Installation and Performance Characteristics of High Capacity Helical Piles in Cohesionless Soils-Article

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The paper reports a full-scale testing program evaluating large-capacity helical piles in dense to very dense, cohesionless soils in northern Alberta. Eleven piles with shaft diameters of 324–508 mm and single or double helices were tested: seven under compression and four under tension. Installation torque, pile movement, soil conditions, and load capacity were assessed.<br /><br />The piles were successfully installed in dense sand, with end-of-installation torques reaching approximately 338 kN·m. Compression and uplift load–displacement curves generally showed an initial linear response, a nonlinear transition, and a later, less-steep linear response. No compression test showed plunging failure. Maximum measured capacities, using a displacement criterion of 5% of helix diameter, were about 2,920 kN in compression and 2,900 kN in uplift. Uplift capacity was generally about 80% of compression capacity. Quick load tests were considered adequate for dense sand because most movement occurred within five minutes and creep was minor.<br /><br />Capacity estimates based on individual helix bearing resistance plus shaft friction generally agreed reasonably with test results. The commonly used Davisson criterion produced substantially lower capacities than the 5% displacement criterion, highlighting the need for care when selecting a failure criterion for large helical piles. Predrilling could reduce shaft friction and disturb soil supporting a helix; trimming helixes to aid installation through cobbles also reduced bearing resistance. Installation torque was useful for identifying relative changes in soil resistance, but torque–capacity correlations were variable and should not be relied on alone.<br /><br />The study concludes that high-capacity helical piles can perform effectively in dense cohesionless soils, with capacity strongly influenced by pile geometry, soil profile, and installation methods.

Keywords

helical piles

dense cohesionless soils

load testing

uplift capacity

compression capacity

installation torque

bearing resistance

shaft friction

Davisson criterion

northern Alberta