UBC Theses and Dissertations
Monotonic and cyclic shear loading response of natural silts Soysa, Achala Nishan
An experimental research program comprising constant-volume direct simple shear (DSS) tests was conducted to study the monotonic, cyclic shear and post cyclic consolidation response of natural silts. Relatively undisturbed samples of silt which were obtained from three different locations in the Lower Mainland area of British Columbia were used for this purpose. Plasticity indices of the natural silt samples which were considered for the study were 5, 7, and 34. Monotonic shear response of the natural silt was studied with the constant volume DSS test results that were conducted with different vertical effective stresses and different overconsolidation ratios (OCRs). Stress-strain response of normally consolidated silt at different consolidation stresses were found to be stress-history-normalizable where as higher OCR and higher plasticity resulted greater shear strength. Normally consolidated silt specimen, despite of their difference plasticity, exhibit gradual strain accumulation without abrupt loss of shear stiffness during cyclic loading with different cyclic stress ratios (CSRs) at different consolidation stress levels. The potential and rate of strain accumulation and development of excess pore-water pressure (Δu) were noted to be increased with higher CSRs at all tested consolidation stress levels. The cyclic shear resistances of silt, derived from cyclic direct simple shear (CDSS) tests, were not sensitive to the tested range of different consolidation stress levels, whereas higher plasticity resulted greater cyclic shear resistance. Relative undisturbed specimens exhibit comparatively higher cyclic shear resistance than the reconstituted specimens despite of comparatively denser particle arrangement in reconstituted specimens. However, during the constant-volume monotonic DSS tests, relative undisturbed specimens exhibit comparatively lesser shear resistance than the reconstituted specimens implying that soil fabric / microstructure plays a significant role in governing the shear loading response of silt. The examination of consolidation responses of silt specimens that were initially normally consolidated and subjected to constant-volume CDSS loading revealed that the post cyclic consolidation volumetric strain increases with the maximum cyclic pore-water pressure ratio developed during constant volume CDSS loading for all tested silt specimens with different plasticity.
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