Doctor of Philosophy
Civil and Environmental Engineering
M. Hesham El Naggar
Large-capacity helical piles can provide immense construction and performance advantages over the conventional concrete and steel piles. Nowadays, there is significant interest in using large-capacity helical piles to support foundations that would be subjected to both dynamic and static loading.
The main objectives of this thesis are to: investigate the dynamic response and impedances of large-capacity helical piles; develop an analysis methodology for their dynamic response; and investigate their static axial compression and lateral behaviour, considering installation effects on their dynamic and static performances. The thesis presents the first full-scale vertical and horizontal dynamic field testing program executed on large-capacity helical piles, which involved 190 full-scale field load tests on nine instrumented large-capacity helical piles and two driven steel piles with different geometrical configurations installed in cohesive soils. Six piles were tested two weeks after installation and four piles were tested after allowing a recovery period of nine months following installation.
One hundred and seventy six field experiments were conducted to evaluate the dynamic response characteristics of single helical piles and driven piles under different levels of vertical and horizontal harmonic excitations. The effects of various parameters, namely: pile length, number of helix plates and inter-helix spacing, excitation intensity, and soil thixotropy on the dynamic response were investigated. The experimental results were compared to the theoretical predictions of the continuum theory considering linear and nonlinear approaches. Reasonable match was found between the predicted response using the nonlinear approach and the measured response for both vertical and horizontal vibrations. The results demonstrated the significant effects of pile installation on forming weak soil zone around the pile, which stiffened with time following installation. This stiffening was manifested in an average increase in pile stiffness of about 43% and in pile damping of 25 to 90% within a nine month period. In addition, the dynamic response of the helical piles was similar to that of the driven piles.
The load transfer mechanism of large-capacity helical piles was found to be predominantly through the helical plates and pile toe end bearing. Based on the results of the pile load tests, it is proposed to define the ultimate load of helical piles as the load that corresponds to pile head movement equal to the pile elastic deformation plus 3.5% of helix diameter. The helical piles displayed a superior axial performance with capacities higher than driven pile by about 17 to 85% based on pile configurations.
The effects of attached helices and inter-helix spacing were found to be negligible on the pile lateral capacity and performance. The lateral pile load tests were examined numerically using the p-y approach incorporated in LPILE program. The mobilized soil shear strength parameters and soil moduli of subgrade reaction were back-calculated.
Elkasabgy, Mohamed A., "Dynamic and Static Performance of Large-Capacity Helical Piles in Cohesive Soils" (2011). Electronic Thesis and Dissertation Repository. 193.
Available for download on Sunday, December 31, 2017