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Cyclic shear loading response of Fraser River Delta silt Sanin, Maria Victoria

Abstract

The cyclic shear response of silt obtained from a natural channel-fill soil deposit in the Fraser River Delta of British Columbia, Canada, was investigated using the direct simple shear (DSS) test apparatus. Constant volume cyclic DSS tests conducted on undisturbed silt specimens consolidated to a vertical stress similar to, or above, the in situ effective overburden stress exhibited cyclic mobility type progressive strain development and equivalent pore water pressure rise. The observations are similar to the behaviour observed for dense sand by others. The cyclic resistance ratio (CRR) of the tested material indicated no significant sensitivity to the initial confining stress level for stress levels below 200 kPa, suggesting that the response is similar to that of normally consolidated clay. The cyclic resistance ratio of the silt increased with increasing overconsolidation ratio (OCR) for OCR greater than 1.3. The silt specimens that developed high equivalent pore water pressures during cyclic loading, suffered significant volumetric strains during post-cyclic reconsolidation, indicating considerable changes in the particle fabric under cyclic loading. Repeated cyclic loading (re-liquefaction) tests conducted after post-cyclic reconsolidation displayed a reduction in CRR in comparison to that noted under first cyclic loading. The decrease in CRR due to the degradation of particle fabric as a result of previous shearing appears to have overshadowed any gain in CRR that would have taken place due to the reduction of void ratio during re-consolidation. The commonly used empirical liquefaction criteria displayed limitations in describing the cyclic response of the tested silt. Laboratory element testing that allows capturing the effect of most controlling parameters seems to remain as the more reliable approach for estimating liquefaction susceptibility of fine-grained soils.

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