Undrained Cyclic Shear Behavior of a Low Plasticity Alluvial Silt

The undrained cyclic shear behavior of low-plasticity, fine-grained soils transitions from liquefaction to cyclic softening over a narrow range of plasticity index (PI). The uncertainty associated with characterizing the cyclic behavior of low plasticity silts has a significant impact on seismic vulnerability studies in the Pacific Northwest region of North America and other parts of the world. This study presents the results of laboratory cyclic shear tests on a low plasticity silt from a major infrastructure project in the Portland, Oregon, area. The soils in this study are defined as silty sands (SM) to sandy silts (ML) using USCS classification methods, with PI ranging between nonplastic (NP) and 5. The cyclic behavior of the soil is examined in the context of published relationships and data on similar soil types. The effects of pore water pressure during cyclic loading on post-cyclic reconsolidation strains and post-cyclic shear strength degradation are explored and compared to published studies on similar soils of the region. The results of this study contribute to the body of knowledge for cyclic behavior of low-plasticity silts and reduce uncertainty in design.