Effect of initial static shear on liquefaction and pre-failure deformation of sand under bidirectional cyclic loading

The effect of initial static shear τ_s on the dynamic characteristics of soil has been studied through extensive laboratory experiments over the past decades, but the influence of radial cyclic stress on its effect has rarely been considered. In this paper, a series of undrained triaxial tests were conducted to the saturated medium dense sand under axial and radial cyclic loadings with a phase difference of 180°. It is shown that the medium dense sand mainly performs two typical deformation response modes, including the accumulated plastic strain and cyclic mobility. The compressive static shear is conducive to the liquefaction resistance, while the extensional one is on the contrary. Such effects are magnified as p'₀ increases, though the absolute value of CRR reduces. The radial cyclic load leads to a higher N_f compared with the loading in the single axial direction, and significantly affects the generation of the pore pressure and its relationship with the accumulation of the axial strain. The samples with the compressive static shear are more sensitive to the strain failure criteria. But for samples with extensional static shear, the limiting residual pore pressure ratio u_{r, lim} precedes before the strain failure. For geotechnical applications, it is necessary to comprehensively consider both the pore pressure and strain failure criteria.