ABSTRACT
The construction of a new subway line in any urban area is a complex task and typically needs large investment of hundreds of million dollars (in some cases billions) over a few years. Selection of the alignment with low-level risks is usually taken place at the environmental impact statement stage or at the planning design level. It should be proven by the completion of the alignment selection process that the selected alignment is fully feasible and economically cost-effective investment for the client. In almost all mechanized excavation, a circular shape is used with only variable being the dimension of the tunnel diameter. In this paper, a comprehensive comparison of advantages and disadvantages of single tube versus twin tubes for a subway system is outlined. Different aspects such as tunnel diameter (tunnel spaceproofing), configuration of subway line and stations, impacts on passenger safety and evacuation, ventilation system as well as rail system for each solution are evaluated and compared in depth. The practical example of a mega project has been discussed and the details of the decision-making process has been explained. A multi-criterion approach is established in the end to facilitate the decision making for these two different subway systems in any urban areas.
