Research on the Mechanical Behavior and Stability of Shallow Tunnels Crossing through Adversely Varying Surrounding Rock Masses

With the increasing transportation demand in the Yangtze River Delta region of eastern China, shallow-buried large-section tunnels have become the preferred solution for expressway construction in hilly areas because of their technical and economic advantages. In such shallow-buried tunnel sections, the surrounding rock is significantly affected by weathering, fractures, and joint development, which can easily induce plastic deformation, face instability, or even collapse. Furthermore, tunnel excavation may cross through varying rock grades, meaning the tunnel face can encounter abrupt geological changes at interfaces. This study investigates a tunnel along the section of the Yixing–Changxing Expressway by establishing a three-dimensional finite element model to examine deformation characteristics during excavation under varying surrounding rock interfaces. The results demonstrate that the rock interfaces can be approximately treated as a new set of joint slip surfaces, with tunnel stability being significantly better at interface angles of 15° and 30° compared with 0° and 45° conditions. The excavation-induced surface settlement disturbance extends 15 m in Class IV surrounding rock and 25 m in Class V rock, while the maximum crown settlement location shifts gradually from the tunnel’s right side to the left as a result of joint dip direction. For joints with a 76° dip angle and 47° dip direction, the maximum deformation initially concentrates on the right side of the tunnel centerline near the portal section, then progressively shifts leftward with advancing excavation, ultimately localizing on the left side at the exit section, clearly indicating the significant influence of joint orientation on overall tunnel displacement.