A study on the failure mechanisms of layered soft rock tunnels considering the random distribution of rock mass mechanical parameters and the characteristics of anchor bolt failure

To investigate the asymmetric deformation and stress characteristics of tunnels and support structures in high geostress layered fractured rock, this paper establishes two refined modeling methods: a numerical model for anchor bolt failure and a model for fractured layered surrounding rock, while considering the spatial variability of soil. The study analyzes tunnel deformation and bolt tensile-shear fracture mechanics under varying bedding angles. The results indicate that: (1) the most unfavorable stress position for tunnel structures in layered fractured rock typically occurs normal to the bedding planes; (2) the tunnel's asymmetric deformation is due to normal compressive and tangential sliding effects of geostress on the bedding planes. When the bedding angle is gently inclined, significant extrusion deformation occurs at the tunnel crown and invert; when steep, substantial tangential sliding forces cause maximum deformation at points where the bedding direction is tangent to the tunnel profile. (3) Fracture development in the surrounding rock primarily occurs normal to the foliation planes, similar to maximum displacement deformation patterns, while other areas propagate outward due to joint shear slip. (4) In layered fractured rock, failed bolts predominantly show tensile-shear fractures, influenced by bedding angle, particularly near the left shoulder to the crown and right invert. Finally, based on the deformation characteristics of layered fractured surrounding rock and the mechanical properties of anchor rod fracture, reasonable differential support optimization measures were proposed, and the simulation results were applied to the Yangjiaping Tunnel of the Chenglan Railway in China.