Water content of moist-tamped nonplastic specimens for constant-volume direct simple shear testing

Constant-volume direct simple shear (DSS) tests on dry soils are an efficient means to evaluate the behavior of cohesionless soils. However, cohesionless soil specimens prepared by dry techniques (e.g., air pluviation) may not be sufficiently contractive to directly evaluate the critical-state shear strength mobilized in monotonic loading. As an alternative, moist tamping allows preparation of loose specimens that exhibit contractive behavior throughout monotonic loading which, where feasible, can maximize testing efficiency in studying static liquefaction. This paper presents a theoretical background for this specimen preparation protocol and validates the approach using DSS test results for three nonplastic soils with various fines content (0 %, 15 %, and 60 %). The results illustrate that if the specimen preparation water content is selected to minimize initial soil suction within the moist-tamped specimen, then the effect of initial soil suction on shear resistance measured in constant-volume DSS testing is negligible when the specimen reaches the critical state. Thus, constant-volume DSS testing performed on moist-tamped nonplastic specimens without saturation appears to provide a feasible alternative to maximize testing efficiency for critical-state behavior studies.