Capacity to Torque Correlations for Large Diameter Helical Piles-Paper

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The paper reevaluates the relationship between helical pile installation torque and axial load capacity, commonly expressed as the capacity-to-torque (Kt) factor. Earlier research and the ICC-ES AC358 standard largely relied on data from smaller pile shafts, leaving limited guidance for increasingly common large-diameter piles.<br /><br />The authors analyzed 416 full-scale tension and compression load tests, including 86 tests on shafts 5.5–12.75 inches in diameter. They found that the existing AC358/Perko equation increasingly underestimates measured capacity for larger shafts. They propose a revised empirical curve using a nonzero horizontal asymptote. The new correlation closely matches existing Kt values for smaller shafts, while increasing estimates for larger shafts by roughly 5–20% or more. Unlike the existing equation, which approaches zero as shaft diameter grows, the proposed relationship approaches a nonzero limit.<br /><br />The authors caution that the database may still underestimate actual capacity. Many tests stop at twice the design load, before the pile reaches the displacement limit or shows plunging, so its true maximum capacity is not measured. They recommend extending tests until failure criteria are reached, or determining test loads from the measured installation torque and the designer’s selected Kt. A practical option is to complete the usual acceptance test, then reload the pile in an “excess test” to establish its ultimate capacity.<br /><br />More complete testing would improve the database and support more accurate, optimized designs, particularly for large-diameter helical piles.

Keywords

helical piles

installation torque

axial load capacity

Kt factor

AC358 standard

large-diameter piles

full-scale load tests

empirical curve

design optimization

ultimate capacity