Cyclic threshold shear strains of sands based on pore water pressure buildup and variation of deformation characteristics

Abstract

The cyclic threshold shear strain gamma(c)(th) is defined as the cyclic strain amplitude above which the pore water pressure in saturated sandy soil increases and/or the modulus and damping values vary with the number of loading cycles during cyclic loadings. In this paper, the cyclic threshold shear strains are thoroughly investigated on sands using torsional shear tests. Toyoura sand is tested at various densities, drainage conditions (dry, saturated drained, and saturated undrained), and effective confining pressures. Based on the test results, the cyclic threshold strains above which the excess pore water pressure increases and/or shear modulus and damping ratio vary with the number of cycles are determined. Cyclic hardening in dry and saturated drained conditions, and cyclic degradation in an undrained condition, are observed for the same sample, using identical testing equipment. gamma(c)(th) for cyclic degradation is compared with gamma(c)(th) for cyclic hardening. The variation of damping ratio with the number of cycles is observed at different drainage conditions, and gamma(c)(th) for damping ratio is also investigated.