Abstract:
ABSTRACT
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 support larger loads by using larger diameter shafts. 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 focus on smaller shaft sizes typically ranging from 1.5 in [38 mm] square to 3.5 in [89 mm] round. Current trends have motivated the helical pile industry to utilize helical piles with larger diameter shafts that 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 shaft sizes ranging from 1.5 in [38 mm] square up to 12.75 in [324 mm] round.
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 shaft 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 the capacity-to-torque ratio for piles with larger shaft diameters on the order of 5% to 20% while still presenting strong correlation to prevailing capacity-to-torque ratios for the smaller shaft sizes. Also, presented are some key insights that the authors offer for consideration in further development of this critical relationship.
Keywords: HELICAL PILE, HELICAL PIER, HELICAL ANCHOR, SCREW PILE, CAPACITY-TO-TORQUE, Kt FACTOR, EMPERICAL DATA, LOAD TESTS