Profound 2-D Amplification Effects for Sites with Soft Soil Underlying Stiff Soil

Abstract eng:
For a 30m deep 1-D soil site of about 0.5s period with a soft soil layer at the surface overlying a stiff layer above bedrock, the short-period amplification characteristics are similar to those of the 1-D model with a single soft soil layer, in that the cross-over point separating the amplification and deamplification range is about 0.3-0.5g for peak ground accelerations (PGA). However, for a soil site with a soft layer between a stiff surface layer and the underlying bedrock, the short-period amplification ratios are drastically reduced from those of a site with a uniform soft soil layer or a site with a soft soil layer at the surface, with a cross-over point for PGA amplification ratios as small as 0.1-0.2g, depending on the thickness of the stiff soil layer. Significant differences between these two 1-D models are observed at periods up to 1.0s. We investigate the 2-D effect from a 2-D soil basin with identical vertical shear-wave velocity profile at the basin center to that of the 1-D model. We model six 2-D trapezoidal basins that have a symmetric soil/bed rock interface with width/depth ratios of 3, 6, and 10, subjected to a suite of rock site records. For the first set of three basins, the soft soil layer is over the stiff soil, while the second set has a soft soil layer between the stiff layer and the bedrock. We find that the differences in response spectral amplification ratios between the two sets of basins are reduced from those of the two 1-D models, and show profound 2-D effects.

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