Experimental Investigations on Noncoaxiality of Unsaturated Weathered Red Mudstone Subjected to Monotonic and Cyclic Simple Shearing

Noncoincidence of principal stress and principal plastic strain rate (or total strain rate) axes has long been observed by extensive experimental and micromechanics-based evidence, in terms of saturated soils and proportional loading. However, influences of noncoaxial behaviors on unsaturated soils under seismic loading have not been fully addressed. This paper presents a series of monotonic and cyclic simple-shear testing on unsaturated weathered red mudstone (WRM), to investigate the effects of initial static shear stress, shear stress amplitude, shear strain amplitude, matric suction, number of loading cycles and three-stage stress-controlled amplitudes, on the noncoaxiality of unsaturated WRM. For cyclic simple shearing, both stress-controlled and strain-controlled loading conditions are applied to simulate the cyclic loadings. Conclusions can be drawn that the noncoincidence between the principal stress direction and the strain rate direction is evident and validated by the monotonic simple-shear tests, where the noncoaxiality is obvious at the initial stage but decreasing during shearing. With respect to cyclic simple shearing, the initial static shear stress, shear strain amplitude, shear stress amplitude, number of loading cycles, and matric suction exhibit significant impacts on the peak noncoaxial angle, whereas the vertical stress demonstrate negligible influences on the peak noncoaxial angle. These findings can help develop advanced dynamic constitutive models for unsaturated soils.