• Hingeneering

Soil Liquefaction Considerations at Burrard Inlet, False Creek, and the Fraser River

Liquefaction is the process by which the soil sediments below the water table temporarily lose Its strength due to the application of earthquake-induced cyclic shear stresses, and behave as a viscous liquid rather than soil (1). Liquefaction phenomenon can lead to slope instability, lateral spreading of ground, settlement, increased lateral loads on retaining walls and piles, and loss of foundation support.

Preliminary assessment on liquefaction susceptibility, i.e. the ability of the soil to liquefy when subjected to an applied stress, can be carried out through existing literature, and maps, for example the GeoMap Vancouver (2), or the Liquefaction Susceptibility map included in the Seismic Design Guidelines for Dikes (3).

Generally, because of the type of sediments found in Burrard Inlet, False Creek, and the

Fraser River, the potential for soil liquefaction should be investigated. The Commentary on the Canadian Highway Bridge Design Code (1), and the Seismic Design Guidelines for

Dikes (3), describe several methodologies to determine the potential for soil liquefaction

under a seismic event of a certain magnitude. Where necessary, the design of Waterfront

Structures shall allow for the effects of liquefaction.

Liquefaction and lateral spreading for Jetty Foundation

In the case of the jetty foundation, liquefaction induced by seismic activity may cause

uncontrolled or differential settlement and temporary loss of bearing capacity. To investigate the risk of any of these mechanisms to affect the stability of the jetty and its foundation, a liquefaction potential (LP) analysis of the soil is carried out according to the seismic events as outlined in Canadian Highway Bridge Design Code. The LP of a soil is defined as the ratio between the Cyclic Resistance Ratio CRR (the capacity of the soil to resist liquefaction) and the Cyclic Stress Ratio CSR (the seismic demand on a soil layer). A commonly used safety factor worldwide for this susceptibility ratio is 1.2.

[1] CSA-S6.1, 2014, Commentary on CSA-S6: Canadian Highway Bridge Design Code.

[2] Geological Survey of Canada, 1998, GeoMap Vancouver: Geological Map of the

Vancouver Metropolitan Area.

[3] Golder Associates, 2014, Seismic Design Guidelines for Dikes 2nd Edition.

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