Time and technique of rehabilitation for large deformation of tunnels in jointed rock masses based on FDM and DEM numerical modeling

When large deformation occurs in a tunnel, resulting in the primary lining intruding into the tunnel clearance, an appropriate treatment method is critically important to ensure that the tunnel can be completed on time and safely. This paper investigates the optimal treatment timing and technique for replacing the deformed primary lining. For this purpose, a two-stage numerical approach is employed for a severe tunnel collapse accident occurring on the Jinhong Highway. In the first stage, a global tunnel model is constructed using the finite difference method (FDM). The parameters are calibrated by the synthetic rock mass (SRM) approach, laboratory testing and field data. The continuously yielding (CY) joint model is used in the SRM, and its parameters are derived by the joint direct shear test. In the second stage, a local model encompassing the discrete fracture network (DFN) is constructed using the discrete element method (DEM) in the collapsed zone. Based on the DEM, the influence of the primary lining rehabilitation on the failure of the rock masses is analyzed. The local model is used to study the optimal rehabilitation timing, the optimum rehabilitation duration and the reasonable length of each replacement cycle. The following three conclusions are drawn. (1) The optimal rehabilitation timing is when the deformation of the rock masses reaches 90% of the total deformation. (2) There is a time threshold for the duration of the rehabilitation; if the duration is less than this value, the impact on the surrounding rock is small and almost the same. However, if the duration exceeds this value, the damage to the rock mass will increase as the duration increases. (3) The reasonable length of every replacement cycle should be less than the steel support spacing.