Cyclic Lateral Performance of Helical Tapered Piles in Silty Sand-Article

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The study evaluates the cyclic lateral performance of a proposed foundation pile for solar panels: a short, hollow spun-cast ductile-iron tapered shaft fitted with a lower helical plate. Seven full-scale piles were installed by mechanical torque in medium-dense to dense silty sand: five tapered iron piles and two straight steel helical piles. After axial testing, piles underwent monotonic lateral loading followed by two-way cyclic lateral tests. Three-dimensional finite-element models were also developed to interpret the field results.<br /><br />Both tapered and straight helical piles performed satisfactorily under the applied cyclic loads. The helical plate contributed to lateral resistance by restraining rotation of the pile’s lower section. Cyclic loading produced soil gaps and cave-in behind the piles, creating looser soil zones and pinched load–deflection loops. These effects were more pronounced for the tapered piles, whose larger diameters near the ground surface compacted more soil around the area where gaps formed.<br /><br />Prior monotonic lateral loading shifted subsequent cyclic load–deflection curves toward the direction of the initial loading and reduced soil stiffness. Overall, cyclic loading caused only minor changes in measured pile stiffness; stiffness sometimes decreased and sometimes increased as soil recompacted. Numerical simulations likewise indicated substantial degradation of soil shear stiffness near the piles, with reductions influenced by prior loading and gapping.<br /><br />The results suggest that these helical piles can provide effective lateral support under the tested conditions, but their cyclic response depends on soil disturbance, loading history, and pile geometry. The findings are specific to the tested piles, soil, and loading levels.

Keywords

solar panel foundation

cyclic lateral loading

helical pile

tapered pile

ductile iron shaft

soil gapping

load-deflection response

finite element model

lateral resistance

silty sand