Stope filling has been discussed by Jackson in an earlier paper. Filling long has been recognized as the most effective method of permanently supporting the walls of mine workings and minimizing movement and subsidence. It is used alone in cut-and-fill stoping and in conjunction with framed timbers in square-set stoping for support of weak walls during the working of the stope and for permanent support. In cut-and-fill stoping its function is to support the walls and to provide a working floor; loose filling cannot be packed tight enough against the back to hold great downward pressure and under subsidence of the back it is compressible. Dry-pack walls have been employed widely, especially in Europe, both to afford direct support to the back and to retain loose filling.
Stope filling may be introduced (1) at frequent regular intervals during stoping as one of the operations in a cycle, as in cut-and-fill stoping, (2) after completion of a stope to afford permanent support, or (3) to fill stopes in preparation for mining adjacent blocks of unmined ore, such as pillars between stopes and crown or floor pillars. Filling also is employed in emptied shrinkage stopes for the dual purpose of providing permanent support to the walls and a rising floor from which tongues of ore in the walls can be reached and the walls can be prospected for blind lodes at various horizons. Mill sands have been employed at Franklin, N. J., to fill the space above top-slice caves in pillar mining between filled stopes and to provide a cushion above the top slices to absorb the shock of any masses of rock capping that may break loose. Waste filling is used for a similar purpose in the Colorada ore body at Cananea, Mexico. Filling also may be employed under some special conditions for extinguishing mine fires.
Various filling materials are employed, such as waste rock from development or from sorting in the stopes, waste shot off the walls of the stope or broken in waste raises and crosscuts driven into the walls, rock from surface quarries or sand and gravel pits, mill-tailing sands, and granulated or crushed slag.
Rock from development often can be disposed of most cheaply by using it for stope filling and may make it unnecessary to break waste, especially for filling. In other instances the supply of development rock may be inadequate and have to be supplemented by filling material from other sources.
Where movement and subsidence over and around the workings must be prevented, a filling that will settle or shrink the least after placement is required. Sand filling usually shrinks the least of any filling material in common use, whereas coarse rock will shrink con-siderably by reason of the gradual filling of the voids. Eaton states that coarse waste may compress as much as 10 to 25 percent before the fill develops its full strength. At the Tintic Standard mine 42 filling in square-set stopes settles as much as 7 feet between levels 120 feet apart.
Coarse filling is objectionable in cut-and-fill stoping, as it increases the difficulty of laying an even floor under the next cut of ore.
The introduction and spreading of fill in the stopes constitute important operations in cut-and-fill and square-set stopes. In rill stoping, gravity is employed for this purpose, and hand work usually is required only for spreading the fill immediately under the hanging wall and for filling corners.
In flat stopes, filling may be placed by hand, by power scrapers (fig. 112), or by wheelbarrows or cars (fig. 93). Sand is the easiest filling material to spread, especially if scraping is employed, and, in addition to conventional methods of handling, blowing or flushing may be used. Thus, at Copper Range, stamp sands were at one time blown by compressed air from a hopper through a 4-inch pipe for distances up to about 250 feet and for 20 feet beyond the end of the pipe; air pressure was 75 to 80 pounds. By blowing, sand can be placed above the level of the intake and under bad ground where it is unsafe for men to work.
At Matahambre, Cuba, about half the fill tonnage was made up of mill sands in 1929. Mill tailings are pumped to a 30-foot bowl classifier on surface near a raise from the mine workings. The sand or rake product, to which water has been added, is taken through a special rubber-lined pipe down the raise and into the stopes to be filled. Classified sands are also employed for filling in some of the mines on the East Rand in Africa. The sand from the classifier falls into a hopper above a borehole, and enough water—1 part to 2 or even 3 parts of sand—is added to make it flow down the hole (6 inches or more in diameter). Underground the sand-water mixture is distributed in launders set at 5- to 30-percent gradient. The sand packs solidly and provides almost perfect support for the hanging wall. Costs are given as $0.185 per cubic yard of sand placed.
Wet sand that drains quickly and perfectly forms a fill that will shrink very little. One objection to the use of water, of course, is the cost of handling it as it drains from the fill. Another is the difficulty in sealing off the fill so that the water will drain immediately but without carrying sand with it.
Costs of filling at different mines are not available on a comparable basis; some reports give the cost of procurement of filling but not the cost of placement, whereas a number of others give only the cost of placing; some give the cost of filling per ton of ore mined and others the cost per yard of filling. At the Tintic Standard mine, waste was broken in an open-cut and delivered to the shaft at a contract price of 20 cents per ton. In one open-cut at Cananea, 9 to 10 men delivered 310 tons of waste rock per day at a cost of 13 cents per ton. The filling cost at Cananea during August and September 1929, in square-set, horizontal cut-and-fill, and rill cut-and-fill stopes, averaged 17 cents per ton of ore mined; in 1937 the cost of filling over top slices between completed cut-and-fill stopes was $0.1704 per ton mined by top-slicing.
In 1928 the cost of filling at Matahambre, including both rock and sand fill, was $0.177 per ton mined.
In 1929 the labor cost of filling in cut-and-fill stopes at the Pecos mine was 25 cents per ton of ore mined, in square-set stopes 18 cents, and in underhand pillar stopes 26 cents.
At the Hollinger mine, Timmins, Ontario, sand filling is excavated from a pit and transported to the mine over an aerial tramway 3.6 miles in length. In 1934, the cost of excavating and transportation was $0.1494 per ton of sand, including operation, power, maintenance, and amortization of cable wear. The cost of filling in the stopes (horizontal cut-and-fill, fig. 93) during the same year averaged $0.1221 per ton of ore broken.
At the McIntyre mine, Schumacher, Ontario, in 1930, it cost 50 cents per ton to break waste in stope crosscuts and 50 cents per ton to handle it in cars and spread it by hand. In a large waste stope, waste was broken by sublevel stoping at a cost of 10 cents per ton; secondary breaking on grizzlies, loading, and tramming to the main waste pass cost 25 cents per ton; and waste-stope development, grizzly installation, etc., about 10 cents per ton, a total of 45 cents per ton for waste procurement.
At Matahambre the cost of procuring, handling, and distributing mill-sand filling was as follows:
Actual cost, allowing for remaining useful life of pipe, was probably not over 20 cents per ton.