Model Test Study on the Effect of Quasi-Rectangular Shield Tunnel Excavation on Adjacent Pile Foundation in Sand

Tunneling activity inevitably induces soil stress redistribution and ground deformation, which may affect adjacent existing pile foundations. Since many previous studies have mainly focused on circular tunnels, the effects of quasi-rectangular shield (QRS) tunneling on adjacent existing pile foundations are not well investigated and understood. In this study, a series of physical model tests were carried out to investigate the response of a single pile and pile group subjected to newly QRS tunneling beneath an existing circular tunnel in dry sand. Two distinct underpass cases were considered: an orthogonal underpass (QRS tunnel axis perpendicular to the circular tunnel axis) and an overlapping underpass (QRS tunnel axis aligned with the circular tunnel axis). The test results indicate that QRS tunneling-induced ground surface settlement and single-pile settlement in the overlapping underpass case were 3.6 and 1.2 times that in the orthogonal underpass case, respectively, with a narrower settlement trough. The axial force distribution along the single pile remained qualitatively consistent in both underpass cases, consistently exhibiting a downward load-transfer mechanism, and further leading to a monotonic growth pattern in axial force with progressive QRS tunnel excavation. The additional stress of the single pile was consistently higher in the overlapping underpass case, which had maximum axial force, negative bending moment, and maximum positive bending moment increases of 20%, 13%, and 6%, respectively, relative to the orthogonal underpass case. The front pile in the pile group exerted a pronounced shielding effect on the rear pile, while the restraining action of the pile cap also contributed measurably to the overall pile responses.