Results of Dynamic and Static Load Tests on Helical Piles in the Varved Clay of Massachusetts-Article

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The study evaluated dynamic load testing (DLT) as a faster, lower-cost alternative to static load testing (SLT) for determining the compressive capacity of helical piles in cohesive soil. Tests were conducted on seven piles installed in the varved clay at the University of Massachusetts Amherst site. The piles had identical three-helix configurations and were installed to depths of 3.7 or 5.5 m. Researchers measured installation torque, performed DLTs using instrumented rods and a dropped weight, and conducted SLTs several weeks later. Dynamic results were analyzed with CAPWAP and compared with static load–displacement curves using the Davisson failure criterion.<br /><br />For the five piles with comparable capacity measurements, DLT and SLT capacities generally agreed closely: their ratios ranged from 0.97 to 1.26. The load–displacement curves also showed good agreement. DLT results for two shorter piles were conservative, apparently because the impact energy did not mobilize their full capacity; greater energy was recommended. The analysis indicated that resistance above the lowest helix was largely associated with shearing of the soil trapped between helices.<br /><br />Measured torque-to-capacity factors (Kt) exceeded the commonly recommended value of 29.5 m⁻¹. Average static Kt values were 48.0 m⁻¹ for the shorter piles and 37.6 m⁻¹ for the longer piles, demonstrating that the factor varies with soil layer and pile conditions. The authors conclude that DLT is a viable capacity-testing method for helical piles in cohesive soils, but site-specific load testing is needed to calibrate torque correlations. Further research is required on suitable dynamic failure criteria and performance in non-cohesive soils.

Keywords

dynamic load testing

static load testing

helical piles

cohesive soil

compressive capacity

CAPWAP analysis

installation torque

Davisson failure criterion

torque-to-capacity factor

varved clay