Optimization of depth for subway tunnel excavation considering Technical and economic perspective (case study; the second line of the Tabriz subway)

In this research, the effect of different tunnel depths, has been investigated from both technical and economic perspectives. Then, the optimal depth for the construction of subway tunnels has been determined according to both views. Ground surface settlement with different overburdens is the technical perspective. In this study, the allowable surface settlement limit is considered 25 mm. If the ground surface settlement is more than the allowable limit, It cannot be technically suitable for tunnel excavation. Estimation of tunnel excavation costs as well as construction costs of metro stations, along the second line of the Tabriz subway form the economic perspective of this study, to optimization of depth for subway tunnel. This research has been undertaken to identify and optimization of depth for subway tunnel excavation considering Technical and economic perspective, in the second line of the Tabriz subway as a case study. The Line 2 of Tabriz Metro consists of a double-track tunnel with a total length of 21km and 20 underground stations. The ground of the second line of the Tabriz subway is classified in two modes; The first case is, based on the type of ground and the second case is, based on the amounts of overburdens in the tunnel route. In the first case; The different layers of land along the tunnel route are classified into four general categories. In the second case; Tunnel overburdens are classified into five general categories along the tunnel route based on their height. In this research, FLAC3D V6.00.69 software was used. The amount of ground surface settlement has been investigated in 16 different models. Grounds that have more settlement than the allowable depth limit, cannot be suitable for tunnel excavation. Then, the informations related to the tunnel excavation costs as well as construction costs of metro stations, along the second line of the Tabriz subway have been calculated. Next, the optimal depth is determined from an economic perspective. Depths where the tunnel overburden is less than the diameter of the tunnel, are technically more than the allowable settlement limit, and it is not recommended that the tunnel be excavated in that spatial range of the ground. On the other hand, the investigation of this study showed that with increasing depth of tunnel, the tunnel excavation costs initially decreased, and after that when the tunnel overburdens are more than twice the diameter of the tunnel, it increases again. Also, with the increase of tunnel overburden, the construction costs of metro stations have increased. So that the construction costs of a one-story station is equal to 95 billion tomans. Meanwhile, the construction costs of a four-story station was equal to 250 billion tomans, which is much more than the previous case. Therefore, the optimal depth for the construction of tunnels and stations is when the tunnel has more overburden than the diameter of the tunnel and also less than one and a half times the diameter of the tunnel.