ABSTRACT: Liquefaction and seabed instability in the Fraser River delta

CGRG Bibliography of Canadian Geomorphology
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Flow liquefaction failures of sand deposits in the Fraser River Delta are a major concern for the stability of coastal structures. These failures are believed to be triggered by environmental processes, such as rapid sedimentation, surface waves, drawdown effects associated with large tidal amplitude changes, was in the sediments, tidal currents, dredging and the interaction of these events. An investigation to evaluate the possible contributions of these triggering mechanisms in a major liquefaction failure that occurred in 1985 has been carried out in this thesis. The background for explaining liquefaction failures in the Fraser River Delta (FRD) is based on steady state concepts. Flow liquefaction is the strain softening and collapse of a loose sand to its steady state. Steady state divides sand into two possible states, that is, contractive and dilative. A contractive sand with its driving stresses greater than its steady state strength has a potential for flow liquefaction. A dilative sand requires large strains to reach its steady slate and, hence may not be susceptible to flow liquefaction. The concepts of collapse surface and contractant state boundary are discussed in explaining the triggering of flow liquefaction in contractive sands. Different processes influence the deposition of sand at the mouth of the Fraser River in the delta. Explanation of the triggering of the flow slides in sand deposits of the delta requires knowledge of steady state parameters. Laboratory test results on the Fraser River Delta sand ate presented and discussed. A chart is developed to characterize the in-situ state of deposits of the FRD. In-situ test results, such as Cone Penetration Tests and Seismic Cone Penetration Tests are performed and evaluated for flow liquefaction susceptibility of the deposits in FRD. The evaluation shows that the sediments are prone to flow liquefaction. The potential triggering mechanism, the stress state in the deposits is established using finite element stress analysis. The stress state is evaluated using the chart developed from laboratory tests, conducted as pan of this study. The analysis shows that the sedimentation builds up shear stresses, but can not initiate flow liquefaction failures. The evaluation for the effect of wind generated surface waves in causing deep seated liquefaction flow slide is aided by a field study for wave induced porewater pressures in the delta. The study indicates that no significant porewater pressures accumulate due to surface waves and hence, surface waves can not trigger deep seated flow liquefaction failures. The effect of tides in causing the flow slides in the sediments of FRD is evaluated. The tides can not initiate failure in submerged saturated sands. However, gas induces destruction of the sediments and the effect of tides on gaseous sediments is investigated. It is found that the presence of gas induces residual pore pressures in the sediments during low tides and contributes to the triggering of flow liquefaction failures in the delta. These failures lead to retrogressive flow slides.

Bibliography of Canadian Geomorphology