Influence of existing tunnel on mechanical behavior of new tunnel

By quantifying the displacement and crack propagation during the excavation of a new tunnel constructed near an existing tunnel, the influence of the size of the existing tunnel, the distance between tunnel centers, and the earth pressure coefficient, K on the mechanical behavior of existing and new tunnels was investigated and analyzed. A series of experimental model tests was performed and analyzed. It was found that the displacements decreased and stabilized as the distance between tunnel centers increased depending on the size of the existing tunnel. Consequently, a 3.0D distance between tunnel centers for Model Test I and 1.2D for Model Test II are required conservatively to avoid the tunnels being influenced by each other. It was also found that regardless of the distance between tunnel centers, displacements are reduced and hence the stability of the pillar can be secured as the earth pressure coefficient increases. This fundamental insight provides the basis for a more rational design of closely spaced twin tunnels.