Doctor of Philosophy
Civil and Environmental Engineering
El Naggar, M. Hesham
The thesis investigates the nonlinear soil-pile-structure interaction through three-dimensional nonlinear finite element models (FEM) employing the OpenSees platform. The FEMs were validated with the results of large-scale shaking table tests of model pile groups-superstructure systems in dry and saturated sand and large-scale field tests on single piles installed in cohesive soil. The numerical models correctly predicted the different pile deformation modes that were exhibited in the experiments. The results illustrated that the inertial interaction contributed to the bending moments at the pile top, while the kinematic interaction contributed to the bending moment at the layers interface. In addition, the excess pore water pressure and associated liquefaction during the shaking caused a dramatic decrease in the pile shaft friction resistance. Nonetheless, positive shaft friction was observed through the loose sand layer until the soil was fully liquefied. The larger pile diameters caused higher excess pore water pressure in the dense sand, which reduced the bearing pressure.
The lateral displacements of the helical piles (HP) and adjacent soil decreased compared to the driven piles. Meanwhile, HPs demonstrated excellent performance in maintaining their capacity during and after liquefaction and controlling the post-liquefaction settlement. The response of the HP groups in the saturated test was dominated by the rocking behaviour, while the flexural behaviour dominated the response in the dry tests. Moreover, the FEMs were employed to examine the effect of model scale on seismic response of prototype HPs. A set of scaling factors was suggested for extrapolating the soil and HP responses from 1g shake table model tests.
The seismic performance of bridge-helical pile foundation was explored based on the seismic fragility analysis. The results revealed that the HPs were the most fragile component in the non-liquefiable and liquefiable tests. The liquefiable soil decreased the seismic demand on the column lateral deformation and increase the demand dispersion. The reinforced concrete pier exhibited a large drift response in the non-liquefiable soil. Finally, the dynamic response of driven and HPs was explored in cohesive soil. The HP could replace the driven piles with shorter lengths, and it reduced the required width of the improved soil.
Summary for Lay Audience
This thesis examined the piles' behaviour during earthquakes in dry and saturated sand as well as clay soils. When the soil is loose sand and has water, the water pressure increases and the soil may loose most or all of its strength and acts like water, which is called soil liquefaction. Computer simulations were used to capture the behaviour of the piles and soils from experiments. The simulation was employed to gain deeper insights into the response of piles and soils under different conditions. The results demonstrated that the pile loses its side resistance concurrent with the water pressure generation, yet some pile resistance was observed until liquefaction occurred. The computer models were then used to investigate the response of piles to earthquakes with varying intensity, piles diameters, and the superstructure mass. It was found that the water pressure increased as the piles' diameter increased, which reduced the pile toe resistance.
The models were also employed to analyze the response of different pile configurations called helical piles (HP) in dry and saturated soils. The HP is a straight pipe with helices along its length. The results demonstrated the superiority of helical piles in enhancing the axial and lateral response of the piles in different ground conditions. In addition, different configurations of HPs including single and double helices were analyzed. It was found that the second helix further reduced the HPs settlement but had a negligible effect on the lateral deformation. Furthermore, the scaling effects of extrapolating the results of the helical piles and soils from the small scale of the experiments to prototype size were evaluated. The amount of distortion between the small experiments and prototype results was quantified and a set of scaling factors was provided to scale the results from experiment scale to full size. Furthermore, the seismic performance of bridges supported by HPs was examined through a probabilistic approach. The results showed that the HPs were the most fragile component in the dry and saturated tests and the concrete pier exhibited a large lateral response in the dry soil.
Finally, the dynamic responses of piles installed in clay soil were examined. The results demonstrated that the HPs exhibited excellent lateral and axial responses comparable to that of the regular piles. The HPs have a lower lateral deformation, experienced better axial response, and reduced the improved soil width compared to the regular piles.
Hussein, Ahmed Fouad, "Seismic Response of Driven and Helical Piles in Non-liquefiable and Liquefiable Soils" (2021). Electronic Thesis and Dissertation Repository. 8291.