The fracture pattern of rock mass surrounding a deep tunnel is closely related to the in situ stress, which is generally quite anisotropic. This study focuses on the triaxial stress condition for the formation of ring-like fractures around tunnels through three dimensional numerical modeling. The fracture patterns around both the tunnel and ahead of the tunnel face under different anisotropic triaxial stresses were analyzed, and the influence of tunnel shape on the formation of ring-like fractures was also discussed. A conceptual criterion for the formation of ring-like fractures was proposed based on the simulations. It was found that the ring-like fractures can be expected when the axial pressure coefficient is less than 0.3, regardless of the tunnel shape. Ring-like fractures are more likely to be located near tunnel-free surfaces with a larger radius of curvature. The aim of this article is to shed some light on the interpretation of failure phenomenon in deep underground excavations.
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