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A constitutive model for sand and the analysis of the canlex embankments Puebla, Humberto

Abstract

A numerical analysis procedure including a constitutive model for predicting static liquefaction occurrence and liquefaction-induced displacements is presented in this dissertation. The main features of sand characteristic behaviour, as observed from laboratory element tests, are first identified to establish the most important issues regarding static liquefaction analysis. Based on physics fundamentals and elastic-plastic theory, a relatively simple constitutive model capable of capturing sand characteristic behaviour is then proposed. The model has two independent plastic components, a shear mechanism and a volumetric mechanism. The plastic shear mechanism is governed by a hyperbolic relationship between the stress ratio and plastic shear strain, and a flow rule for estimating shear-induced plastic volumetric strain from plastic shear strain. In addition, it has the ability to handle some of the effects induced by inherent anisotropy and rotation of principal stresses. The plastic volumetric mechanism responds to a power law that relates the cap pressure to the plastic volumetric strain induced by compressive loading. The proposed constitutive model is incorporated into a commercially available computer program (FLAC). The code uses a finite differences method that satisfies dynamic equilibrium using a step-by-step time domain procedure and a groundwater flow technique, allowing coupled stress-flow analyses to be performed. The proposed model is calibrated using data from conventional laboratory tests. The model is shown to capture reasonably well the drained and undrained characteristic response of Syncrude sand as observed from element tests, over a range of confining stresses and relative densities. The calibrated model is subsequently used for modelling the Canadian Liquefaction Experiment (CANLEX) embankments that included a field event in which a test embankment was built over a loose sand foundation layer, and a centrifuge test performed on a sand model of the prototype structure. Both earth-structures were planned to induce a static liquefaction failure and their responses characterized by the observed displacements and pore pressures. The proposed constitutive model and numerical procedure used to simulate the CANLEX embankments are shown adequate for performing analysis of sand liquefaction, triggered by rapid monotonic (static) loading. The results from modelling both embankments are in reasonable agreement with the measured responses.

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