Development of traortation in large cities and the need for rapid transit systems, due to the shortage of urban space, natural barriers require the construction of new tunnels. According to how construction of tunnel, creating the new tunnels near the intersection with the previous tunnel, would be necessary there for the stability analysis of new and previous tunnels is one of the most important topics that will be addressed during the construction of new tunnels, it should be done in the way that the previous tunnels maintain its efficiency. Tehran-Shomal (North) highway in the West of Tehran, starts from Shahid Hemmat crossing and at own path crosses along the Alborz mountain tunnel with a length of approximately 6400 meters (from km 47+800 to 54+200). Great Alborz twin tunnels is one of the largest structures that are constructed in Tehran-Shomal (North) highway. These tunneles begin from Gachsar to the Zangole bridge in Karaj-Chalos Road. Alborz Tunnel consists of two main tunnels (there and back) and an exploration tunnel (between the two main tunnels). Dimensions of width and height of Alborz tunnel are 12.04 and 9.07 meters, respectively. The main tunnels of Alborz are connected to exploration tunnels by the access tunnels. The distance of these access tunnels are approximately 600 m. Dimensions of width and height of access tunnels are 5.2 and 5.33 meters, respectively. complex conditions such as stress and displacement variation due to excavation of exploration tunnel, the main tunnel in parallel with exploration tunnel and access tunnel between exploration tunnel and main tunnel, creates. The main purposes of this study is investigation of the effects of inclined access tunnels excavation on the main tunnels stability and proposition of the new maintenance system due to high stress concentration at the junction. Also, this study showed that the effect of inclined tunnels excavation is 1.5 times of inclined tunnels width from the center of junction. Also, these results indicate that direction of inclined tunnel excavation was affected on main tunnel lining and excavation of inclined tunnel from the main tunnel toward exploration tunnel causes axial forces and bending moments in the lining of the main tunnel to increase.
