Table of Contents
The general conditions to which each of the principal stoping methods is applicable have been stated briefly in the preceding pages. Variations of the principal methods, differences in methods of drilling and blasting, handling ore and waste in the stopes, transferring broken ore from the stopes to the haulageways, and filling and timbering operations, and variations in sequence and direction of stoping, are employed to suit local conditions. The term “stoping practice” is employed in this bulletin to cover details of stoping procedure rather than the broader and more fundamental considerations that are covered by the term “stoping method.”
Variations of Principal Stoping Methods
Some of the variations of stoping methods have been discussed already. Thus, it has been noted that shrinkage, cut-and-fill, and square-set stopes may be mined by horizontal or inclined slices or cuts (angle of working); that square-set stopes usually are mined overhand, from the bottom upward, but may be mined underhand (direction of working); that in cut-and-fill stopes where the walls are very heavy a systematic stringer-set method of timbering may be used, or, if the backs are “scaly,” temporary horizontal stringers on posts may be used (auxiliary support). Square-set stopes may be worked on successive horizontal floors or in vertical panels or sections (sequence and direction of working). In top-slicing, the slices may be driven parallel to each other or radially (figs. 100 and
101). It has also been noted that stopes may be carried longitudinally (along the vein) or, in wide ore, across the vein (direction of working).
Cut-and-fill or shrinkage stopes may be silled out full width on the level and the shrink or fill, as the case may be, carried on timbers, or silled a short distance above the level, thereby leaving a back or “arch” pillar over the drift to support the stopes. Modern practice usually favors the former method if the ore on the level is of good grade. This ore usually can be recovered more completely and at less cost before the stopes are worked out above it than by later robbing operations.
Any of the supported-stope methods may be employed for mining either on the advance or on the retreat (sequence of working). In advance mining, the stopes are started and carried upward or downward as development advances progressively farther and farther from the shaft or main adit. In retreat mining, the levels are developed to the extremities of the vein, deposit, or property boundary and stoping begins at the farthest points and progresses toward the shaft or adit. In advance mining, the haulage-ways become longer and longer and, if the ground is heavy, the cost of maintaining them under stoped ground becomes greater and greater. In retreat mining the active haulage ways are in ground undisturbed by adjacent stoping, are therefore maintained more easily, and become progressively shorter. When mining is done on the retreat, the stoped-out areas can be abandoned and, unless movement and subsidence must be prevented, can be allowed to cave.
A few of the principal variations of stoping methods not heretofore described are those shown in figures 102 to 107, inclusive.
Figure 102 shows shrinkage stoping on drift pillars at the Nevada-Massachusetts mine, Nevada. (Compare with fig. 88.)
Figure 103 depicts inclined cut-and-fill stoping at the Eighty-Five mine, New Mexico, (compare with figs. 91 and 93.) The inclined or “rill” variation of cut-and-fill stoping possesses the advantage of gravity movement of the ore to the ore passes and of filling into the stopes. It thus eliminates hand-shoveling as a major part of the operation and makes it unnecessary to lay track and use cars or wheelbarrows for handling the ore and for placing fill in the stopes. The rill system, however, does not permit much waste to be sorted out and left in the stopes; waste inclusions in the vein and slabs from the walls go into the chutes with the ore. Handling of ore and waste in horizontal stopes by means of power scrapers has increased during the past decade; and in large stopes, at least, it is doubtful whether gravity handling in rills is any cheaper than scraper handling.
Figure 104 illustrates the sublevel inclined cut-and-fill system developed at the Champion mine in Michigan. In changing from an advancing to a retreating system of mining, it was found that stoping had to be done on many levels to produce the necessary tonnage. By splitting the blocks between haulage levels with sublevels and mining as shown, concentrated stoping could be done and the output produced from one-third the number of levels otherwise required to
be active. The figure shows the ore being removed by scrapers; where much sorting had to be done, however, it was necessary to lay track, load the ore into cars, and tram it to the ore chute to obtain best results.
Figure 105 shows a semishrinkage stope that is a combination of shrinkage and cut-and-fill or shrinkage with delayed filling. Instead of cleaning out the ore and running in filling after each cut, two or more shrinkage cuts are made before the ore is drawn out and the stope filled. This system effects a large reduction in flooring expense.
The rilled square-set system is illustrated by the practice at the Bawdwin mine in Burma (fig. 106) described by Calhoun. Inclining
or rilling the stopes largely eliminates hand-shoveling, as both ore and fill run by gravity. The objection to rilling in cut-and-fill stopes, that is, the difficulty of sorting waste, does not apply to the same extent in square-set stoping, as sets can be kept open for the express purpose of providing a space to receive sorted waste.
The square-set system employed at the Bunker Hill and Sullivan mine in Idaho is shown in figure 107. Stoping is begun on the hanging-wall side of the vein, as at A, and proceeds toward the footwall, carrying a nearly vertical stope face and a steeply arched back.
The foregoing and other variations are described and discussed in more detail in Bulletin 390, Stoping Methods and Costs, by Chas. F. Jackson and E. D. Gardner.
How to Handle Ore in Stopes
With some methods of stoping, the broken ore moves by gravity directly to the loading chutes on the haulage level without any handling from the time it is blasted until it is drawn from the chutes.
