Performance of isolation piles in protecting subway tunnels adjacent to foundation pits: Experimental and numerical investigations

Laboratory model tests and numerical simulations were conducted to investigate the control effect of different arrangements of isolation piles on the additional deformation and stress of tunnels adjacent to pit excavation. Four different arrangements of isolation piles were discussed in this work: arched, curved, double-row, and single-row. The results indicated that considering the burial depth, the four arrangements of isolation piles had significant control effects on the horizontal and vertical deformation, as well as the bending moment of the tunnel. Among them, arch-shaped isolation piles have better isolation effects than other types discussed in this work. Subsequently, taking the arch-shaped isolation piles as an example, the effects of the burial depth, pile position, height-span ratio, and the optimal buried depth of the arch-shaped isolation piles on the tunnel were investigated. The arch-shaped isolation piles have a critical height-span ratio of 0.123, and the closer to the tunnel, the better the protective effect on the tunnel. The optimized embedding depth of isolation piles depends on the depth of the tunnel and the relative position to the foundation pit. The results of this study have great guiding significance for the design of isolation piles to protect adjacent tunnels near excavations.