Torsional Resistance of Drilled Shaft Foundations: A State-of-the-Art Review-Article

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This review examines methods for estimating the geotechnical torsional resistance of drilled-shaft foundations. Although shafts often experience torsion from eccentric wind, seismic, wave, or impact loads, design guidance is limited, resistance mechanisms are not well understood, and existing methods rely on sparse test data.<br /><br />Most practical approaches estimate resistance from soil–shaft skin friction, commonly using the α method for cohesive soils and the β method for cohesionless soils. The review also evaluates agency procedures, including methods from Colorado and Florida, by comparing predictions with laboratory, centrifuge, and full-scale test results in cohesive, cohesionless, and layered soils.<br /><br />No method consistently emerged as best. For the available data, the α and β methods used together and the Colorado DOT (CDOT) method gave the most reliable estimates for layered soils, with relatively low variability and errors. However, both approaches—and the other methods reviewed—could poorly represent how shear resistance varies along a shaft. Some methods were notably conservative or unconservative. Toe resistance was generally a small share of total capacity and is sometimes omitted in design.<br /><br />The authors recommend using the α, β, and CDOT methods cautiously until better approaches are developed. They emphasize the need for additional full-scale and centrifuge testing to clarify resistance and load-transfer mechanisms, as well as validated models that account for shaft rotation and simultaneous lateral and torsional loading. Existing design methods also do not predict the rotation associated with reaching nominal resistance.

Keywords

drilled-shaft foundations

torsional resistance

geotechnical engineering

soil-shaft skin friction

alpha method

beta method

Colorado DOT method

full-scale testing

centrifuge testing

load-transfer mechanisms