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| | burrow - Hutchinson encyclopedia article about burrow (Site not responding. Last check: 2007-10-08) |
 | | Large-section tunnels are often driven in two stages; a pilot heading is excavated in advance which is afterwards enlarged to the full section of the main tunnel. |
 | | The various shots in the tunnel face, constituting the complete round, explode either instantaneously or with a time lag of 2, 3, or 4 seconds according to their position in the rock face. |
 | | Bentonite slurry under pressure is being used to drive sewer tunnels in Hamburg, West Germany, and Warrington, Cheshire; this new development ensures the stability of the ground about to be excavated, and provides lubrication to help with pushing the tunnelling mole forwards. |
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