Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

El Naggar, M. Hesham

Abstract

Full-scale shaking table testing was conducted to evaluate the seismic performance of single and grouped helical piles. Eight circular and one square helical piles with different properties as well as one driven circular pile were installed in dry sand enclosed in a laminar soil shear box that was situated on the shaking table. Dynamic properties of sand bed and its natural frequency, as well as the natural frequencies of single and grouped helical pile-soil systems were evaluated from the collected data during different shaking events. The effects of different pile configurations as well as successive shakings on the characteristics of the sand bed and pile-soil systems were also investigated. In addition, responses of single and grouped helical piles were computed analytically using the software DYNA6, which agreed with measured responses. These analyses accounted for degradation of soil stiffness and gap opening. Furthermore, effects of the earthquake characteristics (i.e., intensity and frequency content) on seismic performance of single and grouped helical piles were evaluated from the measured responses. The performance characteristics of helical pile groups were discussed in terms of the interaction between piles within a group and the contributions of vertical and lateral stiffness of individual piles to the rocking stiffness and the overall capacity of the pile group. The effect of pile head connection to the pile cap (1-bolt connection versus 2-bolt connection) was evaluated and compared. In addition, behaviour of a single pile and a pile within a group were compared in terms of their normalized responses. A three-dimensional nonlinear dynamic finite element model was constructed employing the software ABAQUS to simulate the shake table testing. The numerical model was validated with the experimental results, which was then used to perform an extensive parametric study. The parametric study explored the effect of the level of a single helix, diameter of a single helix, number of helices, spacing between helices as well as the effect of the pile diameter. Results revealed the superior performance of helical piles under strong ground shakings due to the significant contribution of the helices to the rocking resistance of the pile group.

Summary for Lay Audience

Helical piles are considered a reliable and cost-effective alternative to conventional driven piles because of their fast installation, lower cost and lower labour risk. They are suitable for retrofitting existing deficient foundations because they require smaller installation equipment that cause minimal vibration and noise during installation. Observations from recent strong earthquakes have demonstrated excellent performance of structures supported on helical piles with negligible damage. However, some buildings in the same areas that were supported on conventional reinforced concrete piles collapsed. This motivated the research community to explore the reasons why helical piles substantially performed well during seismic events. This study aimed at understanding the behaviour of single helical piles and helical pile groups during a strong earthquake shaking. Results from full-scale testing as well as numerical modelling revealed the superior performance of helical pile groups under seismic loading.

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