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UBC Theses and Dissertations

Shear loading response of sand-silt mixtures Soysa, Achala Nishan


A comprehensive experimental research program was undertaken to investigate the shear loading response of sand-silt mixtures with the objective of addressing the inconsistencies in the understanding of the mechanical behavior of these soils. Natural sand and silts originating from the Fraser River Delta in British Columbia, Canada was used as parent test materials to generate the full spectrum of mixtures needed for the study. The work included triaxial and direct simple shear tests on specimens reconstituted to achieve pre-selected sand-silt compositions. The reconstituted specimens were prepared using water pluviation, or modified slurry deposition/consolidation techniques. New experimental procedures to prepare sample, pour slurry, apply vacuum, and initially consolidate the slurry were developed to equitably accommodate the full range of sand-silt mixture specimens. The effect of fines content was systematically investigated for both monotonic and cyclic shear loading cases. The undrained shear strength mobilized at phase transformation (Su-PT) of the sand-silt mixtures decreased with increasing fines content (CF) in triaxial compression tests. In triaxial extension tests, the Su-PT increased with increasing CF; however, no noticeable upward or downward trend on the effect of CF was noticeable in the results from direct simple shear tests. The coarse-grain-based and fine-grain-based void ratios were observed to serve as better indices to examine the shear loading response of sand-silt mixtures. The cyclic resistance ratio (CRR) decreased with increase in the coarse-grain-based void ratio for predominantly sandy specimens. Similarly, the value of CRR decreased with increasing fine-grain-based void ratio for specimens having a silt dominant matrix. The shapes of typical stress–strain loops in the initial stages of constant shear stress amplitude cyclic loading tests, on both fine-and coarse-grained soils, were noted to be distinctly different from those observable during the later stages. Considering the number of loading cycles corresponding to the commencement of the transition point in this stress-strain pattern change as the instance of unacceptable performance, a new shear stiffness–based criterion was developed to determine cyclic resistance ratio CRR from cyclic shear tests; this provides a more robust engineering basis to determine the CRR than the strain-amplitude based criteria commonly used in current practice.

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