Investigation on three-dimensional large deformation characteristics of tunnel in stratified rock mass with coupled discrete–continuous method

The Xinhuashan tunnel is located in the western region of Hunan Province, China. Although the entrance section is shallowly buried and free from tectonic stress influences, severe large deformations occurred during excavation due to the softening effect of carbonaceous shale under water-rich conditions. The deformation characteristics include large deformation magnitude (on the meter scale), high deformation rate, and pronounced asymmetry. A three dimensional (3D) coupled discrete–continuous model that can modeling such deformation characteristics is proposed based on the knowledge of the tunnel deformation characteristics in carbonaceous shale strata. A parameter calibration method for the numerical model is developed based on the experimental results. The model effectively reflects the dip angle and strike on the large deformation of the tunnel. The results showed that the interlayer deformation in carbonaceous shale contributed nearly 50% to overall deformation. Joints also affect deformation patterns and damage modes. Interlayer deformation is most pronounced at joints intersecting the tunnel profile. Deformation concentrates at the arch location with slow inclination, while steep inclination increases sidewall damage risk. Larger dip angles result in more significant deformations, e.g., a 47% increase at 90° compared to 15°. As dip angle increases, there’s a transition from bending damage to slip deformation. Analyzing tendency and deformation reveals that strike of 45° is the most stable direction for tunnel excavation. Deformations are inhibitory within 0°-45° but rapidly increase for strike higher than 45°. The upper tunnel arch waist is particularly sensitive to shifts in structural tendencies and exhibits significantly higher rates of deformation increase than the arch foot. Strike influences asymmetric deformation in surrounding rock: sidewall deformation intensifies as strike increases, while arch foot deformation decreases.