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a. Set of timbers used in a shaft outside the regular shaft set, as extra
protection in heavy ground. Pryor, 3
b. Set like the larger shaft set with dividers omitted, except that the
wall and end plates are broken at all joints to facilitate renewing. Used
in heavy ground to protect the regular shaft timbers. The jacket set is
placed outside the regular timbers, separated from them by short blocks,
and is blocked and wedged against the rock. Lewis
Source:
Dictionary of Mining, Mineral, and Related Terms
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