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Tunnel Boring Machines are specially designed machinery to build tunnels with a circular cross section. Tunnels boring machines can “dig" though a variety of soil types including hard rock. Although these machines can be used and are used in any type of tunnel construction they are mostly needed when doing Urban Tunneling, that is when building an underground tunnel beneath a city. Using a tunnel boring machine (TBM) has many advantages over the traditional “drilling and blasting" method of constructing a tunnel. Using a TBM makes a smooth circular tunnel wall reducing the work needed to align the tunnel walls and thus reducing tunneling costs. Also, a TBM causes fewer disturbances on the surface than hand mining and blasting.
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How a Tunnel Boring Machine Works
Tunnel boring machines have one or two large metal cylinders (shields) mounted on a trailing support mechanism. The front of the shield has a rotating cutting wheel. Following the cutting wheel there is a chamber where the excavated material (sand, rock, or any soil mix) is either mixed with water to make a slurry or left as it is. A system to remove the excavated material completes the tunnel boring machine.
In basic terms, the TBMs work like an earthworm with cycles of digging forward and dragging the rear end afterwards. A series of hydraulics systems pushes the TBM forward (excavating the soil) while the rear end of it is braced against the tunnel wall. When the TBM head has excavated at its maximum length (this depends on may variables including TBM type, soil type, etc) the front end of the TBM is braced against tunnel wall and the rear end is pulled forward. These cycles continues until the complete tunnel has been excavated.
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Tunnel Boring Machines Parts
Rotating cutting head: this is located in front of the TBM. It is the part that does the actual “digging" although it is more of a cutting action. The cutting wheel will rotate at different speeds (1 to 10 rpm) according to the soil type. Since the invention of the rotating cutting head there has been a significant reduction in the need of replacing the cutting parts. These, although not infinite, have quite some long lasting lifespans.
Metal shields: this is a large metal cylinder used to brace the machine against the wall tunnels. There can be one or two shields depending on the TBM type and type of soil to be excavated. Rock type soils can be excavated with a one-shield machine while sand soil digging requires a two-shield TBM
Supporting mechanisms: Behind the cutting wheel and the shields TBMS have a vast array of supporting mechanisms such as dirt removal, control rooms, rails, transporting system, pipelines (if slurry is created), etc.
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Wagner and Schulter. 1996. Tunnel boring machines: trends in design & construction of mechanized tunneling.Taylor and Francis.