That means mainly lack of surface place which is now the case not only in mega cities but in nearly all town and cities around the world.

The ground is massive and opaque and provides a variety of advantages in terms of isolation, such as climate, natural disasters and earthquake, protection, containment. Isolation is an important reason for placing facilities underground.

The ground also provides a variety of advantages in terms of protection of the environment, such as aesthetics or ecology. These are notably important aspects in designing facilities with a low environment impact.

In hilly or mountainous areas, the use of tunnels improves or make feasible various tranport options uch as roads, railways, canals, etc.

Conventional Tunnelling can be defined as the construction of underground openings of any shape with a cyclic construction process.

As opposed to conventional techniques, these are all the tunneling techniques in which excavation is performed mechanically by means of teeth, picks or disks. These tunneling techniques comprise then a wide range of different machines

Surface tunnelling encompasses all construction methods of underground structures built by a temporary or permanent open trench in the surface.

Immersed tunnels have been in widespread use for about 100 years. Over 150 have been constructed all over the world, about 100 of them for road or rail schemes.

A typical example of underground structure where a combination of methods is employed is a deep base transalpine tunnel, like Gotthard and Lotschberg tunnels in Switzerland.

Installations and equipment are an important part of underground structures. They are complex, and usually comprise a quite large part of the general budget and planning of the whole structure.

Several factors influence the evolution of underground facilities from their opening. Basically, they are: age, material quality, construction method,and environment, characteristics of the traffic or fluids carried.

Due to the high value of tunnels a good maintenance scheme must be planned for all underground structures during its life of operation

Rehabilitation of an underground structure could be described as its restoration to good condition, operation, or capacity.

On some occasions, the objective of rehabilitation works is to increase the functionality of the underground structure or its capacity.

Before any design or planning work begins, an occupational health and safety strategy for the project should be drawn up and set out in a health and safety plan.

Training, equipment, safety signs, fire fighting, welfare, athmospheric monitoring, lighting.

Following several dramatic accidents in road tunnels, between 1999 and 2001. Tunnel safety became a concern, which led to a strong demand for improvements from tunnel users and politicians.

With large concentrations of people in limited space, it has become necessary to utilize the subsurface for locating a growing number of services in urban environments.

In a growing number of cases, public buildings are being built underground in some of the major cities all over the world.

One of the big advantages of underground spaces is the high thermal inertia and the relatively limited cost of construction when the rock mass is of an adequate quality.

Even if it is not very common to locate industrial activity underground, on some occasion’s strategic or hazardous activities are carried out underground.

The need of secrecy and the high strategic value of military installations make the use of the underground space of particular interest.

The Trans-Tokyo Bay Highway, also known as the Tokyo Bay Aqualine, is a 15.1 km marine crossing through the middle of Tokyo Bay connecting Kawasaki City in Kanagawa Prefecture with Kisarazu City in Chiba Prefecture on the Boso Peninsular.

The Madrid M30 motorway, the inner ring road of the city, has been at the centre of a major urban renewal project where the motorway has been redirected underground.

The Gotthard Base Tunnel (GBT) is a railway tunnel under construction in Switzerland

Recognized as the largest, most complex, and technologically challenging highway project in the history of the United States, the Central Artery/Tunnel Project significantly reduced traffic congestion and improved mobility in one of America's oldest and most congested major cities.

Greater Cairo underground metro is considered one of the most important national projects executed in the Republic during the second half of the twentieth century.

In 1954, a typhoon sank five ferry boats in Japan's Tsugaru Strait and killed 1,430 people. In response to public outrage, the Japanese government searched for a safer way to cross the dangerous strait.

The Seoul Metropolitan Subway is one of the most heavily used rapid transit systems in the world, with well over 8 million trips daily on the system's ten lines totaling 287 km.

Malaysia Kuala Lumpur’s Stormwater Management and Road Tunnel (SMART) is a unique solution to the Malaysian capital’s long-term traffic and stormwater management problems and the first tunnel of its kind in the world.