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Boards from 1 to 4 in (2.5 to 10 cm) thick and of sufficient length to
bridge the space between timber or steel sets or between rings in skeleton
tubing. Usually, planks 9 to 12 in (23 to 30 cm) in width are used. Round
poles, either whole or split, light steel rails, ribbed sheet metal, and
reinforced concrete slabs are sometimes used in place of planks. Backing
deals tighten the supports against the ground and also prevent the
collapse of material between the timber or steel sets or rings.
See also: lagging
Source:
Dictionary of Mining, Mineral, and Related Terms
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