We present a method to assess the robustness of underground tunnels against brittle failure by creating vulnerability functions based on rock mass quality and static loading intensities. Using a Monte Carlo Simulation and FLAC3D, we simulated multiple models of a tunnel in rock masses with varying qualities and subjected to different static loads. Fragility curves were constructed using a stress-based failure criterion to measure the severity of brittle damage. Mathematical approximations were used to generate vulnerability functions linking occurrence probabilities of damage states to loading intensities. Results showed that fragility curves offer a numerically developed tool for design engineers to predict different damage states based on variations in rock mass quality and in situ stress state.
To ensure the stability of underground excavations, it is important to assess the potential for damage or failure. This assessment, known as fragility assessment, determines the likelihood of damage at different severity levels (Kaiser 2020 and Heidarzadeh et al. 2021). In underground tunnels, stability is greatly influenced by the quality of rock and the variabilities associated with estimation / measurement of in-situ stress state. At shallow depths, blocky to disintegrated rock masses (moderate quality) fail under structurally controlled gravity-driven modes such as gravity falling or sliding of wedges. By increasing depth, hence the magnitude of in-situ stresses, the dominant mode of failure shifts to a combination of structurally controlled and stress-assisted (or stress-driven), i.e., localized brittle failure of intact rock with the unraveling of rock blocks. Ultimately, under high in-situ stresses, a jointed rock mass may behave as a massive rock, hence brittle failure occurs around the entire excavation area (Villaescusa 2014 and Kaiser 2020). Understanding the level of damage within the rock mass is crucial for modifying and adjusting ground support. Fragility curves and vulnerability functions developed through probabilistic numerical analysis can aid in this assessment. While fragility curves have traditionally been used to evaluate seismic vulnerability, this study aims to introduce a methodology for assessing the robustness of underground tunnels against brittle modes of failure based on rock mass quality and static loading intensities.
