Cyclic Axial Performance of Helical-Tapered Piles in Sand-Article

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The study evaluates the cyclic axial behavior of spun-cast ductile iron (SCDI) tapered piles fitted with a lower helix. The system combines a tapered shaft, rough surface, and torque-based installation. Seven instrumented piles—five tapered helical piles and two straight-shaft helical piles for comparison—were installed in silty sand and tested under cyclic compression and uplift. The researchers examined how prior monotonic or cyclic loading affected performance and used three-dimensional finite-element models to interpret soil stiffness changes.<br /><br />In cyclic compression, tapered piles accumulated less displacement than the straight-shaft piles when tested without prior cyclic uplift. Their stiffness generally increased during the early cycles as surrounding and bearing soils compacted. Numerical analyses indicated that compression cycling reduced soil stiffness around the shafts, especially around tapered piles, while stiffness beneath the helix and pile toe showed little degradation.<br /><br />Prior cyclic uplift weakened the tapered piles’ subsequent compressive performance. Uplift reduced soil confinement around the shaft, lowering shaft resistance; in this loading sequence, the straight-shaft piles performed better. Conversely, prior monotonic compression improved tapered piles’ cyclic uplift response: increased soil confinement raised shaft resistance and reduced displacement compared with the straight-shaft piles.<br /><br />After monotonic uplift, both pile types showed satisfactory cyclic uplift performance. Overall, the proposed tapered helical system performed especially well in cyclic compression, but its response depended strongly on loading history. The findings highlight the importance of considering prior loading and changes in soil confinement when assessing pile performance under repeated axial loads.

Keywords

spun-cast ductile iron

tapered helical piles

cyclic axial behavior

cyclic compression

cyclic uplift

silty sand

soil confinement

shaft resistance

finite element modeling

loading history