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Behaviour of Helical Pile Groups and Individual Piles under Compressive Loading in a Cohesive Soil
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- Author / Creator
- Lanyi, Stephen A
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Helical piles are becoming increasingly common as a result of their wide range of foundation applications. This pile type consists of a steel shaft with one or more helical plates welded near the toe. The axial behaviour of this pile type is difficult to predict because the failure mode is dependent on many factors, including: pile geometry, pile load, soil stiffness, and the degree of installation disturbance. There is a lack of studies that have evaluated helical pile behavior while considering all of these factors. To resist large loads, helical piles are commonly installed in closely spaced groups. However, the engineering behaviour of pile groups, such as their: load-settlement response, installation-induced pore pressure response, effects of soil setup, and failure mode, has not been investigated in current literature. In the present study, the axial behaviour of single and grouped helical piles under compressive loading was investigated by conducting full-scale field tests at a cohesive soil site in Edmonton, Alberta. In the first phase of the test program, six single piles, instrumented with strain gauges along the pile shaft, were tested. The inter-helix spacing ratio (s/D) of the piles was varied at 1.5, 3, and 5, where s is the inter-helix spacing and D is the helix diameter. The pile failure mechanism was estimated by comparing the measured load distributions to predicted distributions. The results indicate that at loads below the ultimate state the individual bearing model dominated pile behaviour regardless of the s/D ratio; however, as the pile load increased, significant cylindrical shear resistance might develop. The bearing capacity factor and the adhesion coefficient were estimated based on the measured pile component loads and compared to recommended values. In the second phase, seven 2 × 2 helical pile groups were tested. The pile group spacing was varied. The results indicate that group interaction, resulting in a reduction in group performance, increased as group spacing decreased and as the group load increased. The group interaction of helical piles was less than that predicted for equally spaced conventional piles. The measured load distributions indicate that individual bearing failure occurred to a grouped pile with an s/D ratio of 5; however, the lower-helix resisted more load than by the upper-helix, compared to a single pile with the same s/D ratio. The measured group capacities and load distributions of the instrumented piles indicated that the grouped piles failed individually, as opposed to as a block. The effects of soil setup on the behaviour of single and grouped helical piles were evaluated by comparing the load-settlement response of tests occurring 2 to 5 hr after pile installation to comparable tests occurring at least 7 days after installation. Piezometers were installed at the center of selected groups and near a single pile in order to measure the excess pore pressure response to pile installation. The results show that excess pore pressure significantly reduced the performance of groups; however, the effects of excess pore pressure on single piles were limited to the soil very near the pile shaft and did not affect pile performance at the ultimate state. At the center of groups, the magnitude of excess pore pressure increased and the pore pressure dissipation time decreased as group spacing decreased.
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- Subjects / Keywords
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- Graduation date
- Fall 2017
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.