Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. Mohamed Hesham El Naggar

Abstract

The performance of a novel piling system is investigated, which involves a spun-cast ductile iron (SCDI) tapered shaft fitted with a lower helical plate. It combines the efficiency of the tapered section, the competitive cost, effectiveness and durability of spun cast ductile iron with a rough surface and the construction advantages of helical piles. The system is installed using a fast, low vibration and reduced noise process. Seven instrumented piles including five SCDI tapered and two steel straight pipes were installed in sand using mechanical torque. The piles were subjected to cyclic and monotonic compression, uplift and lateral load tests. Different loading sequences were adopted to assess the effect of prior cyclic/monotonic loading on the piles’ performance. The installation torque was monitored and the resulting capacity-to-torque ratio was compared to the literature reported values. The compaction of the previously disturbed sand from the helix penetration due to the pile taper resulted in superior compressive behavior of the proposed system compared to the straight shaft piles. The tapered piles exhibited higher stiffness at lower displacements compared to the straight shafted piles and the helix increased their uplift resistance. In addition, tapered shafts enhanced the lateral stiffness and the helix provided fixation due to the passive bearing pressures on the helix surfaces, which further improved the lateral performance of the short helical piles. A three dimensional finite element model was established and calibrated using the experimental data. The model was then used to simulate the response of SCDI piles with different configurations when subjected to different loading conditions including axial and lateral as well as combined moment-horizontal loads. Under cyclic loading, the tapered helical piles exhibited better compressive performance while the straight shaft helical piles performed better in uplift loading. The proposed system stiffness remained practically unchanged through the cyclic lateral loading applied in the current study. The monotonic performance of the tapered helical piles in clay was numerically simulated. The results showed an increase in axial and lateral capacity and stiffness of the tapered piles over the straight shaft ones, with greater uplift-to-compressive capacity ratio than in sand.


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