THE EFFECTS OF STRAIN RATE ON THE BEHAVIOR OF SANDS IN CYCLIC DIRECT SIMPLE SHEAR TESTS

ABSTRACT: Cyclic direct simple shear tests are often performed to evaluate the liquefaction susceptibility of cohesionless soils. The test enforces a condition of constant volume on the specimen by laterally confining it with either a series of metal rings or a wire-wrapped membrane, while the height of the specimen is controlled by locking the top platen in place, thereby preventing any change in specimen height. The specimen is subjected to either an applied shear stress or an applied shear strain and undergoes shear deformations. The decrease in vertical stress measured during loading is equal to the decrease in effective stress that would be measured in an undrained cyclic simple shear test on an equivalent specimen. A study was performed to evaluate the effect that the rate of load application has on a sand’s liquefaction susceptibility. Strain-controlled cyclic direct simple shear tests were performed on specimens of Ottawa C-20/30 sand prepared to 40% relative density. The strains were applied at frequencies of 0.1, 0.2, 1 and 2 Hertz. The results were evaluated in terms of the number of cycles of loading required to trigger liquefaction, the quantity of normalized dissipated energy per unit volume required to initiate liquefaction and pseudo energy capacity, the calibration parameter for the Green, Mitchell and Polito pore pressure generation model. From the study, it was concluded that, over the range of frequencies tested, the frequency of loading had no statistically-significant effect on the number of cycles of loading required to trigger liquefaction, or the quantity of normalized dissipated energy per unit volume required to initiate liquefaction or the pseudo energy capacity.