Alan J. Lutenegger
Installation torque has been used in the design of helical anchors (Screw-Piles) since the late 1960s. KT factors released by the manufacturer relating ultimate capacity of Screw-Piles to installation torque allow engineers to calculate a design installation torque which is necessary to achieve the design capacity in the field. These KT factors have been based on shaft geometry alone (Hoyt and Clemence 1989). Recent full-scale uplift tests in both clay and sand have shown that the traditional methods of analysis for estimating uplift capacity based on microscale tests are not representative of macroscale behavior. A soil wedge does not fully develop in many cases and failure is a result of local bearing capacity in the soil immediately above the lead helix and side resistance along the pipe shaft. The relative contribution of these two components to the uplift capacity depends on the specific geometry of the Screw-Pile, not only the shaft geometry, but also the configuration of the helices, the soil type, and depth of embedment of the helical plate (Lutenegger 2015). Full-scale installations and load tests were performed on anchors of varying lead section geometry, shaft geometry, number of helices, soil type, and depth of embedment. Both direct (TORQ-PIN and Chance Digital Indicator) and indirect (hydraulic pressure) methods were used to monitor torque during installation. The direct methods were used to evaluate the reliability of hydraulic pressure readings and how different combinations of machine, torque head, and operator can affect the torque during installation. This paper will investigate how these factors affect KT and also determine the factors that affect the accuracy of torque measurement in the field.