Effect of Spacing on the Capacity of Helical Anchors-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines how center-to-center spacing affects the uplift capacity and group efficiency of helical anchors. It reviews prior research and reports full-scale field tests at three sites: Houston, Texas (1983), Centralia, Missouri (2022), and Caseyville, Illinois (2022).<br /><br />The new tests used vertically installed pairs of anchors in side-by-side arrangements and ASTM D3689 tension testing. Centralia tests were conducted in stiff clay; Caseyville tests were conducted in loose-to-medium-dense granular fill. Anchor spacing was varied, with three tests at each spacing at the 2022 sites. The results compare uplift capacity at several displacement criteria, torque correlation factor (Kt), and group efficiency.<br /><br />Across the studies, greater spacing generally improved capacity and group efficiency, but the spacing at which interaction becomes negligible depended on soil and anchor configuration. Prior research suggests a critical spacing-to-helix-diameter ratio (S/B) of about 5 in dense sand and 4 in hard clay, while soft-to-firm clay may show no critical spacing. Prior group efficiencies ranged from 60%–95% for single-helix anchors in loose-to-medium-dense sand at S/B ratios of 2–3; double-helix piles in stiff clay achieved 91%–97% at ratios of 2–5.<br /><br />The current tests indicate no critical spacing within the Houston test range of 2–7 ft for three-helix anchors in very stiff clay. At Centralia, the critical S/B ratio for single-helix anchors in stiff clay appears to be about 2. At Caseyville, group efficiency ranged from 50%–90%, with a critical ratio of about 3. Overall, spacing recommendations should account for soil conditions and anchor design rather than rely on a single universal ratio.

Keywords

helical anchors

uplift capacity

group efficiency

center-to-center spacing

spacing ratio

ASTM D3689

torque correlation factor

stiff clay

granular fill

field tests