Transactions . off as the hanging wall is reached so it is the usualpractice only to undercut the stopes to a point about 20 ft. (6 m.) fromwhere the values cease. The undercutting of the stopes is then com-menced from these raise cross-cuts, this undercutting being carried for the a. T. JACKSON 475 full length of the stope. The width depends on the values, as shown bythe cross-cut raises; the height is about 7 ft. (2 m.) and at an angle ofabout 40°, so that broken ore will gravitate to the chutes. There are two methods of stoping, one in which a small amount of theore occurs in the foot-wall
Transactions . off as the hanging wall is reached so it is the usualpractice only to undercut the stopes to a point about 20 ft. (6 m.) fromwhere the values cease. The undercutting of the stopes is then com-menced from these raise cross-cuts, this undercutting being carried for the a. T. JACKSON 475 full length of the stope. The width depends on the values, as shown bythe cross-cut raises; the height is about 7 ft. (2 m.) and at an angle ofabout 40°, so that broken ore will gravitate to the chutes. There are two methods of stoping, one in which a small amount of theore occurs in the foot-wall gabbro and the majority in the slate, andthe other in which the ore occurs in a schist formation. The methodadopted in the gabbro and slate stopcs was introduced by the late JohnR. Mitchel when he was superintendent of the Perseverance mine. Itconsists of a combination of shrinkage stoping and caving. From 15 to30 per cent, of the ore is broken by drilling and blasting, and the balanceobtained through Fig. 9. This method is described as follows: After the stope is undercut,machines are started from both ends of the stope, taking a cut along thefoot wall 7 ft. ( m.) high and 12 ft. ( m.) wide, until the cuts meet inthe center of the stope, after which the machines are brought back to thepillars and a cut of the same dimensions is taken across the pillars to thehanging wall. A small amount of caving will then commence, but it isgenerally necessary to take successive cuts of the same dimensions alongthe foot wall, and afterward across the pillars. Caving of the orebody ata right angle to the stratification takes place following these cuts, inquantities from a few tons to several thousand. The men working on thefoot wall or along the pillars are perfectly safe, and continue to work up thestopes in this manner until the back of the stope approaches from 40 to50 ft. of a worked-out stope above, at which time the caving becomes more 476 MINING METHODS IN ALASKA
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
