基于上海轨道交通工程实践，采用离散元软件PFC2D模拟盾构隧道的正交下穿施工，分析了存在既有隧道情况下盾构开挖面塌陷失稳破坏的失稳模式、地表沉降及支护压力比，并与现场监测数据进行比较验证，沿开挖纵向进行关于开挖面塌陷失稳影响的普适性规律研究.结果表明，当开挖面距离既有隧道较近且未穿越时，既有隧道对失稳模式存在显著影响，表现为烟囱状失稳区扩大、地表沉降槽不连续、极限支护压力比减小；当开挖面穿越既有隧道后，有既有隧道开挖的失稳模式基本与无既有隧道开挖一致.模拟过程中测得2种工况极限支护压力比分布为0.4~0.6，其中下穿工况极限支护压力比较小，即较不容易发生塌陷破坏.综合对模拟结果的分析，当开挖面位于既有隧道前2倍隧道直径范围内时，开挖面的失稳模式将受到既有隧道的影响.
Based on the tunneling case for Shanghai railway transportation line, undercrossing tunneling simulation was performed by using the DEM (discrete element method) software PFC2D. The instability mode, surface subsidence and support pressure ratio were investigated to analyze the coverage from existing tunnel to the instabilities of the shield tunnel face, and were compared with the monitoring of the surface subsidence, to discuss the universal law of the face collapse along the longitudinal excavation. The consequences of DEM simulation showed the instability mode was varied when the tunnel face was close to the existing tunnel before crossing, performing the expansion of the chimney, the discontinuity of the surface subsidence tank and the decrease of the limiting support pressure ratio. When the tunnel face passed through the existing tunnel, it had very little influence on the instability mode. The limiting support pressure ratio lied between 0.4 and 0.6 in both situations of single shield tunnel and undercrossing shield tunnel. And the ratios in the undercrossing were lower than the one in the single so that it was more difficult to collapse in the undercrossing. According to the comprehensive analysis of simulation results, the existing tunnel had a significant impact on the instability mode when the tunnel face was in the range of twice the tunnel diameter in front of the existing tunnel.
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