Determining the best depth of subway tunnel excavation considering ground type, support system characteristics, and tunneling cost: case study of Tabriz subway, Line 2

The cost of subway construction is especially dependent on stations' construction cost. In order to minimize these costs, an optimum depth for subway tunnels should be adopted. This study intends to investigate the best depth of tunnel construction for Tabriz subway, Line 2. The study is carried out in two categories of technical and financial studies. In technical studies, surface settlement is the ruling criteria, which its allowed limit is 25 mm. Results showed that depths less than 1.5 times the tunnel diameter bear settlement above the limit; therefore, tunneling in such depths is not recommended. Financial studies consist of a cost analysis of tunneling and subway station construction. To do so, the depth of station construction and their distance from each other have been analyzed. Results indicated that the construction costs are highly dependent on the stations' depth and their distances. To put it more clearly, the least subway costs are achieved when the overburden is at minimum and station distance is at maximum. To be more specific, along the 17 km of the route, when the station depth is greater than twice the tunnel diameter, and their distance is 750 m, the unit cost of construction reaches 29 M$, while it is at its minimum (7 M$) when the stations' depth is less than tunnel diameter, and their distances are 3000 m. Eventually, the optimum depth for Line 2 of the Tabriz subway is 1.5 to 2 times the tunnel diameter, while obviously, more distanced stations cost less.