Helical Anchor Pullout Capacity in Sand Overlain by Clay: Experimental and Numerical Insights-Paper

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This ongoing study examines the pullout capacity of circular anchors embedded in sand beneath a soft clay layer, a soil profile common in reclaimed areas such as Kooragang Island, Australia. Current design guidance for anchors in these layered conditions is limited.<br /><br />Researchers combined scale-model pullout tests with finite element limit analysis (FELA). The experiments used loose sand overlain by consolidated clay of varying undrained strength, with different anchor embedment depths. FELA modelled the sand and clay using Mohr–Coulomb and Tresca criteria, respectively. Image analysis was used to compare observed and predicted failure mechanisms.<br /><br />FELA capacities were generally within 25% of experimental measurements, and simulated failure patterns agreed well with observations. Shallow failures reached the ground surface and showed a discernible failure angle; transitional and deep failures were more diffuse, with deep failure zones remaining below the surface.<br /><br />Preliminary numerical analyses indicate that weak clay can reduce pullout capacity by roughly 10–30%, particularly where sand friction angles are high (about 35–45°) and clay strength is below approximately 10 kPa. For lower sand friction angles, the clay’s influence was generally smaller. In some cases, however, stronger clay increased predicted capacity—occasionally to about twice that of an equivalent uniform-sand profile. This possible benefit may reflect greater mobilized resistance, but boundary or confinement effects could also contribute and require further investigation.<br /><br />The findings are preliminary. Planned work includes expanded parametric studies, an analytical design model, field-scale validation, and investigation of installation, loading-rate, and cyclic effects. The ultimate goal is a practical design method for circular anchors in layered soils.

Keywords

circular anchors

pullout capacity

layered soil

sand over clay

finite element limit analysis

Mohr-Coulomb

Tresca criterion

anchor embedment

failure mechanism

reclaimed land