Cyclic threshold shear strain and deformation properties of sands under various drainage conditions

Abstract

Deformation properties which are shear modulus, G, and damping ratio, D, were measured using the Stokoe type torsional shear testing equipment under dry, saturated drained, and saturated undrained conditions. The existing Stokoe-type torsional shear equipment was modified using the quick connector and back pressure system to saturate the specimen and to maintain the B-value above 0.99 during the saturated test. The modified equipment can perform the undrained torsional shear tests on fully saturated sand as well as the drained tests on dry and saturated sands. Two types of sands, Geumgang and Toyoura sands, were collected and TS tests were performed at various densities, drainage conditions, and effective confining pressures. The cyclic threshold shear strains above which pore water pressure increases and/or shear modulus and damping ratio vary with the number of cycles were determined by fitting test data using nonlinear model. The cyclic hardening of shear modulus in the dry condition and the cyclic degradation of shear modulus in the undrained condition are observed on the same soil using identical testing equipment, and the cyclic threshold shear strain for cyclic degradation based on pore water pressure buildup is compared with the cyclic threshold shear strain for the cyclic hardening on shear modulus. Additionally, the variation of damping ratio with the number of cycles is observed at the different drainage conditions, and the cyclic threshold shear strain for damping ratio is investigated and compared to the cyclic threshold shear strain for cyclic degradation and hardening of shear modulus. Variations in shear modulus, $G_max$, and damping ratio, $D_min$, at small shear strains ($γ_c<10^{-3}%$) and G and D at medium shear strains ($10^{-3}%<γ_c 10^{-1}%$) shear strains were characterized under various conditions, and test results were intensively compared considering drainage conditions, void ratio, isotropic confining pressure, and overconsolidation ...