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Total stress dynamic analysis of Coquitlam Dam Dharmasetia, Charissa W.

Abstract

Earthquake-induced liquefaction of loose saturated sands can cause large deformations to occur resulting in flow slides. The near catastrophic failure of the Lower San Fernando Dam due to the 1971 earthquake is one of the most well known examples of a flow slide. However, the concern is not limited to flow slide situations in which the driving stresses are greater than the residual strength after liquefaction. Significant deformations can occur for a non-flow slide condition. These are caused by a reduction in stiffness of liquefied materials as well as from inertia forces caused by the earthquake motion. Limit equilibrium type analyses have been used reliably in the prediction of the occurrence of a flow slide. However, difficulty has been encountered in the prediction of earthquake-induced deformation in the event that a flow slide is not predicted to occur. The problems have arisen from the modeling of the behaviour of liquefied soils. Soil exhibits stiff behaviour under cyclic loading and the strains associated up to the point of triggering of liquefaction are small. Upon liquefaction triggering, soil behaves like a liquid and initially strains under very small shear stresses. However, upon further deformation, the soil dilates and regains stiffness and strength. Most deformation analysis procedures overly simplify the effects of earthquake inertia forces and do not adequately model the stress-strain behaviour of liquefied soil. The proposed total stress procedure attempts to take account of both of the above effects. The procedure is separated into three main phases of cyclic-induced liquefaction behaviour of sands: the pretriggering, triggering, and post-triggering response of soils. The triggering of liquefaction in each element of a soil structure is predicted by weighting the cyclic shear stresses induced by a prescribed base motion. Upon triggering of liquefaction, liquefied stress-strain parameters are assigned to zones predicted to liquefy as they occur. The pre-triggering and triggering phases of the procedure were verified using SHAKE analyses. Similarly, the post-triggering phase of the procedure was compared with results obtained from Bartlett and Youd's empirical equation. In both cases, reasonable agreement was found. The method was finally applied to the Coquitlam Dam and the results compared with two of the more commonly used deformation analyses such as variations of the Modified Modulus method and Jitno and Byrne's extended Newmark method. The predicted results from all three methods are in reasonable agreement.

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