Enhanced analysis of load transfer mechanism and deformation estimation for ground improvement using concrete injected columns

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This thesis presents analytical solutions to predict the response of the load transfer platform (LTP) on columns stabilised soft soil subjected to any shape of pressure loadings. The effect of the bending and shear deformations of LTP and the nonlinear stress strain behaviour of soft soil are incorporated into the analytical model. The cracked reinforced Timoshenko beam is proposed and implemented to model LTP to consider the shear and flexural deformations. Soft soil is idealised by spring-dashpot system to include the time-dependent non-linear behaviour. The columns and geosynthetics are modelled with linear Winkler springs in the applied range of stresses and rough elastic membrane, respectively. Influence of negligible tensile strength compared to the compressive strength of granular materials in LTP is also considered. Furthermore, a parametric study has been conducted to investigate how the parameters such as the column spacings, the thickness of LTP, the tensile stiffness of geosynthetics, and the degree of consolidation of the soft soil affect the response of LTP on improved soft soil. Moreover, the results from the proposed cracked Timoshenko beam theory (capturing the combined shear and bending stiffness of LTP) have been compared with results from the Euler-Bernoulli model (capturing deflection due to bending only) and the Pasternak model (capturing deformation due to shear only). This research also provides rigorous solutions to estimate the settlement of the soft soils under embankment load when double layer of geosynthetics reinforcements have been used in the load transfer platform. The response function of the system in plane strain condition has been attained by developing governing differential equations for the proposed mechanical model and its solutions. To develop analytical equations, the basic differential equations of a Timoshenko beam subjected to a distributed transverse load and a foundation interface pressure, generated from the Kerr foundation model is applied. Furthermore, the suitability of the Kerr foundation model for engineering calculations of LTP are evaluated. In addition, the results from the proposed model simulating the soft soil as the Kerr foundation model are compared to the corresponding solutions when the soft soil is idealised by Winkler and Pasternak foundations. Additionally, to assess the overall behaviour of the multilayer geosynthetic reinforced granular layer as well as that of the single layer geosynthetic reinforced granular layer parametric studies are also carried out. The developed analytical model can be applied by practicing engineers to predict the deflection of the LTP and mobilised tension in the geosynthetic reinforcement. In addition, this research presents the results of a numerical investigation into the performance of geosynthetic-reinforced column-supported embankment in soft ground. A three-dimensional finite-element model was employed to compare the results with a case study on a number of governing factors such as the downward and lateral movement of soft soil, the stress transferred to column, and the developed excess pore water pressure. The soft soil is represented by the Modified Cam-Clay model (MCC) while the linear elastic and perfectly plastic model adopting the failure criterion of Mohr-Coulomb is applied for medium dense to dense gravel, cobble soil, the granular platform and the embankment. By adopting Hoek-Brown model (HB) to simulate concrete injected columns, non-linear stress-strain relationship is considered in this study. It should be noted that the geometry and other physical properties of the soils and columns considered in this study have been adopted from Gerringong upgrade project, a ground improvement mission taken place in New South Wales, Australia.
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