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 soil’s 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.