Rill Stoping.—This method, also known as inclined-cut-and-fill mining, is a form of overhand stoping in which inclined slices are broken, as is shown at R, fig. 4. The method works well where the ore is hard and the walls are firm and regular. The slices or cuts are made either from the top down or the bottom up, the miners standing on the broken ore. After a slice has been shot down and the ore has drawn off at the level, waste is run in from the raise. The waste runs in under the influence of gravity and takes its own angle of repose. A layer of planks is laid on the waste, another slice of ore is shot down and drawn off, and the planks are removed for the next layer of waste filling. Both ore and waste require a minimum of handling. At T, fig. 4, stulls and posts are used as support. Here, the broken ore is shown lying on the plank-covered waste. Still stronger support is given by using square-sets. As the stope increases in height the lower point or toe moves away from the raise. The danger of falls of rocks from the back limits the length of the stopes, which in some cases may be only so to 6o feet. An ore chute is started from the level at the toe of the slope and is carried up, keeping pace with the waste filling. If two chutes are used, one for waste and the other for ore and a grizzly is placed over the ore chute, the larger pieces of waste may be sorted out. At the Pilares copper mine in Mexico no planks are used in the stope ; water is used to wet the surface of the waste before the ore is shot down. After drawing off the ore, the fine material remaining is carefully scraped off the waste before additional filling is run in.
Filled Square Sets.---These offer a flexible method of mining heavy ground. By varying the number of sets opened on the bottom or sill floor of a stope and by limiting the vertical number of sets left unfilled, this system can be made to suit the nature of the ground being mined. If no great pressure is brought on the timbers, a sill floor of say 12 to 15 sets or even more may be opened. In bad ground, the sill floor may be only two or three sets. This area of horizontal cut is carried up to the level above, and then another similar cut is started next to the filled sets, and the operations are repeated. Thus a large block of heavy ground may be safely mined in a series of small vertical slices. In very heavy ground, the filling must be carried as close as pos sible to the working face. Chutes and manways are made by lining square sets tightly on the inside with vertically placed plank or lagging. Waste is confined within sets by placing lagging horizontally on the inside of the posts and spacing the plank a few inches apart unless the filling is very fine.
Resuing or Stripping.—This term is applied to the mining of rich narrow veins or of Veins in which rich ore lies in a streak of less width than the vein. The object of resuing is to mine
the ore clean and free from waste, and thus save diluting the ore with waste and sending a larger tonnage of lower grade ore to the mill or smelter. In fig. 5 A is the rich streak and V is the lean part of the vein. A level L is opened and the stoping is car ried upward. The level is driven largely in the worthless rock, which is first removed and then the rich streak is shot down, canvas or hides being commonly used to catch the ore since the richest part is generally the finest.
The ore should separate easily from the wall of the vein; if it adheres tightly to the wall some waste will be mixed with the ore.
The ore should work easily down the stope when it is being drawn. Soft walls are likely to slab off and dilute the ore with barren rock. Sorting is not practised except under unusual con ditions, and little timbering is required. Ore that packs in the stope must be loosened by blasting. If the ore breaks in large blocks, these must be &Hied with shallow holes and blasted; this is called block holing.
One disadvantage of this method lies in the long time the ore is left in stopes, this being two or three years in some cases, and entails high interest charges for carrying the ore.
Fig. 6 shows shrinkage stoping as applied to a large ore body. The lower level is protected by a pillar through which short raises, C, for chutes are put up at 25 ft. intervals. Raises are driven through the pillars at the ends of the stope. Short-cuts, driven through the pillars at small vertical intervals, connect with the working space in the stope, as shown at M. To prevent large pieces of ore from clogging the chutes, grizzlies (bar screens), G, are placed at the top of the chutes C. A pillar of ore, as indicated at P, may be left to protect the upper level. Later, pillars are mined by another method. Fig. 7 illustrates shrinkage stoping in a steeply dipping vein. The ore passes through the grizzly G into the chute C along the haulage H. B is a bulldozing chamber in which men can work at blasting the large pieces which do not pass the grizzly.