Coal mining in Illinois . approximately $ An 8-inchround white-oak leg 6 feet long costs about 80 cents at the pitmouth. Inasmuch as sets fail in the crossbars when not sub-jected to lateral presure and the labor cost of replacing a legis small the use of steel legs may entail an unnecessary ex-pense. Old railroad and streetcar rails are used as crossbars inDistricts IV, VI, VII and VIII. Old rails have been pur- 134 COAL MINING INVESTIGATIONS chased in one district for $12 per ton. When bought forroof support their weight varies from 50 to 75 pounds peryard. In District VIII 60-pound rai
Coal mining in Illinois . approximately $ An 8-inchround white-oak leg 6 feet long costs about 80 cents at the pitmouth. Inasmuch as sets fail in the crossbars when not sub-jected to lateral presure and the labor cost of replacing a legis small the use of steel legs may entail an unnecessary ex-pense. Old railroad and streetcar rails are used as crossbars inDistricts IV, VI, VII and VIII. Old rails have been pur- 134 COAL MINING INVESTIGATIONS chased in one district for $12 per ton. When bought forroof support their weight varies from 50 to 75 pounds peryard. In District VIII 60-pound rails used as crossbars failedunder the roof weight. No rail lighter than 70 pounds peryard should be used under heavy roof presure and even heavyrails are inferior to I-beams because their carbon content ishigh causing them to break more easily than the I-beam andtheir section is not adapted to the purpose. Shaft bottoms at many mines in all districts are crudelytimbered usually with 16 to 24-inch framed 3-piece sets carry-. Fig. 40. Solid concrete pier at branch ing 2-inch lagging or with round timber legs and in nearly all new mines is being used for roof-supportsat shaft bottoms. Concrete in all large mines is coming into general useas a substitute for close or massive timbering as at shaftbottoms or at pillar points at partings. Fig. 38 shows a shaftbottom at a mine in District VI where the roof is supportedby steel I-beams resting on concrete walls and fig. 39 showsthe inby end of a shaft bottom in District VIII which is linedthroughout with concrete. The walls of the lining are 24inches thick at the bottom and the thickness of concrete is TIMBERING 135 gradually reduced till at the crest of the arch it is 12 gob filling is packed between the arch and the roof. Thelength of concrete bottom on each side of the shaft is 165feet. The concrete was made in the following proportions:1 Portland cement; 1 sand; 4 washed gravel. Proportionsfor concrete which
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcoalmin, bookyear1915
