Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor(s)

Dr. M. Hesham El Naggar

Abstract

Micropiles are used in various applications, including low capacity micropile networks, underpinning and seismic retrofitting of existing foundations and high capacity foundations for new structures. They facilitate fast installation with a high degree of ground improvement. The current Federal Highway Administration (FHWA) design guidelines designate hollow bar micropiles as Type B micropiles, even though their construction technique is different than typical Type B, which results in overly conservative design. In addition, the current practice for construction of hollow bar micropiles is limited to drilling bit/hollow bar diameter ratio of 2.5 or less. In this thesis, full-scale load tests were conducted in order to: evaluate the suitability of FHWA design guidelines to hollow bar micropiles installed in cohesive soil; and to evaluate the performance of hollow bar micropiles constructed with drilling bit/hollow bar diameter ratio of 3. The load tests included axial monotonic and cyclic axial loading and monotonic lateral loading. Eight micropiles were constructed using 76 mm (3 in) hollow bars (76 mm OD and 48 mm ID) with air/water flushing technique and advanced to a depth of 5.75 m: six micropiles were installed using 228mm (9 in) drill bit and two micropiles were installed using 178 mm (7 in) drill bit. All micropiles were instrumented with vibrating wire strain gauges to measure the axial strain at three stations along the micropile shaft. The axial load test results are presented and discussed in terms of load-displacement curves and load transfer mechanism. The load tests results showed that the grout/ground bond strength values proposed by FHWA (2005) for Type B micropiles grossly underestimate the bond strength for calculating the ultimate capacity. In addition, the toe resistance can be significant for micropiles resting on sand due to the increased toe diameter. No stiffness degradation was observed in the micropile capacity after applying 15 load cycles. Finally, lateral capacity of micropiles is moderate due to their small diameter. However, the larger drilling bit resulted in enhanced lateral performance and increased capacity due to the larger diameter. In addition, using fibre reinforced grout can increase the micropile lateral capacity and enhance its ductility.


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