Great Alborz twin tunnels with 6450 meters in length, located in 100 km distance from north of Tehran, are the largest structures in the Tehran-North highway construction. The Alborz tunnel passes rugged mountains with high and low reliefs. These tunnels consist of two main tunnels and an exploration tunnel. The width and height of main tunnel tubes are 12.4 and 8.9 meters respectively. Main tunnels connected to each other every 600 m by shortcut (traffic) tunnels. These shortcut tunnels have width and height of 6.2 and 5 m respectively. Near the intersections, the openings of the main tunnel widen and expand to 12.4 width and 8.9 m height. Numbers of shortcut tunnels in the Alborz tunnel are 10 sets. This study analyzes the impact of excavation of three shortcut tunnels which are located in different formations and geological conditions on the stability of main tunnels with Flac 3D. The first one located 600 meters from north span, is embraced in Shemshak formation and includes 230 meters of overburden. Displacement and support systems (Shotcrete with 25 cm thickness and rock bolt System) of this area is in a stable condition. The second region with a distance of 3000 meters from north span, located between Dorood and Karaj formations and the area is located under 850 meters of geological formations. Final displacement for roof of the main tunnel is 63.6 mm. This value indicates an unstable condition in Sakurray?s criterion and also shows the stable condition in comparison to Hoek?s diagram. Support systems (including Shotcrete with 30 cm thickness, steel sets with 0.5 meters spacing and rock bolt System) provide stability of this intersection area. The third area is situated in 5850 meters from north span and is located in Karaj Formation. Depth of this region is 323 meters. Displacement and support systems (Shotcrete with 25 cm thickness and rock bolt System) of the third region is in a stable condition. It has been concluded that the impact of widening on the main tunnel is more than implementation of shortcut tunnels over displacement and support system.
