Shaking Table Test on Response of Prestressed Concrete Hybrid-Reinforced Solid Square Piles in Soft Versus Stiff Clay

Prestressed concrete hybrid-reinforced solid square (PCHSS) piles combine strands and rebars for improved ductility. This study presents large-scale shaking table tests (1:2 scale) on PCHSS pile–superstructure systems embedded in soft and stiff clay, subjected to four ground motions with distinct frequency contents. The experimental program systematically captured the evolution of natural frequencies, damping ratios, dynamic earth pressure distributions, bending moment profiles, curvature ductility demands, and post-test cracking patterns. Results reveal that ground stiffness governs the degradation pathway and energy dissipation mode: soft clay exhibited 37.3% frequency degradation (1.33× that in stiff clay), while stiff clay showed a 101% damping increase (>3× soft clay). Long-period ground motions generated maximum curvature ductility demands and peak bending moments 2–3 times those of short-period records under identical PGA, attributed to near-resonance coupling. Pile–soil interaction transitions from compatible deformation to progressive separation with increasing seismic intensity, with gap spacings of 40–53 mm observed in soft clay. Notably, the code-specified 4D reinforcement zone was found insufficient for soft clay foundations, where crack distributions extended to 4D–7D, warranting an extended zone up to 7D. The findings provide experimental benchmarks for numerical model calibration and offer practical guidance for extending hybrid-reinforced precast piles into moderate-to-high-seismicity regions.