Effects of Initial Direction and Subsequent Rotation of Principal Stresses on Liquefaction Potential of Loose Sand

The effects of the initial orientation of principal stress axes and subsequent rotation of principal stresses on liquefaction susceptibility of sands were investigated. Monotonic and cyclic hollow cylinder torsional shear tests were carried out on Fraser River sand specimens consolidated to different initial principal stress orientations and subjected to principal stress rotation during loading. Cyclic loading was applied with constant amplitude cyclic deviator stress, but along stress paths that impose different magnitudes of principal stress rotation. Test results demonstrate that the cyclic resistance ratio (CRR) is influenced by both the initial orientation of principal stresses and the magnitude of stress rotation during dynamic loading. These results suggest that the degree of stress rotation influences CRR more significantly than the initial principal stress orientation. Yet, the effects of the degree of stress rotation are not considered in current liquefaction assessment practice. The only available mechanism to account for principal stress directions is the use of the Kα factor, which focuses on the initial principal stress orientation only. Irrespective of the initial inclination of the major principal stress axis, the weakest cyclic resistance was noted in tests with a principal stress rotation of ±45°. The increased susceptibility to liquefaction is possibly due to factors such as the inclination of the plane of maximum shear stress with the bedding plane, inclination of major principal stress with the bedding plane, the presence of horizontal shear stress, and the nature of the variation of shear stress on the weak bedding plane

Shear and dewatering behaviour of densified gold tailings in a laboratory simulation of multi-layer deposition

Tailings may undergo desiccation stress history under varied climatic and depositional parameters. While tailings substantially dewatered prior to deposition may experience desiccation under the greatest range of climatic variation, even conventionally deposited tailings may desiccate in arid climates at lower rates of rise. Bench-scale research has shown that the stress history imparted by desiccation substantially improves strength in gold tailings. The present study further investigates this phenomenon by simulating multi-layer deposition of high-density tailings using a modular drying box, 0.7mby 1 m inplan. The box is instrumented for directly measuring evaporation, drainage, water content, vertical volume change, and matric suction. Additional measurements included total suction at the surface as well as observations of crack development. The dewatering behaviour conforms to that predicted by previously published generic modelling, specifically that the presence of partially desiccated tailings initially accelerates, but then decelerates dewatering of fresh tailings. The shear behaviour of samples obtained using buried tubes and by driving thin-wall tubes into the multi-layer simulation are compared with shear behaviour of samples from bench-scale experiments. Shear strength of samples from the multi-layer simulation is independent of the sampling method, and shows higher strength than the bench-scale samples. The higher strength may be due to the greater number of wet-dry cycles or other age-related processes

Confining stress and static shear effects in cyclic liquefaction

Liquefaction resistance of a sand under cyclic loading is assessed and the effects of the levels of confining pressure and static shear on resistance to liquefaction are investigated. Site-specific values of the resistance under specified levels of confining and static shear stresses are measured in the laboratory. The measured values are compared with those which would be predicted by the application of empirical multiplying factors K σ and Kα to the reference resistance at 100 kPa effective confining stress with no static shear. It is shown that Kσ and Kα are not independent, as assumed in current practice. The combined factor Kσ × Kα resulting from the empirical method is shown to underestimate the cyclic resistance ratio regardless of the initial density and confining and static shear levels. The degree of conservatism is most dramatic at looser density states