Uplift Performance of Multi-Helix Anchors in Desiccated Clay-Article

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The study reports full-scale uplift tests on seven square-shaft, multi-helix anchors installed in desiccated Beaumont clay near Baytown, Texas. The site was extensively characterized using piezocone testing, borings, and laboratory strength tests, allowing the authors to estimate undrained shear strength and its uncertainty.<br /><br />The anchors had a plate-spacing ratio of 3 and were embedded about 19 average plate diameters. Load–displacement measurements were corrected for slack in the shaft couplings. Four anchors were loaded to peak resistance, which ranged from 310 to 516 kN; peak resistance was reached after roughly 25–50 mm of displacement. Initial secant stiffness varied substantially, with a coefficient of variation of 36.3%, while capacity had a coefficient of variation of 25.3%. Stiffness and capacity showed no clear relationship to anchor geometry or test timing.<br /><br />All anchors taken to failure softened after reaching peak load, consistent with the post-peak behavior observed in triaxial tests of the site’s fat clay. Excluding one unusually behaving anchor, resistance generally fell by 5% and 10% after about 18 mm and 26 mm of additional displacement, respectively.<br /><br />The authors compared measured capacities with individual-plate breakout, cylindrical-shear, and SPT-based prediction methods. The theoretical failure-mechanism models performed best overall, while the SPT correlation was also reasonably accurate. Prediction variability was about 40–44%, below the estimated 53% uncertainty in undrained shear strength. The results suggest these methods can provide useful capacity estimates for anchors in Beaumont clay, provided the uncertainty in soil properties is recognized and site characterization is adequate.

Keywords

full-scale uplift tests

multi-helix anchors

Beaumont clay

undrained shear strength

peak resistance

load-displacement behavior

initial secant stiffness

post-peak softening

anchor capacity prediction

piezocone testing