Existing studies on the mechanism of isolation piles in controlling the displacement of adjacent tunnels have predominantly concentrated on the ultra-deep foundation pit excavation stage, with an inadequate systematic investigation into the entire construction process. A 29m-level ultra-deep foundation pit project adjacent to a tunnel in Shanghai is taken as a case study in this paper. A coupled analysis model of foundation pit-isolation pile-tunnel is established by PLAXIS 2D finite element software, and for the first time systematically explores the displacement control mechanism and influence law of key parameters of isolation piles in the dual stages of excavation and building surcharge of ultra-deep foundation pits. Research findings indicate that during the whole foundation pit construction process, the tunnel presents a deformation pattern characterized by horizontal stretching and vertical compression. Based on the distribution characteristics of pile shaft bending moments, isolation piles can be divided into upper and lower segments with the inflection points serving as the boundary, and the inflection point has the characteristic of surcharge-induced upward shift under the condition of ultra-deep foundation pits, and the distinct action mechanisms of the upper and lower segments of isolation piles in the excavation and surcharge stages of ultra-deep foundation pits are revealed The optimal design criteria of isolation piles for ultra-deep foundation pits are proposed, with the pile length being 2.0 times the foundation pit excavation depth. The embedded isolation piles have a strengthening effect in the surcharge stage on the control of the horizontal displacement of tunnels in ultra-deep foundation pits. A reasonable pile spacing can form a continuous soil arching effect, with the optimal value being 1.2 times the pile diameter. The research results fill the research gap of the control mechanism of isolation piles in the building surcharge stage of ultra-deep foundation pits, and provide exclusive theoretical support for the design of isolation piles in tunnel projects adjacent to ultra-deep foundation pits.
Chen et al. (Fri,) studied this question.