Abstract: The pull-pile support structure in a deep foundation pit is a three-dimensional spatial structure system composed of front-row piles, back-pull piles, a crown beam, and tie beams. Existing engineering cases demonstrate that the pull-pile support structure effectively resists overturning and controls horizontal deformation, yet lacks comprehensive theoretical support and systematic research. To thoroughly investigate the deformation behavior, internal force evolution, and working mechanism of the pull-pile support system during foundation pit excavation, an indoor scale test was conducted. Results indicate that the horizontal displacement curve of the front-row pile is convex, the bending moment curve is anti-S shaped, the axial force curve is shuttle-shaped, and the axial force on the pile body is compressive force. The bending moment curve of the back-pull pile is bow-shaped, and the axial force curve is spoon-shaped. The upper section of the back-pull pile experiences axial tension, while the lower section experiences axial compression. The horizontal lateral displacement curve of the crown beam presents sine function shape of the short crown beam, and the axial force curve presents an obvious M shape. The double peaks of the M shape correspond to the crown beam atop the front-row pile without back-pull support, marking a weak point for potential failure of the crown beam. The spatial characteristics of the pull-pile support structure allow for a stable tension-compression combined structure, forming a large triangle with two smaller triangles and a three-way door frame. The resulting back-pull pile effect and spatial deformation coordination can anchor the front-row pile and shield against earth pressure, as well as suppress differences in the top inclination angle of the supporting pile and excessive deformation of crown beam.

Key words: pull-pile support structures, deformation characteristics, working mechanism, experimental study