COMBINED METHODS Combined methods may take several forms, but variations of room and pillar mining, a series of alternating narrow stopes and pillars, are most common. The ore is sufficiently hard to permit of using shrinkage stoping in the rooms. The pillars serve to support the capping until the stopes are completed. The pillars are mined in a number of ways ; e.g., by top-slicing, sub level caving or block-caving. In another variation of combined methods the stopes may be square-set stopes, and the pillars are mined by slicing. Figs. 12 and 13 illustrate one system of com bined methods. Fig. 12 shows part of a longitudinal vertical cross-section through a stope, taken at right-angles to the plane of fig. 13. Stopes are about 15 ft. wide by 15o to 175 ft. long. Pillars are io ft. thick. The tramming level at the bottom of the stopes is a series of drifts 25 ft. apart laid out at right angles to the long axis of the stopes. From 75 to 1 oo ft. above the tramming level, manway drifts are run at 75 ft. intervals. Raises for manways are put up in the stopes and connect the tramway drifts with the manway drifts, thus affording good ventilation. As the ore is stoped the manways are cribbed to keep them open in the broken ore. Chain ladders are hung in the manways from the manway drifts. After the stopes in a block are completed, the pillars are cut off at the bottom and cave as the ore is drawn through chutes in the sides of the tramming drifts. The ore is
trammed by hand to the grizzlies over the main chutes which lead to the haulage-level below. Undercutting the pillars and drawing off the ore is carefully controlled to give a steep line of contact between capping and broken ore. As one pillar is under cut the ore is drawn down to capping some so to 75 ft. farther back. Some 8o% of the ore is drawn out through the chutes.
In regard to costs only generalized figures can be given. Mitke states that caving with branch raises costs from 37 to 96 cents per ton; combined shrinkage and caving (with one exception) costs from $o.6o to $1.00 per ton, and top-slicing due to the large amount of timber required costs from $1.00 to $2.50 per ton. To illustrate how conditions must control the choice of a mining method, V. A. Brussolo gives the following factors which deter mine the method to be followed: (I) safety, (2) size and shape of ore body, (3) grade of the ore, (4) quantity of waste admixed with the ore, (5) character of rock surrounding the section to be mined, (6) flexibility. Of the ore mined at the Pilares mine during 1927, the percentage secured by different mining methods was as follows: horizontal cut and fill, 58.6%; rill stoping, 15.7%; shrink age with filling, 14.4%; square-sets, 9.8%.
Top-slicing, caving and combined methods account for the remaining 1.5%.
(See COAL AND COAL MINING; PROSPECTING; DREDGERS AND DREDGING.)