Numerical Simulation on Cavity Forming Mechanism of Mud-Burst in Clay-Altered Rock

Clay-altered rock is fragmented and rich in clayey minerals, making it susceptible to mud-burst disasters during tunnel excavation. Cavities induced by mud-bursts are often hidden beyond and difficult to investigate their size, location, and filling condition. These cavities are highly unstable and prone to triggering subsequence of larger mud-burst disasters. Therefore, it is essential to study the influenced range of cavities in clay-altered rock. A large mud-burst disaster in the clay-altered rock was studied, which occurred during the excavation of the 2nd diversion tunnel of the Yingliangbao Hydropower Station, Sichuan, China. First, a refined 3D numerical model was built using on-site geological outcrop and drilling data. Second, cavity filling rate was introduced to estimate the size of the cavity. Finally, the influence range of the cavity under different cavity filling rates was evaluated. The analyses revealed that the numerical model accurately replicated the distribution of clay-altered rock. The actual cavity size was much larger than the volume of mud-burst debris measured in the tunnel, and the actual cavity size could be defined by the filling cavity rate. For short-term stability analysis, the cavity filling rate ranged from 80% to 90%. Furthermore, influenced by the orientation of faults, the influence zone of cavities tilted and exerted pressure on the 1st diversion tunnel, parallel to the 1st diversion tunnel approximately 40 m away, indicating rerouting to avoid additional disasters. The results provide a scientific basis and technical support for the engineering design and construction management to mitigate similar disasters in clay-altered rock.