In this paper, two parallel horseshoe tunnels of Tehran-Shomal freeway project is analyzed using 3D finite difference code (Flac3d). The project is located in north western of Tehran, which connects capital city of Iran to the north part. Dominant formation in the area consists of volcanic green tuff and has specific gravity less than 2500 Kg/m3. Because of the tunnels location it is important to start excavation from right line or left line. In this research, first, the left tunnel is excavated in two stages then supported and the result of stress concentration and displacements compared with another case that means to start excavation from right. Based on the Flac3d analysis, by excavating from left tunnel the maximum displacement was 3.75 mm on right tunnel crown and inversely displacement was 2.75 mm. Software outputs of displacement has compared to the tunnel convergence data which have recorded during tunnel excavation. The results of numerical analysis were matched to monitoring of tunnel convergence, approximately. According to geological condition of the area and results of analysis, it can be concluded that the effect of excavation in one tunnel on another was ignorable and there is no difference to start excavation from right or left.
Making an opening in the ground will alter the in situ principal stresses, the vertical stress (sv), maximum and minimum horizontal stresses (sH and sh). This leads to stress concentration and redistribution of stresses. Redistribution of stresses will make induced stresses around the opening. These redistributed or induced stresses have a definite zone of influence, which makes rock failure and creating the disturbed or failed zone that depends on the rock strength. According to the texts [1,2,3] beyond of the failed zone about 3–5 times of excavation width or diameter, induced stresses return to the earlier case, which makes excavation boundaries. In this study, an analysis was carried out by numerical analysis code Flac3d  to solve the controversial problem between employer and contractor in regard to starting excavation from left or right tunnel in Tehran- Shomal freeway project. Because of the tunnels geometry and location (Fig. 1) the employer (STA, Chinese group) has insisted on beginning the excavation from the right line but the contractor (PTA, our Iranian company) according to its recourses and equipment planned to start excavation from the left line from point of view of accessibility and its resources. The most important issues in the static analysis of the tunnels was the elevation difference between tunnels (10m) and horizontal difference (8m) that made a slope between the tunnels. During the excavation, no monitoring operation carried out on the slope but in the tunnel surface. In this work, firstly, the left tunnel has excavated and stress concentration, displacements, compression and tension zones were obtained. Considering the influence of the excavation from the left tunnel based on displacements and failure zone it was concluded that there is no difference to excavate from the left or right tunnel and vice versa.