Advanced constitutive modeling of sands and applications to foundation engineering

Dimitrios Loukidis, Purdue University

Abstract

A bounding-surface plasticity constitutive model based on critical-state soil mechanics is used for the mathematical simulation of the mechanical response of sands. Modifications and additions have been made to previously published versions of the model in order to improve its versatility and performance. The model is shown to simulate satisfactorily the mechanical response of sands at small and large strains under various loading conditions, while taking into account the inherent and stress-induced anisotropy of the sand. ^ The constitutive model is calibrated against experimental data for Toyoura sand, clean Ottawa sand, and mixtures of Ottawa sand with non-plastic silt. The constitutive model has been implemented in a finite element code in order to perform analyses of boundary-value problems related to the field of foundation engineering. A semi-implicit stress-point algorithm is used for the integration of the constitutive stress-strain rate equations, which has been enhanced by a sub-stepping scheme with error control for increased accuracy. ^ Finite element analyses of shearing along the shaft of a non-displacement pile are performed in order to examine the development of limit shaft resistance and the changes in stress state around the shaft upon axial loading of the pile. Based on these simulations, a relationship between the initial density and stress state of the sand and the lateral earth pressure coefficient at limit-shaft-resistance conditions is proposed. The finite element simulations also shed light on the issue of scale effects arising in small-scale model pile tests. ^ Finally, finite element analyses are performed to study the factors affecting the bearing capacity of strip and circular footings resting on the surface of a sand layer, with special focus on the effect of the sand density and footing size. The present numerical study provides useful insight on the validity of the bearing capacity factors used in practice, and on the choice of the friction angle to be used in the classical bearing capacity equation. Equations for the bearing capacity factor Nγ and the shape factor sγ that take into account the effects of relative density and pressure level are also established.^

Degree

Ph.D.

Advisors

Rodrigo Salgado, Purdue University.

Subject Area

Engineering, Civil

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