Study on large deformation collapse mechanism and control measures for large section tunnel face in Badong formation mudstone

The excavation face is susceptible to instability and collapse when tunnels traverse water-rich soft strata. Based on the Badong Tunnel of the Zhengzhou—Wanzhou high-speed railway, series laboratory tests on the purple-red mudstone series of the Badong formation were conducted to examine the mechanical characteristics of mudstone softening upon contact with groundwater. Considering the influences of rock mass joint development and construction disturbances, the strength parameters of the intact rock mass obtained from laboratory tests were converted to the field rock mass parameters based on the Hoek-Brown strength criterion. The two-phase single-point coupled material point method(MPM) was employed to simulate the large deformation collapse process of the tunnel face. The effectiveness of advanced curtain grouting pre-reinforcement measures was assessed in conjunction with the on-site assessment. The results show that the entire collapse process of the tunnel face can be roughly divided into three main stages: the uplift deformation stage of the tunnel face, the local tunnel face instability and the accelerated movement stage of the collapse rock mass, and stabilization stage of the collapsed soil masses. As the collapse advances, notable changes occur in the seepage field of the surrounding rock, resulting in a funnel-shaped water seepage pattern with water flowing towards the 'gap' in the tunnel face. Following stabilization, the collapsed rock mass accumulates in a cone shape, with the collapsed body located approximately 25 m away from the tunnel face, closely resembling the final pattern observed at the actual site. The implementation of advanced curtain grouting pre-reinforcement measures significantly improves the stability of surrounding rock, ensuring the safe and smooth progress of tunnel construction. Moreover, the two-phase coupled MPM method can dynamically replicate the entire process of tunnel face collapse and large deformation failure in weak strata subjected to groundwater seepage, providing reference for similar projects. © 2024 Central South University of Technology. All rights reserved.