Methods of tunnel construction

The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the final use and shape of the tunnel.

Drill and Blast is one of the most widely used tunneling methods. It is used when the tunnels are in rock and involves the use of explosives. Explosives and timed detonators are placed in the blast holes. Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further blasting.

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Bored tunneling by using a Tunnel Boring Machine (TBM) is often used for excavating long tunnels. Tunnel Boring Machine (TBM) is specially designed for constructing tunnels which could perform different functions during tunneling works. With a large rotating steel cutter head at the front of the shield, TBMs can pass through different types of soil, rock or mixture of both. The TBM can excavate and remove excavated materials and at the same time install the reinforced concrete lining of the tunnel as it progresses. The use of TBM requires relatively less works area, thus minimizing the impact to the traffic of nearby area.

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A shaft is built for delivering the components of the TBM from ground level to the tunnel level for assembly. As the TBM pushes forward, the excavated materials will be transported to the rear end of the TBM for removal through the vertical shaft.

Shallow tunnels are often of the cut-and-cover type, while deep tunnels are excavated often using a tunneling shield. For intermediate levels, both methods are possible.

Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over to carry the load of what is to be built above the tunnel. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing.

http://www.gautrain.co.za/construction/2008/10/tunnel-excavation-methods/

There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes.

Immersed tube construction of underwater tunnels will have its elements built separately in a dry dock or shipyard. These elements are then taken to the site where a trench has already been made under the water to receive them. The segments are then immersed in the water and then joined to each other to form the tunnel.

Costs for immersed tube tunnels are considerably lower than those involved in boring a tunnel beneath the water. The speed of construction is also greater, mainly because activities are simultaneously carried out for almost the entire length of the tunnel.