An underhand stope, see fig. 3, is started by working outward from the top of a winze in a series of horizontal slices, which tect the lower level and to provide a means of loading cars, the lower level is either timbered, as shown at D, or a pillar of ore, E, is left, through which chutes, G, are cut at intervals of 25 to 5o feet. The face of the ore forms a rough slope rather than a series of steps as indicated at F. The angle of repose of broken ore is from 35° to 45°, the steeper angle being for hard ores. If poss'ble, the slope should be such that the ore runs down under the influence of gravity, but in inclined veins it may be necessary to shovel the ore down the slope or to use mechanical scrapers.
Underhand stoping may be applied to large ore bodies under certain conditions. It generally takes the form of underground glory-holes (mill-holes), one series of holes being separated from another series by an intervening pillar of ore. In these cases, as in the working of a narrow vein, the men are in danger of rock falling from the back or walls of the stope, hence the de sirability of having a firm back and strong walls. In veins, the maximum width worked by underhand stoping using stulls is about 20 ft., since longer timbers have little supporting power.
has been used for mining small bodies of ore. The term square-sets is applied to a flexible system of timber support. A complete set is composed of 12 pieces of timber, four each of posts, caps and girts. As ad ditional sets are added each timber becomes common to four adjacent sets. Round timbers are also used in sets, but they require a different framing. Sets are held in place by short blocks placed at the corners and wedged tightly against the walls of the stope. By extending sets vertically and horizontally this skeleton framework may be shaped to fit a stope of very irregu lar outline. However, it is an expensive method of support and cannot withstand great pressure. If a few of the timbers are thrown out of line, the entire framework is likely to collapse. The ore is first stoped to make room for the top floor of sets, which are covered with lagging to hold back the sandy capping or back, then the excavation for the floor below is made. The second floor sets are supported from the top floor sets by using !ave the ore in step-like form. If the ore is rich, timbers called tulls are covered with prank (called lagging) to form the bot om of the level, as at A, but if the ore is lean, it is more eco Lomical to leave a pillar of ore as at B. Should the level be he lowest level in the mine, the ore must be hoisted to the top ,f the winze. The cost of mining is increased, so this procedure s used only for recovering small bodies of ore below a level. Z.aises and winzes are generally timbered to afford at least two :losed compartments, one to be used as a chute for ore or waste tnd the other fitted with a ladder to serve as a manway. To pro chains and planks nailed from a timber in the top set to another in the floor below. The second set timbers are then wedged in place and work on the third floor is started.
In this system of mining, the stopes are ad vanced upward, the ore being removed in horizontal, vertical or inclined slices. The stopes are started from the bottom of a
raise; either from the top of a protecting pillar of ore, as at P, fig. 4, or directly off the level, as at S, in which case the level is protected by stulls or drift sets which are covered with lagging. Stulls used in narrow veins are commonly from 8 to 12 in. in diameter, though sizes up to 3o in., have been used in heavy ground. The upper ends of the stulls are held in place by
Lagging may be plank or round poles. Scattered pillars of ore may be left to support the walls. Though the attempt is made later to recover this ore much of it may be lost. In wide veins so much ore would be required for pillars that square-set timbering is used.
As the stoping proceeds upward the miners must have some support on which to work. For veins having a dip of 4o° or less, the men can work from the footwall, but on steeper dips stulls or square-sets are covered with lagging to afford temporary working platforms. Strong walls and firm ore require a minimum of support, and then the slope may be left open.
In case the ore is likely to fall as soon as it is undercut and if the walls are weak a stronger support than can be given by timbering alone is required and filling the stope with waste is resorted to. The waste is secured from sorting the ore (the amount is seldom sufficient), from driving small drifts or inclined raises into the walls of the deposit, or outside the mine. In some cases the tailing from concentrating plants is used. This material may be dry or it may be mixed with water and washed into the mine through pipes in which case the water is drained from the filling and is pumped from the mine. This method is called sand filling. The horizontal-cut-and-fill or flat-back sys tem of overhand mining is shown in fig. 4. At H the cut is being made horizontally to the right, the miners drilling holes from the top of the pile of broken ore at 0. Chutes, C, are for drawing off the ore to the level below. These timbered chutes are built up to keep the top just above the waste filling, which is brought in through the upper level and dropped down the waste chute in the raise to be distributed through the stope in cars. As the mining progresses away from the main raise, additional waste raises, B, are driven at intervals to save hauling distance. Before a slice of ore is shot down, the waste filling is levelled and is covered with planks. The planks are removed when waste is to be dumped. After the slice H has been carried beyond the first waste raise, another slice may be started from the main raise. In this way several sets of miners may be working in a stope. In the Michigan copper mines where this system is followed, the ore is sorted in the stope. At the Champion mine about 53% of the material broken in the stopes is left for filling. The ore chutes are built of large pieces of waste arranged in circular form. Some support to the back may be afforded by cribs built on the waste or on the ore as at A, fig. 4, but no heavy weight can be supported in this way. For ore bodies of long horizontal extent, vertical pillars of ore are left at intervals, and additional support is afforded by horizontal pillars left just above and below.