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A tunnelling shield is a protective structure used during the excavation of large, human-made tunnels. When excavating through ground that is soft, liquid, or otherwise unstable, there is a potential health and safety hazard to workers and the project itself from falling materials or a cave-in.
A tunnel boring machine (TBM), also known as a "mole" or a "worm", is a machine used to excavate tunnels. Tunnels are excavated through hard rock, wet or dry soil, or sand, each of which requires specialized technology. Tunnel boring machines are an alternative to drilling and blasting (D&B) methods and "hand mining".
Tunnel Construction. Tunnels are dug in types of materials varying from soft clay to hard rock. 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 logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
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Constructed using a circular tunnelling shield developed by Peter W. Barlow and James Henry Greathead and lined with segmental cast-iron rings, this short tunnel under the River Thames successfully demonstrated new tunnelling techniques that would be used to construct most of the subsequent underground lines in London. [15]
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The term tube railway may refer to: . an atmospheric railway, a railway that uses differential air pressure to provide power for propulsion of a vehicle; an underground railway constructed in a circular tunnel by the use of a tunnelling shield