Abstract: 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.

Key words: undercrossing shield tunnel, face instability, discrete element method (DEM), numerical simulation