAKE using equivalent linear idealization are also presented. The program DCHARM is shown to provide ground response results closer to the recorded data than SHAKE. The results obtained indicate that substantially higher accelerations and durations of earthquakes are needed to cause significant degradation of soft clay.