Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

M Hesham El Naggar

Abstract

Micropiles offer an efficient load transfer mechanism, which facilitates carrying considerable load in compression and tension. Recently, hollow bar micropiles (HBMP) have gained wide acceptance due to their fast installation and small installation equipment, which allows installation in sites with limited access. The HBMP is commonly constructed using a drill bit/ hollow bar diameter ratio (Db/Dh) of around 2, and they are typically designed assuming type B micropiles as classified by the Federal Highway Administration (FHWA). However, increasing the drill bit diameter should increase the micropile diameter and hence may enhance its performance and increase its capacity for a minimal increase in cost. In addition, considering their unique construction method, which is different than other types of micropiles, specific design guidelines for single and groups of HBMP are still largely missing. Thus, this thesis presents the results of laboratory and field testing programs that were conducted to investigate the performance of single and groups of HBMP. In addition, three-dimensional nonlinear finite element analyses were conducted to further explore the different parameters that affect the performance of HBMP. The effects of increasing the Db/Dh ratio from 2.25, which represents the current practice, to 3 as well as the micropiles spacing were investigated. Twenty two full- scale HBMP’s were installed in cohesionless soil, six of which were single micropiles and the remainder were divided into four micropile groups. Each group was comprised of four micropiles arranged in a square configuration. Two single micropiles and two groups contained micropiles constructed with a drill bit of Db = 115 mm. The other four single micropiles and two groups contained micropiles constructed with a drill bit of Db = 152 mm. One group of each set was installed with a spacing to micropile diameter ratio (S/Db) of 3, and the other group with an S/Db ratio of 5. The single micropiles were subjected to compression, tension, and then lateral load tests while the micropile groups were subjected to axial centric monotonic compressive loading.

The results demonstrated that increasing Db/Dh to 3 improved the micropiles performance and increased their compression and uplift capacities. The axial stiffness of single micropiles increased by 38 % and 32% in compression and uplift, while their capacity increased by 17% and 22.5%, respectively. The obtained results from the lateral load tests indicated that increasing Db/Dh to 3 improved the lateral capacity by about 32% and the piles were substantially stiffer. As expected, micropile groups constructed with the large diameter drill bits displayed higher stiffness and load carrying capacity than the groups constructed with small diameter bits, which confirms the effectiveness of using a larger drill bit. The axial group stiffness increased by 41 % and 59 % as the Db/Dh increases from 2.25 to 3 for groups constructed at s/D = 3 and 5, respectively. In addition, the group efficiency ratio values at both the working load and ultimate capacity were found to be close to unity for all groups. Finite element simulations investigated the effect of micropile installation in cohesionless soils. In addition, the model was used to extend the load – settlement curves to the failure load when it was not possible to load the micropiles to failure during field testing because of equipment limitation. The numerical model results confirmed that the micropile group efficiency is equal to unity for micropiles in a 2x2 and 3x3 arrangement.

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