In deep hard rock environments under high-stress conditions, rockmasses behave in a brittle manner resulting in spalling and strain bursting. This paper examines the occurrence of brittle failure and its impact on face stability. Generalised Hoek-Brown failure criterion modified after the Damage Initiation and Spalling Limit (DISL) approach is adopted in 2D and 3D numerical models to capture the mechanical response of brittle rocks in underground excavations. Unsupported circular tunnel models are used to investigate brittle failure at the tunnel face. Excavation induced stress and deformation results show that under anisotropic in-situ stress conditions, models become unstable, and relaxation rapidly increases within 3 m behind the tunnel face, while within a distance of 0.85 times the tunnel radius, the tunnel relaxes 85% to 95%. This demonstrates the rapid loss of confinement in hard rock masses within the face vicinity which yields instabilities in the tunnel.
There has been significant growth in underground development in the last decades due to increased demand in infrastructure, energy storage, mining and resource extraction and the need for safer hazardous waste disposal facilities. Consequently, this has resulted in underground structures covering large areas and depths exceeding several kilometers and excavated in high-stress rock mass environments. Rockmass materials under such high-stress conditions tend to behave in a brittle manner during excavation (Diederichs 2007). Brittle failure manifesting as spalling and strain bursting has been extensively examined by several researchers and identified as a significant subject in deep tunnelling (Kaiser et al. 1996). Underground excavations alter the field stress state with the induced stress redistributions resulting in fracturing, disturbance and deformations around the openings. Hence, it is critical to understand the excavation response and related brittle failure manifesting, such as spalling, which influences the support requirements, excavation advance rates and other design and construction aspects. Various researchers (Kaiser et al. 1996, Diederichs 2007) have contributed in developing mechanistic interpretations, empirical methods and semi-empirical approaches of predicting excavation-induced damage in massive rock masses. However, recent years have seen a growing interest in research and application of numerical modelling in predicting rockmass behaviour under such conditions. In this research the Finite Element Method (FEM) and the Damage Initiation and Spalling limit (DISL) constitutive approach introduced by Diederichs (2007) are used to investigate the excavation response of deep underground openings in hard rock masses.
