Collapse mechanism and treatments of a deep tunnel in the weathered granite fault zone

• A new numerical simulation method for establishing single fissure and random fissure groups is proposed. • A conceptual model of the evolution process of deep tunnel collapse under the combined geological conditions of groundwater and fissure zone is proposed. • A set of treatments for rock tunnel collapse in fissure development areas are proposed and verified. Tunnelling in the fault fracture zone is prone to collapse accidents. The failure characteristics and mechanism of a tunnel collapse in Qinghai were analyzed based on the theory of rock mechanics, discrete element method (DEM), and in-situ test. The results show that the construction disturbance leads to the activation of the granite fracture zone, in which many fissures are constantly eroded and expanded to conduct water, which is the main reason for reducing surrounding rock stability after tunnel excavation. DEM simulates the failure process of the surrounding rock, and the collapse arch is observed. The uneven distribution of the stress field in the whole model is consistent with the tunnel’s asymmetric failure phenomenon. Based on the results, treatments such as grouting, enhanced primary support, and advanced pipe roofs were adopted to improve the mechanical properties of fissures and enhance the stability of the surrounding rock. This case provides valuable lessons for similar tunnels.