Comparing Cyclic Direct Simple Shear Behavior of Fine-Grained Soil Prepared with SHANSEP or Recompression Approaches

Cyclic shear laboratory testing is necessary to evaluate geotechnical earthquake hazards and inform the design of fine-grained, low-plasticity soils that can be sampled intact. The method of preparing the soil specimens for lab testing can impact the measured and post-cyclic response. Therefore, it is important to understand the effects that specimen preparation can have on the test results. This research focuses on the differences between two common preparation approaches for testing fine-grained soils, which are the Recompression approach and SHANSEP-type approach. In the Recompression approach, the soil is consolidated to the in situ vertical effective stress. In the SHANSEP approach, the soil is consolidated beyond its historical maximum vertical effective stress (“preconsolidation stress”) and then unloaded to a vertical effective stress to replicate the in situ OCR at the depth of sampling. Different cyclic responses, including the cyclic resistance ratio (CRR) or post-cyclic shear strength, can result from the same soil prepared with these two methods, raising the question of what is the effect of soil preparation on measured soil behavior? This study compares the cyclic response of low-plasticity silt soils from three different sites in the Pacific Northwest that were obtained via Shelby tube sampling and prepared via Recompression and SHANSEP approaches. The specimens were tested with constant-volume cyclic direct simple shear. This study compares the CRR values, cyclic behavior, and post-cyclic shear obtained from both approaches. The results demonstrate that the choice between SHANSEP and Recompression methods can impact the evaluation of cyclic soil strength and post-cyclic shear strength.

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