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tunnels and underground excavations

Water inflows

Exploring ahead of the path of a tunnel is particularly necessary for location of possible high water inflows and permitting their pretreatment by drainage or grouting. When high-pressure flows occur unexpectedly, they result in long stoppages. When huge flows are encountered, one approach is to drive parallel tunnels, advancing them alternately so that one relieves pressure in front of the other. This was done in 1898 in work on the Simplon Tunnel and in 1969 on the Graton Tunnel in Peru, where flow reached 60,000 gallons (230,000 litres) per minute. Another technique is to depressurize ahead by drain holes (or small drainage drifts on each side), an extreme example being the 1968 Japanese handling of extraordinarily difficult water and rock conditions on the Rokko Railroad Tunnel, using approximately three-quarters of a mile of drainage drifts and five miles of drain holes in a one-quarter-mile length of the main tunnel.

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tunnel - Student Encyclopedia (Ages 11 and up)

When natural obstacles-such as mountains, hills, or rivers-block the path proposed for a railway, highway, or pipeline, engineers bore tunnels through or under the obstacles. Structures built as trenches and later covered are also often called tunnels. Tunnels that bring water from reservoirs to cities may be called aqueducts. Mass-transit railway tunnels constructed under cities to relieve crowded streets are known as subways.

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