An observational excavation control method for adjacent mountain tunnels

An observational excavation control method for a mountain tunnel excavated adjacent to an existing tunnel in active service is presented. The new and existing tunnels both having a width of 11.7 m was separated from each other by a distance of 20 m. Besides the small pillar width, a fracture zone with clayey material due to hydrothermal alteration, which would appear in the middle of the new tunnel alignment, had to be taken into account. Under such circumstances, the excavation control of the new tunnel was required to minimize the adverse effect on the existing tunnel. Prior to the excavation, detailed geological surveys on the fracture zone and FEM analyses incorporating the survey results have been carried out. The relationship between the incremental stresses occurring in the concrete lining of the existing tunnel and the expected wall displacement of the new tunnel is established. It has therefore become possible to control the excavation based on the wall displacement of the new tunnel and to select the necessary reinforcement and support method. With this control method the new tunneling construction has been performed without any adverse effect on the existing tunnel.