Centrifuge Modeling of Axial Failure of Helical Piles in Cohesive Soils-Paper

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The study used geotechnical centrifuge tests at 20 g to investigate the axial failure of helical piles in cohesive soil. Instrumented aluminum model piles—one single-helix and three double-helix designs with inter-helix spacing ratios (S/D) of 1.5, 2.5, and 3.5—were installed and tested under compression and tension in medium and stiff kaolinite clay, with undrained shear strengths of approximately 50 and 120 kPa.<br /><br />The tests identified individual plate bearing (IPB) and cylindrical shear (CS), as well as a transitional failure mode in the medium clay. The transition from IPB to CS appeared gradual, rather than occurring at a single critical S/D ratio. In the stiff clay, cylindrical failure developed at a smaller spacing ratio than in the medium clay. The transitional mechanism, observed around S/D = 2.5 in medium clay, reduced helix resistance and could cause conventional capacity estimates to overpredict axial capacity by about 20%. The researchers also found that the presence of a helix significantly reduced shaft adhesion.<br /><br />The findings show that failure mode depends on both pile geometry and soil strength. They highlight the need for caution when designing multi-helix piles with intermediate spacing in medium clay and demonstrate that centrifuge modeling can effectively investigate pile–soil behavior.

Keywords

helical piles

axial failure

geotechnical centrifuge

cohesive soil

kaolinite clay

single-helix

double-helix

inter-helix spacing

cylindrical shear

shaft adhesion