Effects of Installation Disturbance on Behavior of Multi-Helix Piles in Structured Clays-Article

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The study examines how installation disturbance affects the axial behavior and capacity of multi-helix piles and anchors in structured clays. Unlike remolded laboratory soils, natural clays can lose substantial strength when their structure and interparticle bonds are disrupted during pile installation.<br /><br />The authors analyzed full-scale uplift and compression load tests across different pile configurations and clay deposits. Their back-calculations show that disturbance varies around the pile. Soil beside the shaft is heavily disturbed, with mobilized undrained shear strength often reduced to the fully remolded value. Strength along the cylindrical failure surface between helices is also reduced, generally to near-remolded levels. In contrast, soil mobilized by bearing helices is less affected because much of it lies beyond the installation-disturbed zone; its strength is closer to intact peak strength. Soil beneath the bottom helix of a compression pile may remain largely undisturbed.<br /><br />These differences help explain why conventional capacity predictions can be inaccurate. In structured clay, individual-helix bearing failure is less likely than in remolded soil, and the critical helix spacing for this failure mode may be greater. The authors recommend accounting for installation-related strength reductions and assessing both cylindrical-shear and individual-bearing mechanisms; pile capacity should be governed by the lower estimate. The proposed approach produced generally reasonable agreement with measured capacities in the cases examined.

Keywords

structured clay

installation disturbance

multi-helix piles

axial behavior

uplift capacity

compression capacity

undrained shear strength

cylindrical shear failure

individual helix bearing

pile capacity prediction