Transactions . ofpressure until the working-faces came under the full pressureexerted by the strata in movement, reaching from some distance * Tran-. Inst. M. E., 1907, vol. xxxiv., page 407. DISCUSSION ROOF-WEIGHTS IX MINES. 181 over tlie coal-face to a position in the goaf, where they had come torest. This distance might vary from about 180 feet in thin to up-wards of 300 feet in thick seams. He (Mr. Krrkup) inclined to theopinion that the pressures exerted upon the coal-face at D did notrepresent what would result from the full weight of the superin-cumbent strata, but only that part which
Transactions . ofpressure until the working-faces came under the full pressureexerted by the strata in movement, reaching from some distance * Tran-. Inst. M. E., 1907, vol. xxxiv., page 407. DISCUSSION ROOF-WEIGHTS IX MINES. 181 over tlie coal-face to a position in the goaf, where they had come torest. This distance might vary from about 180 feet in thin to up-wards of 300 feet in thick seams. He (Mr. Krrkup) inclined to theopinion that the pressures exerted upon the coal-face at D did notrepresent what would result from the full weight of the superin-cumbent strata, but only that part which might be represented bya triangle, ABC (fig. 19). The sectional area, ACD, of pressuresacts on the face of the coal-seam and on the adjoining goaf-area,while the sectional area, BCD, of strata, beneath the goaf-area,tends to heave by the action of the face-pressure, AB. The objectof the miner should be to prevent the occurrence of any roof-frac-ture (which would tend to modify this), and by careful attention to. Fig. 19.—Diagram showing Directions of Pressures. packing the goaf to gradually allow the pressures of the roof andfloor to meet, at the same time keeping the working-face steadilyprogressing. He (Mr. Kirkup) was extensively working two seams with thefollowing section : ?—-Depth from the surface, 600 feet; strongrock-roof, 20 feet; coal-seam, 1 foot 8 inches: band, from3 to 12 feet; and coal-seam, 3 feet. Large areas of the lower3 feet seam had been worked by the pillar-and-stall system, andthe band proved to be a very bad roof, where it was about 3 feetthick; it improved where it was thicker. Subsequent to the ex-haustion of the lower seam, the upper thin seam was opened out bythe longwall system, and it was found to be very slightly broken :only sufficient to slightly modify its original hardness. The rock- 182 DISCUSSION ROOF-WEIGHTS IN MINES. roof was practically without a fracture, and gateways were carrieda length of 250 feet without the use of props: timber o
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