Gary Seider, Engineering Manager at Hubbell Power Systems, recently posted an interesting story on CHANCE® Foundation Solutions Blog about the origin and development of the Torque Correlation (Kt) Factor.
The torque correlation method is a well-documented and widely accepted method for verifying the axial capacity of helical piles. In his introduction, Seider explains that It is an empirical method originally developed by the A.B. Chance company in the late 1950’s to early 1960’s. The relationship between torque and capacity was first documented in professional literature in 1989 by Robert M. Hoyt, Chief Engineer at A.B. Chance at the time, and Prof. Samuel P. Clemence, Professor at Syracuse University.
Hoyt and Clemence worked with tension capacity and they found that torque correlation was statistically more consistent than the theoretical capacity methods. They produced a simple and elegant relationship that related the ultimate uplift capacity to the average installation torque:
Qult = Kt x T
Qult = Ultimate uplift capacity [lb (kN)]
Kt = Empirical torque factor [ft-1 (m-1)]
T = Average installation torque [lb-ft (kN-m)]
Subsequent studies demonstrated that the compression capacity was greater – sometimes over 30% more than the tension capacity. However, common practice is to use the same torque correlation factor in both tension and compression.
Seider then focuses on future development and particularly on the work of Howard A. Perko “Helical Piles – A Practical Guide to Design and Installation” in 2009, who developed a formula to determine Kt based on the effective diameter of the helical pile. Through the independent accredited test lab CTL | Thompson and the significant contribution of A.B. Chance regarding high-quality test data, Seider provides a demonstration of how Perko’s regression analysis corresponded fairly well to the work done by Hoyt & Clemence and other researchers.
Seider concludes that there are numerous factors that affect installation torque, such as soil type, operator skill level, advancement rate, crowd pressure, rotation speed, accuracy of torque measurements, shaft size, shape of pile shaft, helix pitch, etc. The torque correlation research continues up to today throughout the world, while Hubbell Power Systems, Inc. / CHANCE continues to lead the effort.
Make sure to visit CHANCE® Foundation Solutions Blog to read the full story.
We also suggest to watch (below) a very interesting conversation organized by Hubbell Power Systems and Gary Seider “America's Foundation: Tension & Gravity Load Testing: Torque Measurement & Calibration (Episode 3)”
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