Assessment of liquefaction characteristics of sand under coupled action of compression and shear waves

The effects of coupled action of cyclic compression and shear waves on the liquefaction susceptibility of sands are investigated. Cyclic hollow cylinder torsional shear tests were carried out on Fraser River sand specimens consolidated to different initial stress states and subjected to cyclic shearing with different ratio between shear stress and normal stress increments (S/N) and phase shift (δ) between the waves. The representative S/N were estimated from the numerical simulations and used as an input for the cyclic tests. Tests results demonstrate that at an initial isotropic stress state, the cyclic resistance of sand decreases with an increase in S/N upto a limiting value of about 1.6 beyond which increasing S/N does not significantly influences the cyclic resistance. The finding that cyclic resistance ratio (CRR) decreases with increasing S/N is consistent with understanding that the cyclic resistance is higher under triaxial loading mode compared to simple shear. Coupled cyclic loading under out-of-phase compression and shear waves will lead to elliptical loading paths. The loading under such non-conventional stress paths, could be initiated along different pathways, and soil response is dependent on the overall path including the initial stress state. At a fixed cyclic stress ratio (CSR), increasing δ reduces the cyclic resistance of sand. Tests with different consolidation stress ratio (Kc) shows that the rate of increase in cyclic resistance with initial static shear stress decreases with the increase in S/N which indicates that the triaxial loading mode yields higher Kα than the simple shear loading mode. Monotonic tests on Fraser river sand sheared along different inclination of principal stress axes under different non-linear volumetric strain path shows that the behaviour of sand systematically transformed from dilative to softening response as the imposed volumetric strain path changed from contractive to expansive volumetric strain. At a given strain path, the strain softening tendency of the water deposited sand specimens systematically increases as the direction of major principal stress changes from direction of deposition to the direction of bedding.