Analytical models of impact force-time response generated from high strain dynamic load test on driven and helical piles

The High Strain Dynamic Test (HSDT) offers an alternative approach to the conventional static load test for estimating the axial capacity of helical piles due to its faster implementation and potential cost savings. In this paper, a mathematical model developed by Clough & Penzien that simulates the force-time response at the top of a pile with uniform cross-section subjected to a hammer impact is evaluated for the HSDT setup. To verify the suitability of the model, the calculated force-time history from the mathematical model is compared with actual field data for driven and bored piles. The model is then used to explore the characteristics of hammer impact on helical piles with attention to estimating the impedance increase caused by the helices. Generally, helices offer additional end bearing resistance, thus increase the mechanical impedance of the pile. The obtained results demonstrate that it is necessary to account for the effect of helical plates in the analysis, especially for large diameter helical piles. Consequently, a method to approximate the increase in the pile impedance for a helical pile with single and double helices is developed using added soil mass model.