Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. M. Hesham El Naggar

Abstract

A micropile is a small diameter “cast-in-place” pile, which was initially used to repair deficient foundations. The overall performance of a micropiled raft (MPR) foundation system is similar to a piled raft foundation where the load is transmitted through both the raft and the micropiles. This thesis explores using micropiled rafts (MPR) as a new highly efficient foundation system that combines the advantages of the piled raft system and the efficient installation of micropiles and associated ground improvement. Currently, there is no guidance available regarding the performance of MPR foundations. Therefore, the main objectives of this research are to evaluate the behaviour of MPRs in sand and clay soils and examine the effects of different parameters on their performance.

The research methodology comprised of three primary aspects: performing a series of geotechnical centrifuge tests on MPRs and comprehensive soil characterization in order to obtain experimental results and necessary soil parameters for numerical modeling, developing, calibrating and verifying a three-dimensional finite element model (3D FEM); and conducting a comprehensive parametric study on the behaviour of MPRs in sand and clay soils using the FEM. Four MPRs centrifuge tests were conducted: three tests in sandy soil and one test in clay soil. In addition, single micropile and isolated raft foundation centrifuge tests were carried out in both sand and clay soils.

The results of the centrifuge tests were used to calibrate and verify the non-linear three-dimensional finite element models for both the sand and clay soils. Subsequently, the verified models were employed to conduct a comprehensive parametric study. The parametric study focused on providing additional insights regarding the performance of micropiled raft system that should be helpful for the design engineers. The physical dimensions of MPR structural components and parameters considered in the parametric study are within the range used in the current practice. The results of the centrifuge tests and numerical parametric study were analyzed to establish design guidelines for micropiled rafts.

It was found that the tolerable bearing pressure of MPRs increased by as much as 191% and 101% compared to isolated rafts in sand and clay, respectively. In addition, the load carried by the raft in a MPR depends primarily on the micropile spacing and ranges between 20% and 80% of the applied load. Equations are proposed to evaluate the percentage increase in tolerable bearing pressure (PIBP) and the load carried by the components of MPR for different types of soils due to change in the micropile spacing and raft thickness. The Poulos-Davis-Randolph (PDR) method was found to be able to evaluate the performance of a MPR system with relatively stiff rafts. However, the error margin increases up to 28% for a MPR with a flexible raft. An adjustment factor is proposed to account for the raft flexibility in the PDR method, which reduces the error in estimating the axial stiffness of MPRs with a flexible raft to only 3%.


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