Site-Specific Dynamic Analysis: 1-D versus 2-D

When the subsurface profile at a given site cannot be simplified as horizontal layers or the depth to bedrock varies across the profile, validation of a typical 1-D soil column analysis is desirable in calculating the site-specific response spectrum. This point is illustrated through a detailed analysis of a multi-story building project site underlain by very soft marine clay deposits. The subsurface profile and the corresponding dynamic soil parameters are determined through field exploration, geophysical testing, and laboratory testing. Consequently, a 2-D dynamic finite element analysis is performed to provide insight into the effects of a sloping rock. The results indicate that the short period response across the sloping rock is substantially magnified in the 2-D analysis. The 1-D soil column underestimates the short period amplification factor of the spectral acceleration Fa while the 1-sec period amplification factor F-v generally increases with the thicker soil profile. The boundary effects of a 2-D analysis are also discussed.