Thus, in shrinkage stoping the ore is drawn from the bottom of the column of broken ore that fills the stope, through closley spaced chutes on the haulage level. In sublevel stoping (fig. 86) the ore is blasted out into an open stope and falls into funneled raises or mill holes, and no handling is needed except where benching down is done in two or more steps, when some shoveling may be necessary to clean off the lower benches. With block-caving, most of the ore broken in undercutting falls by gravity into raises previously driven at close intervals, and very little shoveling is required. After caving starts, the stopes are not entered, of course, and the caved ore is drawn by gravity through draw sets or finger raises and is passed through grizzlies to the loading chutes. Thus, there is no handling of ore in the stope proper. In underhand benching to mill holes considerable hand-shoveling may be required to clean off the lower benches after blasting.
Cut-and-fill and square-set stopes often are carried on an angle slightly steeper than the angle of repose of the broken ore (rill stoping) so that it will run by gravity to the chutes and thus a minimum of shoveling be required.
In flat-back (horizontal) cut-and-fill stopes, square-set stoping with horizontal floors, top-slicing, and sublevel caving some ore handling must be done in the stopes, although the amount may be reduced by providing closely spaced ore passes. The cost of building and maintaining a large number of ore passes may be greater, however, than that of wheeling, tramming, or scraping the ore in the stopes, especially in high stopes in heavy ground.
In mining tabular deposits dipping at angles flatter than the angle of repose of the broken ore, it becomes necessary either to drive drifts in the footwall and put up rock raises to the stopes for gravity handling or to adopt some means for moving the broken ore down the dip. The former entails high preparatory expense for deadwork, and since the development of relatively inexpensive and very efficient
power scraping equipment, it is usually more economical to scrape the broken ore down on the footwall and thus avoid driving drifts and raises in the barren rock of the footwall (fig. 111). When the dip is only slightly less than the angle of repose of the broken ore, stationary steel slides alone or steel slides in which a little water is running are sometimes employed to convey the ore from the stope to the level. Where the dip is still flatter, steel shaking chutes have been used successfully. The installation of steel slides or shaking chutes consumes time and is costly; they are subject to damage from blasting or falls of ground and are a source of annoyance and expense. Where power is available, scrapers are much more flexible and efficient.
Scrapers are employed extensively for dragging broken ore to the ore passes in horizontal or slightly inclined cut-and-fill stopes (fig. 112). Figure 112 shows a scraper spreading filling at the left end of the stope. When the back has been blasted and broken down onto the floor at the right end of the stope the scraper is moved to that side without moving the hoist, and the ore is dragged into the chute. When all factors are considered, horizontal cut-and-fill stoping using scrapers or cars is often cheaper than gravity handling in rill stopes; reduced efficiency of the miners when working on an inclined floor and extra labor for bringing in and placing prop timbers and flooring and for setting up and rigging drills may more than offset the cost of scraping.
In square-set stopes, wing slides are used to divert broken ore from one tier of sets to another that is used as an ore pass. These are merely planks extending from one cap or girt to the next one diagonally below it. Their use considerably reduces the amount of hand shoveling, but to eliminate it entirely by this method when working large horizontal floors a large number of chutes is required. When working in vertical panels two to four sets wide, the use of wing slides will virtually eliminate hand shoveling in the stopes. Wing slides are also employed in stulled stopes (fig. 94).
Scrapers have been used successfully in square-set stopes working large horizontal floors. Thus, at the Park Utah mine it is stated that scrapers have reduced stoping costs 30 percent. There the scraper hoist is mounted on a truck that is moved parallel to the hanging wall of the stope over a 3-inch plank floor to any desired line of sets.
In flat beds or other flat deposits worked by breast stoping or heading-and-bench methods, the ore must be loaded from the floor into cars. Hand shoveling, scrapers, and power shovels are all employed for this purpose. Success with pan and belt conveyors for loading in rooms and entries in coal mines suggests their probable future use in metal mines.
At A, figure 113, is shown an electric shovel loading into cars in a heading-and-bench open stope, and at B scraper loading from an open breast stope into a string of cars on a main haulageway. Figure 114 shows an electric scraper hoist pulling ore into a mill hole in a large open stope.
In top slicing and sublevel caving, as practiced in the Lake Superior iron mines, scrapers are used exclusively for dragging the broken ore into raises (fig, 115 at A), or, where top slicing is done in thin ore bodies of low dip angle, for loading cars direct (fig. 115 at B). When loading directly into cars, a slide is constructed as shown. In thick deposits where haulageways can be driven mostly in ore under most of the slice area, with raises to the sublevels also in ore, the ore is usually scraped to raises, as shown at A.
Mechanical handling reaches maximum efficiency when there is enough broken ore to keep the equipment working at capacity or where quick removal of the ore greatly increases the output of the stope by permitting earlier resumption of ore breaking. One advantage of scraping not possessed by other methods is that scrapers often can be used to clean out ore in portions of stopes that it is unsafe to enter. Thus, a fall of ore sometimes may be cleaned out by a scraper and waste filling dragged in to take its place without endangering the lives of the miners.
Ore handling in stopes is intermittent when considerable sorting of ore and waste is necessary.
Obviously, mechanical methods are greatly handicapped under this condition and therefore hand shoveling usually is preferred. Likewise, if only a small tonnage is to be handled, the expense of installing mechanical equipment is not warranted.
Hand shoveling directly into the ore passes or into wheelbarrows is practiced where the passes are spaced at short intervals. Wider spacing may make it more advantageous to lay track and use cars for transferring the broken ore from the muck pile (fig. 93). Cars are used more generally in the wider stopes and direct shoveling and
wheelbarrows in narrow stopes. In wide stopes scraping often will prove more economical than cars unless considerable sorting is necessary, and, hence, the choice of methods will be influenced largely by the requirements for sorting.