DFI Journal - The Journal of the Deep Foundations Institute

Volume 14, Issue 2, January 2020
DOI: https://doi.org/10.37308/DFIJnl.20190716.208

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Helical Pile Capacity-to-torque Correlation: A More Reliable Capacity-to-torque Factor Based on Full Scale Load Tests

Moncef Souissi, James A. Cherry, and Tom Siller


The capacity-to-torque ratio, Kt, has been used in the design of helical piles and anchors for over half a century. Numerous research efforts have been conducted to accurately predict this capacity-to-torque ratio. However, almost of all these Kt factors are based on shaft geometry alone. The capacity-to-torque ratio described herein was found to depend on the shaft diameter, shaft geometry, helix configuration, axial load direction, and installation torque. In this study, 799 full scale static load tests in compression and tension were conducted on helical piles of varying shaft diameters, shaft geometry, and helix configurations in different soil types (sand, clay, and weathered bedrock). The collected data were used to study the effect of these variables on the capacity-to-torque ratio and resulted in developing a more reliable capacity-to-torque ratio, Km, that considers the effect of the variables mentioned above. The study shows that the published Kt values in AC358 (ICC-ES Acceptance Criteria for Helical Piles Systems and Devices) underestimate the pile capacity at low torque and overestimate it at high torque. In addition, and based on probability analysis, the predicted capacity using the modified Km results in a higher degree of accuracy than the one based on the published Kt values in AC358.

helical pile, capacity torque ratio, ultimate capacity, helix configuration, shaft geometry, compression, tension, AC358