AbstractHelical anchors provide an accepted means for the support of compressive and tensile foundation loading. Despite their prevalence, the load-displacement and capacity of multi-helix anchors has not been adequately documented. Full-scale uplift loading tests on square shaft multi-helix anchors installed in desiccated Beaumont clay were performed at a well characterized test site. The stiffness and strength of the anchors is characterized, and compared against the variability in undrained shear strength determined in consideration of inherent variability, measurement error, and transformation error. The large-displacement softening behavior exhibited by the test anchors was shown to reflect the in-situ stress-strain behavior of the high plasticity clay, and the amount of softening with post-peak displacement was characterized. The prediction accuracy of design models was assessed using the bias and its statistical characterization, and indicated that the variability in model accuracy lies within the ...