Linking the Installation Response of Screw Piles to Soil Strength and Ultimate Capacity-Paper

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The paper examines whether the installation response of screw piles can reveal their ultimate capacity and the strength of the surrounding soil. While practice commonly estimates capacity from installation torque using an empirical torque–capacity ratio, the authors argue that this ratio is not constant: it also depends on crowd force, soil strength, shaft roughness, embedment depth, and pile geometry.<br /><br />Focusing on single-helix piles installed under undrained conditions in saturated clay, the study applies an analytical model that combines resistance along the shaft and at the helix. The model links installation torque and axial crowd force to undrained shear strength and pile dimensions. It predicts that torque varies with crowd force, which is generally not measured in field installations, and that the capacity-to-torque ratio changes with installation and soil conditions. The model broadly follows the empirical relationship between capacity and shaft diameter, but predicts meaningful, systematic variation around it.<br /><br />The authors also explore using installation measurements to estimate soil strength. In small-scale kaolin tests, back-calculated strengths increasingly matched the soil’s remolded undrained strength as embedment depth increased. This is consistent with the substantial soil remolding caused by pile installation; discrepancies near the surface may reflect complex deformation as the pile first enters the soil.<br /><br />The findings suggest that screw-pile installation could serve as a form of in situ soil testing, as well as provide information about capacity. However, the evidence is preliminary: the model and laboratory results require validation through full-scale testing. Further work is also needed for multi-helix piles and soils other than clay, particularly sand.

Keywords

screw piles

installation torque

ultimate capacity

undrained shear strength

saturated clay

single-helix piles

analytical model

crowd force

soil remolding

in situ soil testing