Interpretation of Dynamic Pile Load Testing for Open-Ended Tubular Piles Using Finite-Element Method

For a foundation to perform safely, the ultimate strength of each pile must satisfy the structural and geotechnical requirements. Pile load testing is considered to be a direct method for determining the ultimate geotechnical capacity of piles. In this paper the dynamic and static response of a driven steel pipe pile monitored as part of a highway bridge construction project in New South Wales, Australia, has been simulated and then numerically analyzed using the finite-element method. A continuum numerical model has been established to simulate the dynamic load testing of steel pipe piles with unplugged behavior in which adopting measured soil properties resulted in a reasonable match between the measured and predicted results and without needing random signal matching in an iterative process. Settlement at the head and toe of the pile was then calculated when a static load represented by a dead load plus a heavy platform load of a bridge was applied over the pile head. During the dynamic and static load testing simulation, a hardening soil model with small strain stiffness was used to obtain the best correlation between the large and small strains, which occurred while the pile was under static load and being driven. The numerical predictions obtained using continuum finite-element simulations were then compared with the corresponding predictions obtained from the Case Western Reserve University (CASE) method and CASE Pile Wave Analysis Program (CAPWAP) to evaluate the predictions. The results show that the hardening soil model with small strain stiffness exhibits a reasonable correlation with the field measurements during static and dynamic loading. Moreover, parametric studies have been carried out in the established continuum numerical model to evaluate how the interface properties between the pile and soil and the reference shear strain define the backbone on the velocity at the head of the pile and traces of its displacement. Evaluation of dynamic pile load testing based on the continuum-based finite-element model has many advantages for geotechnical engineers dealing with pile design because an established continuum numerical model can assess pile testing under more realistic conditions. This model can also be used to evaluate the performance of piles under different loading conditions, such as combined vertical and lateral loading on a single pile or group of piles and piles near existing structures. Furthermore, this method retains the continuity of different stages of modeling from simulating pile driving, quality control, and investigating settlement, while all these analyses are carried out using one software.