Transition of a failure mechanism from extensional to shear dominated failure with increasing confinement is widely observed in the rupture of samples in the laboratory and in situ associated with underground excavation in brittle failing rocks. However, common failure criteria such as the Hoek-Brown criterion are not able to capture or explain the change in failure mechanism. Hence, a better understanding of these confinement-dependent failure processes is key in continuing safe underground excavation in brittle failing rocks. The 3D finite difference code, FLAC3D, was adopted to investigate the behavior of underground excavation on a numerical model in terms of failure mode transition associated with the confinement level. It is hypothesized that extensional failure is dominant at low confinement, but shear failure mainly occurs at higher confinement. The Hoek-Brown constitutive model embedded in FLAC3D has been modified in order to simulate extensional failure associated with spalling at lower confinement by mean of ubiquitous joints installation. The results support the observation that the rock strength and its behavior may differ significantly in the direct vicinity of openings where the spalling mainly occurs.

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