Investigation of torque-capacity correlations and new formulas for torque factor based on axially compression-loaded helical piles

The empirical torque factor (K-t) is a critical parameter for predicting the ultimate compressive capacity of helical piles from installation torque. However, current K(t )values may be limited by the derivation from combined axial loads, an exclusive reliance on shaft diameter, and the scarcity of adequate and diverse test data. This study introduces more robust and reliable K(t )correlations by developing models derived solely from compression load tests that integrate both the helical pile's shaft diameter (d) and embedded area (EA). A total of 20 model helical piles were installed and loaded axially compression in loose and dense sand to investigate installation torque-ultimate capacity behavior. The depth-torque profiles and load-displacement curves were obtained from model laboratory tests for various helix diameters, numbers, and spacing. Furthermore, installation torques (T) varying from torque reading methods (max., end pile, and average) were examined, and K(t )values were showed variations of approximately 10% with different T values. These results were synthesized with a comprehensive database compiled from literature to create two extensive datasets correlating K(t )with both shaft diameter and embedded area. The proposed novel equations for calculating K(t )demonstrate a strong statistical fit, with high coefficients of determination (R-2) of 0.94 and 0.90 for the d-based and EA-based correlations, respectively. These validated models offer a significant improvement over existing methods, providing a sounder framework for the design of compression-loaded helical piles.