The three main types of tunneling are
- Cut and cover method
- Bored tunneling
- Immersed Tunneling
Let’s take a look at each briefly…
Cut and cover method:
This is a frequently used method when the required tunnel excavation is not very deep (i.e. up to around 80 feet or so). The method involves excavating a trench in the ground, and not boring the tunnel underground. At what stage this excavation is performed with respect to the tunnel construction makes a difference. Because of this, cut and cover method can be grouped into two as, bottom up method and top down method.
The bottom-up method is the straightforward one. First the retaining walls are inserted into the ground, that will later support the excavation from its sides. After that, The tunnel is built. And finally it is back-filled from top.
In the top-down method, again the side retaining walls are first inserted into the ground but this time the roof slab of the tunnel is poured first, before making the excavation. After that the tunnel is excavated below the roof and completed this way. This method has the advantage of restoration of ground surface much quicker than the bottom up method, but the construction is somewhat more challenging, due to having to excavate below the already poured tunnel roof.
This is tunneling in classical sense, as we understand it traditionally. It can also be called as mining method. In this method, the tunnel is dug underground by various excavation methods, or simply by drilling and blasting.
Excavation can be performed by TBMs, which stands for Tunnel Boring Machines., if it is a long tunnel. For shorter tunnels and traditionally, roadheaders are used. TBMs are much larger and more expensive but once they start drilling, good advance rates per day can be obtained. They can also have shields, to make excavation possible in soft grounds, where the shield protects the excavation area.
Drill and Blast methods are suitable for hard rocks, but in urban areas where the tunnel is not very deep in the ground, it can create problems and may not be allowed at all.
As the name implies, these tunnels are immersed into the water. The sections of the tunnel are built on land and then their ends are sealed. After that, they are carried to their final location over the sea, by being floated on the surface of water and being pulled. When they reach to their final location, they are immersed and connected to the next segment. By definition, these tunnels sit on the river or sea bed. There is a trench that is prepared before the immersion, to serve as a foundation for the tunnel. After the immersed sections are placed and connected, their top is covered with stone, to serve as a protective layer.