Relationship Between Installation Torque and Axial Capacities of Helical Piles in Cohesive Soils-Article

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The paper develops a theoretical model for estimating the installation torque of helical piles in cohesive soils and uses it to derive torque factors for estimating axial capacity. The model treats installation torque as the combined torsional resistance of the pile shaft and helices, based on pile geometry and soil strength. It assumes relatively uniform soil, minimal disturbance, constant advancement equal to helix pitch, and no applied down-pressure.<br /><br />Comparisons with 74 field installation records showed reasonable agreement between estimated and measured torque. The resulting torque factors were also checked against full-scale load tests: 21 tension tests and 53 compression tests. Predictions generally agreed with measured factors, with mean prediction ratios of 0.98 for tension and 1.06 for compression. Tension factors were somewhat more predictable; compression estimates are less certain because soil conditions beneath the bottom helix are not represented by installation torque measurements.<br /><br />The study finds that torque factors depend on loading direction, soil properties, and pile configuration. Compression factors were generally higher than tension factors. Shaft size and shape, helix-to-shaft diameter ratio, and embedment depth had substantial effects; pitch had a comparatively minor effect in the configurations studied. Factors typically increased with embedment for shorter piles and became relatively stable at greater depths.<br /><br />Although the model can help estimate installation torque, select equipment, and inform quality control, torque readings are also affected by installation technique, operator experience, soil disturbance, and measurement accuracy. The author therefore cautions against relying on torque alone to determine pile capacity, particularly when installation procedures and torque-measuring equipment are not carefully controlled.

Keywords

helical piles

installation torque

cohesive soils

torque factors

axial capacity

pile geometry

soil strength

tension tests

compression tests

load tests