With the improvement of construction techniques and urban development, tunnels are now built under our cities.
The most common technique of cut-and-cover is often replaced by the use of mechanized tunneling; a technique that nowadays has made possible the construction of tunnels of huge diameters up to 15 meters (Shangai and Madrid M30 tunnels).
Urban road tunnels may be used to divert traffic from center cities or to ease this traffic in the center. They are also built on some occasions to cross rivers or ports.
From a technical point of view, the construction of tunnels under the cities is particularly challenging. Due to the presence of buildings at the surface and other infrastructure in the vicinity of underground tunnels, parameters like vibrations and ground settlements must be tightly controlled. Therefore, urban tunnels are often made in soils with very low values of overburden. Risks of collapse and large deformations at the surface are very high, and negative impacts on old buildings is likely to occur if measures are not taken in advance when designing and building the tunnel.