NATM-STM Application for Tunnel Construction Underneath the Operating Railway, Case Study: Azadegan Depot Link Tunnel of Tehran Metro Line 3

One of the main challenges in the construction of the Azadegan depot of Tehran metro line 3 was the excavation of a link tunnel under the railways. The link tunnel's diameter was about 10 meters and was passed under south railways to connect the depot with low overburden (about 5 meters). The tunneling experiences in Tehran alluviums and also the results of numerical modelling showed that crossing the tunnel under railways without using any pre-support system was not possible. This article provides a technical approach to designing a pre-support system for the excavation of a shallow tunnel under the railways. According to the train speed and the type of sub-structure, the maximum allowable settlement in 10 meters' length was limited to 20 mm, which analytical settlement was not in the allowable range. To reduce risks caused by ground settlement and overcome the challenges of this project, the shallow tunnelling method (NATM-STM) with a pre-support system was proposed. The results of numerical modelling for the proposed design showed that the maximum ground settlement reduced to 11 mm (approximately); it was within the allowable range. This study provides helpful information for shallow tunneling underneath the operating railways in soft soil around sensitive constructions and excavations under complex conditions.