An experimental investigation of the effect of strain rate and stress path on undrained response of nonplastic soils in direct simple shear mode

Numerous studies have explored the influence of strain rate on the small- and large-strain shear resistance of nonplastic soils, most using conventional drained and undrained (or constant volume) laboratory tests. To supplement the initial states, soil gradations, and modes of shear examined in earlier studies, the authors used direct simple shear (DSS) tests to evaluate the effect of shear strain rate on the shear resistance of three nonplastic soils with fines contents that varied from 0% to 60%. In addition, the effect of stress path on the critical state and uniqueness of the critical state line (CSL) were explored. Results from the DSS tests on these soils indicated that irrespective of the nonplastic fines content: (i) peak undrained shear strength increased by up to approximately 9% for every order of magnitude increase in strain rate; (ii) strain rate had little to no impact on shearing resistance at the critical state regardless of stress path; and (iii) the CSL was independent of strain rate. In addition, the authors postulate that the effect of strain rate on peak undrained shear strength may be related to differences in inertia during shearing as well as particle rearrangement.