SUBLEVEL STOPING

A. In this mining method, the ore is excavated in open stopes, retreating from one end of the stope toward the other. The orebody is developed first by a series of sublevel drifts above the main haulage level. The sublevels are connected by a starting raise at one end of the stope and by a passageway raise for entrance to them and the stope face at the other end. Chute raises connect the haulage level to the lowest sublevel, at which the tops of the chute raises are belled out to form mill holes. Beginning at the starting raise the ore is benched down from the sublevels; the broken ore falls into the mill holes, where it is drawn off through the chutes. The stope face is kept nearly vertical as it is benched backward toward the passageway raise.b. A mining method involving overhand, underhand, and shrinkage stoping. Its characteristic feature is the use of sublevels. The sublevels are worked simultaneously, the lowest on a given block being farthest advanced and the subs above following one another at short intervals. The uppermost sublevel underneath the cover is partly caved. The caved cover follows down upon the caved ore. The broken ore is in part drawn from the level, and a part remains in the stope to give lateral support to the walls and to prevent admixture of cover and ore. The breaking faces are developed by crosscuts, which are extended from wall to wall from the end of the sublevel. The method can also be looked upon as a retreating method, the orebody being worked from the top down and the individual blocks upon a given level being worked from their ends to the center. Modifications of this method are chamber-and-pillar system; chambers without filling; combination of subslicing and stoping; drift stoping; filling system; Mitchell slicing system; pillar robbing; pillar robbing and hand filling; room-and-pillar system; square work and caving; square work, pillar robbing, and hand filling; sublevel back stoping; sublevel method; sublevel slicing system; substoping.c. A method of mining best adapted to steeply inclined deposits that have strong ore and strong walls. The ore is usually blocked out by two horizontal drifts separated vertically by 100 to 200 ft (30 to 61 m) and raises between the two horizontal drifts, the latter separated by comparable distances. Vertical pillars may be left between stopes on the same level, and horizontal ones to support the main haulage. After the main blocks of ore have been completely mined, it is common practice to rob the pillars, and the walls of the stope may collapse after the pillars have been robbed d. Of lodes, open-stope mining in which ore is blasted and drawn through footwall openings to a gathering level in the country rock below. Used with strong containing walls and wide lodes e. Of massive deposits, working simultaneously of a series of sublevels echeloned vertically, the lowest leading and the uppermost being partly caved as the covered rock descends.