Group Interaction Effect of Helical Pile in Soft Clay under Uplift and Lateral Load-Paper

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This study investigates the interaction effects of helical pile groups in soft clay under uplift and lateral loading. Three group configurations—two, three, and four piles in rectangular, triangular, and square arrangements—were examined at spacing ratios (pile-center spacing to helix diameter) of 2–5.<br /><br />Small-scale tests were conducted in undrained soft clay using groups spaced at 4D. A three-dimensional finite-element model in Plaxis 3D was validated against the tests, predicting uplift capacity within 8% and lateral capacity within 3% of measured values. The model was then used to assess the other spacings and configurations.<br /><br />Both uplift and lateral capacity generally increased with group size and pile spacing. Group efficiency was lowest at 2D and increased as spacing widened, indicating reduced interaction between piles. Displacement contours showed overlapping soil failure zones at close spacing, while at wider spacing the piles increasingly behaved independently. Overall, group effects were generally insignificant at spacing ratios of 4 or greater, where efficiency reached or exceeded the study’s 95% threshold. For uplift, capacity gains also became limited beyond 4D.<br /><br />The findings depend on the adopted failure criteria: uplift failure was defined at displacement equal to 10% of helix diameter, while lateral capacity was assessed at 25 mm pile-head displacement. The authors note that the model simplifies installation-related soil disturbance, pore-pressure changes, soil-stiffness variation, and loading-rate effects. Thus, its results offer design guidance but should be interpreted with these limitations in mind.

Keywords

helical pile groups

soft clay

uplift loading

lateral loading

group efficiency

pile spacing ratio

finite element model

Plaxis 3D

soil failure zones

pile interaction effects