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

Article

Pdf Summary

The study evaluates the installation and axial performance of large-capacity helical piles in very stiff to very hard clay and clay till in northern Alberta. Ten full-scale load tests were conducted: six in compression and four in tension, on single- and double-helix piles with shaft diameters of 324–508 mm.<br /><br />The piles were successfully installed, including through hard soil layers. Predrilling helped installation but could disturb the soil and reduce uplift resistance if it extended below the upper helix. Measured installation torque generally reflected changes in soil conditions, but it did not provide a reliable, universal estimate of pile capacity.<br /><br />Using a displacement-based failure criterion of 5% of helix diameter, measured compressive capacities reached 2,450 kN (about 275 tons), substantially exceeding traditional helical-pile load ranges. Uplift capacities reached 2,100 kN and were generally lower than compression capacities; in tested piles at Sites 6 and 7, compression capacity was about 13–14% higher. Double-helix piles generally provided greater resistance than comparable single-helix piles.<br /><br />Load-test interpretation methods produced different capacity estimates. Davisson’s criterion was notably conservative, particularly for shorter piles, while the 5% criterion better reflected observed performance. Creep significantly affected compression-test results in cohesive soils at high loads, leading the author to recommend longer load increments—about 20 minutes—for quick tests.<br /><br />Overall, the results support helical piles as a viable foundation option for heavy axial loads in cohesive soils. Conventional bearing-capacity methods estimated measured capacities reasonably well, but further work is needed to establish reliable torque-to-capacity correlations.

Keywords

helical piles

axial performance

clay till

load testing

compression capacity

uplift resistance

installation torque

predrilling

double-helix piles

bearing capacity