Liquefaction resistance of sand under cyclic loading is always an evolving topic in earthquake geotechnical engineering. Conventionally in laboratory, the cyclic response of saturated coarse-grained soil is usually investigated either by using a cyclic triaxial or a cyclic simple shear apparatus, which subjects the soil to relatively simpler loading paths. Several studies have been carried out in the past using these devices to identify the factors influencing the liquefaction characteristics of soil. The major objective in these studies is to characterize the dynamic behavior of soil due to the action of vertically propagating shear waves generated during an earthquake. This propagation of pure shear waves is based on the assumption that the soil strata are perfectly horizontal and homogenous. However, in reality, the soil media is highly heterogeneous, and the plane of stratification is not always horizontal. Therefore, the soil element will be inevitably subjected to simultaneous action of both compression and shear waves during an earthquake loading.

In this study, liquefaction susceptibility of loose Fraser River sand subjected to coupled action of compression and shear waves (or Rayleigh waves) is evaluated. This coupled loading condition better replicates the actual seismic loading condition in-situ than the one-dimensional loading from conventional laboratory tests. The nature and degree of principal stress rotation caused by this coupled loading are significantly influenced by the initial consolidation stress state and shearing parameters such as the ratio between shear stress and normal stress increments (S/N), and the phase shift between the waves. Cyclic hollow cylinder torsional shear tests were carried out on water pluviated Fraser River sand specimens. The specimens were isotropically consolidated to identical relative densities and subjected to coupled cyclic loading with representative values of S/N . Test results demonstrate that for a given CSR and initial confining pressure, the number of cycles to liquefaction decreases with an increase in the S/N  upto a limiting value of about 2 beyond which increasing S/N  does not significantly influences the cyclic resistance. The decrease in the cyclic resistance of sand with the increase in S/N  is primarily due to the coupled action of increase in shear stress on the weak horizontal bedding plane and close alignment of maximum shear stress axis with the bedding plane.