Modeling of sand columns in soft clays under drained loading conditions

Abstract

Sand columns are used as a ground improvement method to enhance the mechanical properties and speed up the consolidation of weak cohesive strata. Some of the research studies on sand columns have used the finite element method (FEM) in analyzing the behavior of reinforced clays, where the sand was modeled using conventional constitutive models which do not account for the post peak strain softening that generally occurs in compacted sands. Most of the studies to date adopted the elastic perfectly-plastic Mohr coulomb model, with a few using hyperbolic constitutive models. The recently developed hypoplastic model provides the ability to account for the post peak strain softening in sands. To our knowledge, the hypoplastic model for soils has never been used in the analysis of problems involving sand columns in soft clays. The objective of this thesis is to investigate and predict the drained load response of clay specimens that are reinforced with sand columns of different diameters, heights, and confinement conditions using the FEM. The Hypoplastic soil model will be the primary model to be used in the FEM, where the Mohr Coulomb and Hardening soil models will be used in some parts for comparison purposes. The first stage of the study is comprised of modeling in Plaxis 2D a series of triaxial tests that were previously performed on normally consolidated Kaolin clay specimens reinforced with Ottawa sand columns, where the area replacement ratio, the column penetration ratio, and the confining pressure were varied. This will be the first research study to use the hypoplastic model in modeling sand columns using the FEM software Plaxis2D and to compare the FE-predicted load response to that measured in the laboratory. In the second stage of the study, a comprehensive 2D FEM analysis will be conducted to predict the load-response of a wider range of area replacement ratios and sand column penetration ratios. The main goal of the 2D FEM analysis is to generate representative soil models that can predict the behavior