Geological and hydraulic conditions are important factors for analyses of tunnel face stability. When a tunnel with high overburden is excavated under such condition that the fault zone containing high water pressure appear in front of the tunnel face, hydraulic fracturing occurs, and the tunnel face may collapse suddenly (Shimizu et al., 2019). In addition, pre-existing fractures in rock mass would also play an important role for tunnel face stability because high water pressure in pre-existing fractures induce propagation and connection of these pre-existing fractures (Sekino et al., 2019).

To accurately evaluate such phenomena, it is necessary to use an evaluation method that can consider pre-existing fractures in rock and fracture propagation by hydraulic pressure. However, conventional continuum analysis methods such as the Finite Element Method (FEM) are insufficient to consider individual fractures and reproduce the fracture propagation by water pressure. Therefore, the Distinct Element Method (DEM) was extended to be able to simulate coupled hydraulic-mechanical processes for a detailed analysis of the hydraulic fracturing in this study. DEM can directly represent grain-scale microstructural features of a rock, and it is possible to express propagation behavior from individual microcracks to macro fractures.

2D-DEM tunnel excavation simulations considering pre-existing fracture were conducted for horizontal cross-section of tunnel face. Our simulation successfully reproduced the tunnel face stability affected by hydraulic fracturing. DEM simulation results with three patterns of pre-existing fractures indicate that the interaction between the pre-existing fractures and high water pressure strongly affects the tunnel face stability. When the tunnel face approach the fault zone containing high water pressure, the excavation released the compressive stress around the high water pressure zone and induced hydraulic fracturing. Hydraulic fracturing connected each pre-existing fractures and new tunnel face were created. The new tunnel face released the stress again and this process repeated until the tunnel face completely collapsed.

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