Topographic reasons
In hilly or mountainous areas, the use of tunnels improves or makes feasible various transport options such as roads, railways, canals, etc. Tunnels are also an important option in river, straits and harbour crossings .
Generally speaking, underground space use off e r s many advantages with regard to the layout of facilities and infrastructures. These advantages derive 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.