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A synthesized approach for estimating liquefaction-induced displacements of geotechnical structures Beaty, Michael Hugh

Abstract

Liquefaction has caused many failures of buildings, bridges, and dams during earthquakes. Damage is caused by the soil displacements that follow the initiation of liquefaction. The potential for liquefaction-induced failures in seismic areas is a widespread concern since many structures were built when liquefaction and its effects were not well understood. Damage observed in a number of earthquakes, including 1971 San Fernando, 1989 Loma Prieta, and 1995 Hyogoken Nanbu (Kobe), has prompted a re-examination of a number of these structures. Many are being retrofitted at great expense. A key factor in such examinations is the magnitude and pattern of displacements arising from soil liquefaction. A total stress dynamic approach is presented for estimating these displacements from seismically induced liquefaction. The approach is derived from widely accepted assumptions for evaluation of liquefaction triggering, flow slide potential, and limited displacements. These different evaluations are combined into a single analysis while eliminating some of the inherent simplifications in current procedures. An explicit finite difference model is used with the earthquake motion applied to the base. Triggering of liquefaction in each element is continuously assessed by weighting each cycle of shear stress. Postliquefaction stiffness and strength properties are assigned to an element when sufficient cycles of shear stress have accumulated. Elements continue to liquefy and respond to inertia loads as the shaking proceeds, causing the displacements to increase with the duration of shaking. The proposed method is used to evaluate case histories and geotechnical structures. The approach is found to give reasonable predictions of displacements. The importance of various input parameters is investigated. Both residual strength and the character of the earthquake motion are found to be critical variables. Limitations of the method are also reviewed. The desirability of using a more fundamental and complex effective stress model to supplement this total stress approach is discussed. Such a combination of analyses may help identify key characteristics of the liquefaction response, particularly when the flow of pore water is a potential concern.

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