Study on the mechanical behaviour and stability control of rock mass ahead of the tunnel face in an ultra-deep soft rock tunnel

Ultra-deep soft rock tunnels are commonly affected by complex geological conditions, particularly near the tunnel face, where stress redistribution in the rock mass can induce instability and collapse. This study investigates the mechanical behaviour of the rock mass ahead of the tunnel face and proposes effective stability control measures. By integrating field monitoring and numerical simulation, the face squeezing deformation, loosened zone, and plastic zone were systematically analyzed. Several advance support schemes, including forepoling pipes, forepoling umbrellas, and grouting, were evaluated in terms of their stabilising effects. The results show that face squeezing deformation increased sharply when the tunnel face approached within 15 m of the monitoring section. After the implementation of the stabilisation measures, deformation was reduced to 0.15 m, corresponding to a 62.5% reduction relative to the uncontrolled condition based on field monitoring. The proposed support optimisation strategy improves tunnel face stability, reduces construction risk, and provides theoretical and practical guidance for similar ultra-deep soft rock tunnel projects.