Master of Engineering Science
Civil and Environmental Engineering
Dr. M. Hesham El Naggar
Helical piles are currently considered a preferred foundation option in a wide range of engineering projects to provide high compressive and uplift resistance to static and dynamic loads. In view of the large capacity of large diameter helical piles, there is a need to determine their capacity using accurate and economically feasible testing techniques. The capacity of piles is usually determined by conducting a Static Load Test (SLT). However, the SLT can be costly and time consuming, especially for large capacity piles. The High Strain Dynamic Load Test (HSDT) evaluates the pile capacity using dynamic measurements generated through subjecting the pile to an axial compressive impact force by means of dropping a hammer at its head. The objective of this study is to investigate the performance and effectiveness of HSDT of helical piles using mathematical and numerical methods. Several case studies were examined to validate the mathematical model. The calculated pile responses were compared with the observed behavior during the actual HSDT. The mathematical model was then modified to investigate the impact response generated at the head of helical piles with different geometries. A method to approximate the pile impedance of helical piles with single and double helices were developed using added soil mass model. Furthermore, two-dimensional (axi-symmetrical) nonlinear dynamic finite element analyses were conducted using Plaxis 2D to investigate the response of helical piles during HSDT. The finite element models were verified against two case histories. The verified models were used to perform a comprehensive parametric study to better understand the aspects of the soil-pile-hammer system on the dynamic response of helical piles during axial impact loads. Finally, the results of mathematical and numerical investigations were used to formulate guidelines for the design of effective HSDT on helical piles as well as on driven piles.
Summary for Lay Audience
Helical piles are composed of circular or square shafts fitted with one or more helical plates attached near the bottom of the shaft. They are used to transfer structural loads at the surface to stiffer and stronger soil. Helical piles are installed using a rotary hydraulic head that generates a torque and vertical force capable of pushing the pile into the ground. In order to determine their load carrying capacity, a Static Load Test (SLT) is conducted, The SLT involves applying loads increasingly at the pile head and measuring the movement at the pile head. However, SLTs is relatively expensive and time-consuming, thus limiting the number of helical piles that can be tested. As an alternative, the High Strain Dynamic Test (HSDT), which involves applying an impact load at the pile head through a falling mass, has been recently applied to determine the capacity of helical piles. This thesis investigates the performance of helical piles during the HSDT and provides guidelines for proper design using HSDT for helical piles.
Alwalan, Mohammed Fahad, "High Strain Dynamic Test on Helical Piles: Analytical and Numerical Investigations" (2019). Electronic Thesis and Dissertation Repository. 6725.
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Available for download on Monday, March 01, 2021