Inverted Arch Replacement Scheme of Two-Lane Tunnel with Grade-IV2 Surrounding Rock: A Feasibility Study

Based on the grade-IV2 surrounding rock of a Hebei two-lane tunnel, this paper examines an alternative scheme of the tunnel bottom support system. The long-term monitoring and analysis of the initial lining pressure of the arch foot, the strain of the steel arch frame, and the contact pressure of the secondary lining from the arch foot to the vault in the test section were performed by field tests. The FLAC3D numerical simulation method was used to study the surrounding rock deformation, stress distribution, and plastic zone range of the alternative scheme with a flat bottom tunnel. Through field tests and numerical simulation analysis, it was found that the supporting structure of added crossbeams at the bottom of the two-lane tunnel in the grade-IV2 surrounding rock could effectively suppress the arching of the tunnel arch bottom. The overall horizontal and vertical stresses were small, and the plastic zone range could be better reduced. Therefore, the supporting structure with the added crossbeam at the bottom can replace the inverted arch. This scheme can not only reduce the disturbances of the surrounding rock caused by the excavation of the inverted arch and ensure that the lining closes as soon as possible to ensure the overall stability of the tunnel, but also improve the construction speed and reduce the construction costs.