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Excavation for rock chambers generally starts with a horizontal tunnel at the top of the area to be excavated and progresses down in steps. Rock is excavated by drilling and blasting, carried on simultaneously in several headings. This procedure may give way, however, as moles gain in their ability to cut hard rock economically and as a rock saw or other device is developed for squaring up the circular surface normally cut by the mole. High geostress can be a real problem (causing inward movement of the chamber walls) unless handled by a careful sequence of ... (100 of 18283 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.
"tunnels and underground excavations." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica, 2011. Web. 25 Mar. 2011. <http://www.britannica.com/EBchecked/topic/609297/tunnel>.
tunnels and underground excavations. (2011). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/609297/tunnel
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