Cyclic failure of low plasticity silt

摘要

Samples of a low plasticity silt were isotropically and anisotropically consolidated and subjected to cyclic triaxial loading in order to establish the liquefaction characteristics. The development of instability in the accumulation of strains and pore water pressures under cyclic loading was related to characteristic points on monotonic undrained stress paths. A sample preparation method using sedimentation was developed compared to a conventional one-dimensional pre-consolidation preparation technique. Sedimented silt behaved like a lightly overconsolidated material while 1-D pre-consolidated samples had heavily overconsolidated characteristics with an associated risk of optimistic predictions of seismic susceptibility. The cyclic shear behaviour of the silt samples prepared using the sedimentation method was examined for samples consolidated at various initial sustained deviator stress ratios q_s/p′_c. Two different failure criteria were defined: double amplitude axial strain ε_(a,DA) = 5% for reversal conditions; or axial plastic strain ε_a, P = 5% for non-reversal. The peak excess pore pressure Δu_p increased almost linearly with the number of cycles until it showed a sudden increase just before reaching a terminal value which decreased with increasing q_s/p′_c. The sudden increase in Δu_p was associated with flow or limited flow deformation of samples. For isotropically consolidated samples the deviator stress ratio at the initial phase transformation in compression determined from undrained monotonic extension tests was the boundary between the stable and contractive behaviour. When the deviator stress ratio exceeded the (q/p′)IPT in extension, the sample liquefied shortly afterwards. For anisotropically consolidated samples under stress reversal cyclic loading liquefaction failure was induced when the stress path intersected the bounding surface formed by undrained monotonic compression tests. For anisotropically consolidated samples under non-reversal cyclic loading the stress path showed a marked contractive behaviour when intersecting the bounding surface formed by undrained monotonic compression tests. This led to cyclic compressive failure.