CAVING Sub-level Caving.—This method is suitable for the large scale mining of ore that is moderately hard, that has fairly regular limits and a capping which caves readily. The method is used chiefly in the iron mines of the Lake Superior district. Fig. 9 illustrates the principles of the method. Details vary at different mines. From the haulage level (levels may be 75 to zoo ft. apart) raises are put up at so ft. intervals, and sub-levels are driven from the raises at vertical intervals of from i 5 to 18 ft., leaving a horizontal pillar of ore some 9 ft. thick between the sub-levels. The development work on each sub-level consists of longitudinal drifts, marked No. i sub-level, No. 2 sub-level, etc., and cross drif is marked D. These are spaced about so ft. apart. Mining is started on the top sub-level, No. 4 in fig. 9, by driving a cross drift parallel to D across the block of ore at the end of the sub-level. The drift is timbered with three-piece sets of two slightly inclined posts and a cap and is lagged overhead. A second drift is driven alongside the first one, the feet of the posts in the two drifts overlap slightly. By removing a little of the lagging over the first drift, shallow holes can be drilled in the back and the ore blasted. Men in the second drift shovel the ore into cars for tramming to the chutes in the raises. After the ore is removed, the floor of the first drift is covered with a layer of timber, and a third drift is driven alongside the second drift and the operations are repeated. The capping caves and, as it is caught on the timber mat, enough pressure is developed by this weight on the ends of the horizontal pillars of ore to reduce considerably the amount of blasting needed to break the ore.
In fig. 9 the ore over drifts A has been shot down. Shovellers in drift B have removed this ore and have laid planks on the floor. As soon as a new drift has been driven alongside drifts B, mining the ore above B will be started. As in sub-level stoping, mining is done in a series of steps. From half to two-thirds of the ore is broken by caving. In some miaes where mechanical scrapers are used, cuts are not taken at right angles to the main drifts but at a smaller angle to avoid working the scrapers around corners. The scrapers pull the ore to the chutes. Sub-level cav
ing requires less timbering than top-slicing and the cost of mining is lower. However, the percentage of extraction is not so high and some capping becomes mixed with the ore.
Block-caving.---In this method a block of ore is undercut, and subsides the height of the undercut. This relatively small movement of the ore is sufficient to break it into small pieces.
Suitable ore bodies should be large, and the ore should be firm enough to stand well during the work of undercutting and yet be brittle enough to break up well during the process of settling. The capping should cave readily. The broken ore is drawn off at the bottom of the block until capping appears. Variations of block-caving are due to the methods of undercutting the ore and drawing off the ore. If undercut from the main level, the block is cut by intersecting drifts and cross drifts into a series of pillars, which are as small as safety permits. The pillars are drilled and blasted, and time is allowed for the ore to settle. At the Pewabic iron mine in Michigan, levels are from ioo to 125 ft. apart; the height of undercut is 7 feet. A block requires several weeks to settle, but continues to work for months. It was found that at the end of 6 to 8 months about 8o% of the ore was fine enough to pass through a 3 in. ring. The broken ore is mined by driving a series of closely timbered drifts into the broken material. The ore runs in at the ends of the drifts and is shovelled into cars. When waste appears a few sets in the drift are blasted down and drawing is resumed at a new point, thus following a retreating system. If a block is undercut from sub-level, the general features are similar to those just described. From the main haulage-level, drifts are turned off at right-angles at say 5o ft. intervals, and are driven to the limits of the block. A sub-level is driven above the main level, and the block is undercut by intersecting drifts and cross drifts on the sub-level. Chutes are put up to the sub-level from the haulage level. The pillars are blasted and the ore is worked into chutes. In caving into chutes, the chutes are close together and their tops are widened into funnels. The ore as it caves falls into these funnels.