UBC Theses and Dissertations
Cyclic loading response of Fraser River sand for validation of numerical models simulating centrifuge tests Sriskandakumar, Somasundaram
Cyclic loading response of Fraser River sand was investigated using the U B C direct simple shear (DSS) device as input to numerical simulation o f centrifuge physical models. A simple air-pluviation method was developed to reconstitute laboratory sand samples replicating the soil fabric anticipated in centrifuge specimens. Constant-volume (undrained) tests were conducted with and without initial static shear stress condition at loose and dense density states. While the observed trends in mechanical response were similar, the loose air-pluviated samples were more susceptible to liquefaction under cyclic loading than their water-pluviated counterparts. The differences arising from the two sample re-constitution methods can be attributed to the differences in particle structure, clearly highlighting the importance of fabric effects in the assessment of the mechanical response of sands. Densification due to increasing confining stress (stress densification) significantly increased the cyclic resistance of loose air-pluviated sand with strong implications in relation to the interpretation of observations from centrifuge testing. This effect, however, was not prominent in the case of water-pluviated or dense samples. The initial static shear stresses reduce the cyclic shear resistance of loose air-pluviated sand in simple shear loading, in contrast to the increase in resistance reported based on data from triaxial testing. Dense sands indicated a increase in cyclic resistance in the presence of initial static shear stress. Previous cyclic loadings generating high excess pore water pressures (or significant shear strains) reduced the liquefaction resistance of sand against future cyclic loading, while cyclic loading generating small excess pore water pressures (or small shear strains) increased the liquefaction resistance of sand. However, densification due to post-cyclic consolidation, sometimes, contributed to increasing the liquefaction resistance by compensating for the weakening in the liquefaction resistance due to the large pre-shearing. The volumetric strains accumulated during drained cyclic loading were independent of the time rate of shear strain, and they increased with increasing shear strain amplitude and the number of cycles. The proportionality of shear-induced volumetric strain to the cyclic shear-strain amplitude, based on drained cyclic shear tests with small shear strain amplitudes, was not observable when the material is subjected to relatively large amplitudes of cyclic strain. A three-parameter shear-volume coupling model was developed for cyclic loadings associated with large strains.
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