In hilly or mountainous areas, the use of tunnels improves or makes feasible various transport options such as roads, railways, canals, etc. This is especially true when it comes to the construction of high speed transportation lines, as low values of steepness and radius are needed. Tunnels are also an important option in river, straits and harbour crossings, they are sometimes the only option available to cross water expanses (high currents, large depths, large maritime traffic, etc.).
Therefore, the layout of underground facilities and infrastructures benefits from several advantages. These advantages are derived essentially from the freedom (within geological, cost, and land ownership limitations) to plan a facility in three dimensions and from the removal of physical barriers on the land. These advantages are essential when planning infrastructure in very dense urban environments, where Cities that are capable of functioning both in social and hygienic terms, form the prerequisite for a decent life in built-up areas. Underground space has an important role to play in this respect, i.e. in the achievement of environmentally-friendly development, whether it be in the reduction of pollution or noise nuisance, the efficient use of space, economic development, the preservation of the living environment, public health or safety. In these fields, it offers numerous advantages. There is a big lack of space available and multiple geographical constraints.