Experimental Study on the Behavior of Spiral Grouped-Piles Installed in Sand Soil under Lateral Cyclic Loading

Background: Spiral piles are a promising foundation solution for onshore renewable-energy structures; however, their lateral cyclic performance, particularly in grouped configurations, remains insufficiently understood. This study investigates the influence of key design and loading parameters on the behavior and stability of steel spiral pile groups under lateral cyclic loading. Methods: A 1-g physical modeling program was conducted in medium-dense sand (relative density, Dᵣ ≈ 50%). Two group arrangements (2×1 and 1×2) were tested with three slenderness ratios (L/B = 9, 13.5, 18) and three batter angles (θ = 0°, 15°, 30°). Reversed (two-way), displacement-controlled lateral cyclic loading was applied for up to N = 200 cycles across a range of loading frequencies (f). The response was evaluated using normalized load–displacement curves, stiffness degradation trends, and accumulated residual displacements. Results: The tests reveal systematic dependencies of lateral performance on the number of cycles (N), frequency (f), slenderness ratio (L/B), batter angle (θ), and group arrangement. Increasing N and f amplifies stiffness degradation and residual drift. More slender piles exhibit greater lateral compliance, while battered groups (15°–30°) demonstrate reduced deformation and higher normalized lateral resistance compared to vertical groups, depending on the arrangement and loading direction. Group layout (2×1 vs. 1×2) significantly influences load sharing and failure mechanisms, resulting in distinct normalized response envelopes. Conclusions: The experimental results identify the primary geometric and loading factors governing the stability of spiral pile groups and provide normalized response data to support preliminary design and parameter selection. These findings contribute to the broader adoption of steel spiral pile foundations in onshore renewable-energy infrastructure, where reliable performance under lateral cyclic loads is critical. Keywords: Spiral pile; grouped-piles; 1-g model; sand; soil rocking; cyclic loading.