Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, (gamma)t, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, (gamma)c, larger than (gamma)t residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at (gamma)c<(gamma)t such changes are negligible even at large N. To augment limited experimental data base of (gamma)t in cohesive soils, five values of (gamma)t for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold (gamma)t was also tested on one sand, with the results comparing favorably to published data. The test results confirm that (gamma)t in cohesive soils is larger than in cohesionless soils and that it generally increases with the soilʹs plasticity index (PI). For the silts and clay having PI=14-30, (gamma)t=0.024-0.06% was obtained. Limited data suggest that (gamma)t in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.