Large-scale model tests on inclined steel pipe pile retaining structures upon excavation

The inclined alternating combination steel pipe pile retaining structure (IACSPPRS) is a cost-effective, environmentally friendly excavation-support system that offers advantages such as ease of construction and reusability. Despite its demonstrated performance in practice, research on its deformation and load-bearing mechanisms remains limited. This study presents large-scale model tests aimed at evaluating pile head displacements, bending moments, deformations, and axial force distributions during excavation. The findings indicate that, at equivalent excavation depths, IACSPPRS piles exhibit significantly reduced deformations compared to conventional cantilevered piles (CP), thereby demonstrating superior retaining performance. Furthermore, IACSPPRS benefits from the combined effects of tie-back action, gravity, and spatial structural interaction, leading to lower internal forces, reduced displacements, and improved overall stability. The system forms a spatially rigid frame, wherein the outer piles function as tensile anchors and the inner piles act as compressive struts, analogous to an internally braced support system in both load-bearing and deformation behavior. The combined effects of active friction on the outer piles, passive friction on the inner piles, and soil gravity contribute to enhanced anti-overturning capacity. Increasing the inclination angle between piles improves deformation resistance, with an optimal angle of about 20° under spatial constraints. The order of retaining performance effectiveness is as follows: vertical-inclined alternating composite piles (VICP), inclined-vertical alternating composite piles (IVCP), and inclined alternating composite piles (IICP).