UC Berkeley

Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated compacted soil during earthquake shaking. Existing seismic compression analysis procedures are based on laboratory test results for clean uniform sands, and their applicability to compacted soils with fines is unclear. We evaluate seismic compression from cyclic simple shear laboratory testing of four compacted soils having fines contents that are sufficiently large that fines control the soil behavior, but possessing varying levels of fines plasticity. Each soil material is compacted to a range of formation dry densities and degrees-of-saturation. The test results show that seismic compression susceptibility decreases with increasing density and decreasing shear strain amplitude. Saturation is also found to be important for soils with moderately plastic fines (plasticity index, PI » 15), but relatively unimportant for soils with low plasticity fines (PI » 2) across the range of saturations tested (³54 %). The saturation effect appears to be linked to the presence or lack of presence of a clod structure in the soil, the clod structure being most pronounced in plastic soils compacted dry of the line-of-optimums or at low densities. Comparisons of test results for soils with and without low- to moderately-plastic fines suggest that fines can decrease the seismic compression potential relative to clean sands.