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A soft-ground tunnel below the water table involves a constant risk of a run-in—i.e., soil and water flowing into the tunnel, which often results in complete loss of the heading. One solution is to lower the water table below the tunnel bottom before construction begins. This can be accomplished by pumping from deep wells ahead and from well points within the tunnel. While this benefits the tunneling, dropping the water table increases the loading on deeper soil layers. If these are relatively compressible, the result can be a major settlement of adjacent buildings on shallow foundations, an extreme ... (100 of 18267 words)
Aspects of the topic tunnels and underground excavations are discussed in the following places at Britannica.
Articles from Britannica encyclopedias for elementary and high school students.
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 (see aqueduct). Mass-transit railway tunnels constructed under cities to relieve crowded streets are known as subways.
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