ABSTRACT
The construction of a new tunnel in any city environment is an immense task, often involving the investment of hundreds of millions of dollars over a few years. Selection of the alignment with low-level risks is a decision to be taken at an early stage, usually the Preliminary Design stage, of development of an urban tunnel project. The environmental context and the tunnel-excavation method influence the definition of the alignment to a great extent and impose various constraints that are rather different and often more important than those connected to a structure on the surface in an area outside the town. Road tunnels have much larger excavation diameters than railway tunnels because the roads always have at least two lanes and the sidewalks. When selecting the tunnel route for any urban tunnel project and subsequently defining the horizontal and vertical alignment, the constraints may cause hazards for the tunnel design and construction.
