The limitation of established road networks above ground has been an issue in overcrowded cities. In order to solve this problem, new road networks have recently been planned and constructed under the ground instead of above ground due to environmental issues. Consequently, in urban areas, the development of new road tunnels inevitably involves neighboring existing underground structures. In this case, the stabilized ground near an existing underground structure could be relaxed by the excavation of a new tunnel. This will create issues in terms of stability of existing and new tunnels. One major factor determining the stability of existing and new tunnels can be the size of the existing and new tunnels, the distance between tunnel centers, in-situ stress, ground condition, joint orientation and location, and so on.
Most of the previous studies mainly focused on the variations of joint orientation and location and layered ground with only one tunnel size. They also did not simulate each excavation step of tunnel during the model tests. In this study, the influence of the size of the existing tunnel located near the new tunnel, the distance between tunnel centers, and the earth pressure coefficient, K on the mechanical behavior of existing and new tunnels is investigated and analyzed. Especially, each excavation step of tunnels is modeled and analyzed. A series of experimental model tests are performed and analyzed under the condition of homogeneous material. Finally, an appropriate distance between tunnel centers according to the tunnel size for the stability of existing and new tunnels is suggested.