Coal mining in Illinois . uire no timber-ing or there may be an alternation of rock top and clod asshown in fig. 43. Where the transition from limestone roofto clod is abrupt it is productive of many accidents from rooffalls. At a mine in District VII there is a thick shale depositoverlying the coal and the roof on both sides of the shaftcaved to a height of 42 feet from the floor. This cave ex- TIMBERING 137 tended 110 feet along the main entry. Fig. 44 shows themethod of timbering the entry in the caved area. The framesshown were set on 4%-foot centers. In many of the mines examined in Distr
Coal mining in Illinois . uire no timber-ing or there may be an alternation of rock top and clod asshown in fig. 43. Where the transition from limestone roofto clod is abrupt it is productive of many accidents from rooffalls. At a mine in District VII there is a thick shale depositoverlying the coal and the roof on both sides of the shaftcaved to a height of 42 feet from the floor. This cave ex- TIMBERING 137 tended 110 feet along the main entry. Fig. 44 shows themethod of timbering the entry in the caved area. The framesshown were set on 4%-foot centers. In many of the mines examined in Districts II, IV, VI,and VII top coal was left where the immediate roof over thecoal was thick black shale. Top coal prevents variations oftemperature and humidity from affecting the shale of theroof proper, which spalls badly when exposed to the air. Asa rule where no top coal is left the shale falls with the coalor is drawn. Where there is less than four inches of shalebetween the coal and the cap rock it is drawn. Where the. #fei Fig. 43. Alternation of ^ood and bad roof shale is over 4 inches thick it is propped in some mines, butin others it is drawn unless it is over 2 feet thick. In District II where the two benches of the bed are unitedand the coal is over 6 feet thick, top coal is left up in entriesand the roof is arched. In this district no timber is usedin entries under top coal except where it is broken by the lower bench only is mined the roof is supportedby three-piece timber sets having 8-inch crossbars and 6-inch legs. White oak is generally used for entry timbering. In rooms in Illinois the variations in propping are as wideas are those in entry timbering. The roof may be limestone 138 COAL MINING INVESTIGATIONS or hard shale requiring no propping or top coal may be leftor the roof may be clod with such slight cohesion that it breaksat the prop or it may be black shale so difficult of supportthat it requires cross-bars on props. A mine prop is supposed t
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcoalmin, bookyear1915
