Mesoscopic instability mechanism and grouting reinforcement of deep roadway surrounding rock with weak interlayers

Constructing deep underground tunnels faces severe challenges when the surrounding rock contains weak, unstable layers. This study investigates how the angle and spacing of these weak layers lead to the deformation and collapse of deep tunnel structures. A particle flow simulation, based on a 1,000–m-deep tunnel, was developed to recreate the physical behavior of the surrounding rock and observe how it cracks, shifts, and loses its internal support under various geological conditions. The findings indicate that steeper weak layers cause the rock’s internal support network to break down more severely, resulting in a peak crack density of approximately 1,250 per square meter and a fractal dimension of 1.5848 in the highly fractured zones. Conversely, a larger distance between these weak layers preserves more solid rock, significantly reducing overall damage, restricting the maximum displacement of the roadway roof to between 0.3 m and 0.4 m and maintaining a lower fractal dimension of approximately 1.33. The primary cause of tunnel failure is the widespread internal fracturing which in field conditions led to cumulative floor heaving exceeding 6 m and rib shrinkage exceeding 4 m prior to reinforcement. To control this instability, the injection of a grouting slurry into surrounding rock. This cementing slurry intervention glues the broken rock fragments back together, successfully rebuilding the internal support network and optimizing the fractal dimension of crack distribution. Ultimately, using this grouting reinforcement strategy fundamentally repairs the rock’s internal connections, transforming the broken material back into a stable structure.