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Monotonic and cyclic shear loading response of natural silts from British Columbia, Canada Verma, Priyesh


An experimental research program comprising constant-volume monotonic and cyclic direct simple shear (DSS) testing was undertaken to advance the knowledge on the mechanical response of fine-grained silts and silt-clay mixtures. Natural silts originating from different geographic locations within British Columbia, Canada were used as basic test materials. The effects of vertical effective consolidation stress (σ'vc), initial static shear stress bias (α), shearing direction, soil plasticity, and clay mineralogy on the mechanical behavior of silts and silt-clay mixtures were systematically investigated. Under monotonic shear loading, low-plastic, non-sensitive, normally consolidated (NC) silts under varying σ'vc and α values were found to normalize by σ'vc. This normalized response was not observed for high-plastic, sensitive, NC silt up to a threshold σ'vc value - due to ‘destructuration’ of initial soil fabric due to shearing; as the σ'vc increased beyond the threshold, the normalized response seemed to gradually emerge – suggesting full ‘destructuration’ of the initial soil structure under higher σ'vc. With respect to cyclic shearing, the NC silts and silt-clay mixtures displayed a cumulative increase in excess pore-water pressure with associated progressive degradation of shear stiffness with increasing number of loading cycles. This cyclic-mobility-type response was generally observed in all the tested fine-grained soil specimens regardless of the magnitude of σ'vc, α, shearing direction, PI, clay-type, and applied cyclic stress ratio. The cyclic resistance ratio (CRR) of low-plastic, NC silts was found to be insensitive to the σ'vc level, whereas the CRR was found to decrease with increase in α. The CRR of the silt-clay mixtures appeared to be relatively insensitive up to 20% clay proportion by weight regardless of the clay type. Beyond this threshold, the CRR increased with increase in illite content, and it decreased with increase in kaolinite content. The CRR of silt-clay mixtures of similar composition was found to decrease with increasing α for the tested range of 0 ≤ α ≤ 0.10. Complementing the above, the equivalent number of uniform cycles versus earthquake magnitude (Mw) relationships were also examined for fine-grained soils, especially including the strong ground motion time histories from latest subduction zone earthquakes with Mw > 8.0.

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