Abstract:
The installation 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 torque correlation factor. However, almost of all these Kt factors are based on shaft geometry alone. Recent full-scale tests (axial compression and tension) have shown that the traditional Kt used, based on shaft diameter, does not properly account for many application conditions. 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 was used to study the effect of these variables on the capacity torque correlation factor and resulted in developing a more reliable capacity-torque ratio, Km, that considers the effect of the variables mentioned above. The new formula 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 reliability analysis, the predicted capacity using the new modified Km assures a higher degree of success than the one based on the published Kt values in AC358.