Liquefaction Characteristics of Sand under Complex Seismic Loading Paths

Abstract

The effects of phase shift (δ) between compression and shear waves, and consolidation stress ratio (Kc) on the liquefaction resistance of sand under simultaneous compression and shear wave loading is investigated using a hollow cylinder torsional shear apparatus. The differences caused by prior stress history (assessed using drained versus undrained preshear) highlight the importance of test protocols in undertaking research to explore the effects of complex dynamic loading paths. The liquefaction resistance of sand was highly dependent on the loading path. However, it was not affected much by variations in δ, if the horizontal shear stress ratio (τzθ/σmc′) and the ratio between shear stress increment and normal stress increment (ΔS/ΔN) were held constant. In tests with constant cyclic stress ratio (CSR), an increase in δ decreases the cyclic resistance because the increase in δ causes a reduction in the rate of deviatoric stress increment per degree of principal stress rotation. At a given CSR, for δ≠0 cases, a change in ΔS/ΔN does not affect the liquefaction resistance of sand because the magnitude and pattern of rotation is not affected much by ΔS/ΔN (for the ratios explored in this research). Increasing Kc or static shear stress ratio (αst) increased the cyclic resistance of the tested sand for ΔS/ΔN<1. The rate of increase in cyclic resistance with increasing αst decreases with the increase in ΔS/ΔN and was essentially unchanged for ΔS/ΔN=1. This observation, that the rate of increase in cyclic resistance with αst decreases with increasing ΔS/ΔN, is consistent with the observation in the literature that the cyclic triaxial loading yields higher static shear stress correction factor (Kα) than cyclic simple shear in loose sand.