Tunnels under water obstacles are often built instead of bridges where bridges can prevent ships from passing. They also build tunnels to avoid the intersection of different traffic flows at the same level (subways, tunnels instead of railway crossings, tunnels as part of road junctions).
A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power. or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely.
The main part of a metro as well is laid in tunnels. To avoid intersections, subway lines are laid at different depths.
Development is necessary for construction of a tunnel, that means an artificial void in the earth’s crust. In stable rocks the development is usually left without any additional fortifying structures. In unstable rocks they build first temporary fortification and then facing. Facing is a critical element of a tunnel, perceiving pressure of surrounding rocks and providing tunnel waterproofing. Tunnel parts near to its exits are called portals. Portals give architectural view to the tunnel entrances against environmental landscape.
Types of construction:
1 . Closed methods of construction of tunnels are used for building of tunnels with deep and shallow bedding.
1.2. Stable rocks of medium fortress and solid:
mining with using of drilling-and-blasting works;
harvester method of penetration, it is development of the soil with the help of special tunnel sinking harvesters.
1.3. Burst and soft rocks:
new Austrian way of penetration, it is penetration using pliable arch;
shield tunneling method, they develop soil with the help of tunnel shields, then build tunnel facing.
1.3. Unstable, flooded soils and severe atmospheres:
tunneling methods using compressed air, freezing, water fall or grouting with special solutions;
shield tunneling method.
2. Open methods, as a rule, are used for construction of tunnels of a shallow bedding. Compared with closed methods, the construction of open ones is much cheaper, but it is necessary to rearrange roads and the communications that are above a tunnel.
method of excavation;
trench method, in this manner they often build pedestrian tunnels;
If construction of tunnels is made in difficult engineering environment they use special methods: drainage, soil freezing, caisson method with compressed air.