Serviceability Performance Evaluation of Helical Piles under Uplift Loading

Abstract

The use of helical pile foundations has widely expanded in recent years. However, the prediction of the load-displacement performance of helical piles under uplift loading for service-level displacements is still a challenge for designers. In this work, a probabilistic power law model is used to characterize the random nonlinear uplift load-displacement relationship of deep helical piles. Model parameters are found from a database of 30 uplift loading tests conducted on helical piles in Brazil. Using structural reliability concepts, a practical example of a transmission tower foundation is provided to illustrate the use of the developed probabilistic model for serviceability limit state (SLS) evaluation of helical piles. The investigation takes into account the uncertainties of three parameters: one parameter of the load-displacement curve, the empirical correlation between pile capacity and installation torque, and the wind loading on foundations of transmission lines. For the example evaluated, the reliability indexes increase with allowable displacements. Additionally, the shape of the load-displacement curve is shown to be relevant when small allowance displacements are specified. In addition, when the allowable displacements approach the failure criteria for the ultimate capacity of the foundation, the developed service limit state model converges naturally to the ultimate limit state model. The structural load is found to be the random variable with the largest contribution to failure probabilities, especially for larger allowable displacements.