High Strain Dynamic Test on Helical Piles: Analytical and Numerical Investigations

Abstract

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.