a. A pillar mining system in which a series of entries, panel entries, rooms, and crosscuts are driven to divide the coal into blocks of approx. equal size, which are then extracted on retreat. Development openings are most commonly driven between 15 ft and 20 ft (4.6 m and 6.1 m) wide. Pillars are most commonly 40 to 60 ft (12.2 to 18.3 m) wide and from 60 to 100 ft (18.3 to 30.5 m) long. Woodruff
b. A system of control in which a number of units, for example, powered supports, are operated as a group. NCB
Source:
Dictionary of Mining, Mineral, and Related Terms












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