Capacity to Torque Correlations for Large Diameter Helical Piles

Since the development of the ICC-ES AC358 acceptance criteria, the use of helical piles as deep foundation solutions has grown in popularity and expansion of desired applications. With this has come a desire to use larger diameter shafts to support larger loads. Helical piles employ a unique method to predict performance by correlating the final installation torque to the ultimate load carrying capacity for both tension and compressive loads. This capacity-to-torque factor is commonly denoted as the ‘Kt’ factor. Previous publications from multiple authors have estimated the capacity-to-torque relationship both empirically and theoretically for helical piles.  Most of these prior studies focused on smaller shaft diameters from 1.5-inch square to 3.5-inch round.  Current trends have motivated the helical pile industry to utilize larger diameter pile shafts than are not well represented by the prior publications, including AC358. This paper presents an expanded body of empirical data comprised of full-scale load test results of helical piles with diameters ranging from 1.5 inches up to 18 inches. The curve fitting methods used to develop the AC358 equation are re-examined and offers an alternative that better fits the test data. The current methodology uses a simple exponential curve with a limit of zero as pile shaft diameter increases. This implies that at some point a helical pile’s diameter would become so large that it would have zero capacity, which is illogical. Utilizing a horizontal asymptote, other than zero, in the curve fitting analysis provides a better fit to the data.  The better fit increases to the capacity-to-torque ratio for larger diameter piles on the order of 15% to 30%.  Also, presented are some key insights that the authors offer for consideration in further development of this critical relationship.