Table of Contents
In tabular, dipping lodes and thick massive deposits the ore bodies are developed laterally at several horizons spaced at more or less regular vertical intervals. In areas of high relief each horizon may be developed by driving an adit into the side of the hill, from which branch headings are driven as required.
When the main extraction opening is a shaft, the horizons or “levels” are developed from stations cut into the side of the shaft. Shaft stations usually are partly cut and timbered during shaft-sinking operations to avoid later damage to the permanent shaft timbers from blasting against them when level development is begun. Special shaft timbering is required at the stations, and by installing it at the time the shaft is being sunk, the work of tearing out regular shaft sets and replacing them with station sets later on is obviated. By cutting the stations during shaft sinking, level development may be begun on one or several levels at a time with minimum delay as soon as the shaft has been completed.
A level comprises lateral workings (stations, drifts, and crosscuts) at approximately the same horizon or elevation, though the term “level” is often employed loosely to designate all workings tributary to the level proper. Level development provides haulage ways and means of access to the working places. The main drifts and crosscuts serve as a base from which to drive auxiliary drifts, crosscuts, raises, and winzes for the purpose of exploring and blocking out the ore bodies and preparing them for economical extraction by stoping. They also serve as airways for ventilation of the workings, afford drainage facilities, and carry compressed-air and water lines and electric wiring for operation of mining equipment.
The vertical distance between main levels is commonly 100, 125, or 150 feet, although intervals as low as 50 feet and as high as 250 or 300 feet sometimes are employed. In many instances arbitrary intervals of 100 to 150 feet are satisfactory, but in others the most suitable and economical interval can be determined only by careful weighing of a number of factors governed by the shape and dip of the lode, position of the shaft in relation to the ore bodies, distribution of and distance between separate ore bodies, persistence of the ore between levels, and physical characteristics of the ore and wall rocks, which, in turn, determine the mining method to be employed.
These and other factors are discussed in an earlier Bureau of Mines publication. In this publication the author has produced a set of curves showing the relationship between cost per ton of ore mined and level interval for shrinkage stoping, horizontal cut-and-fill stoping, and square-set-and-fill stoping based upon estimates of development-costs and of the number of tons of ore developed per vertical foot for different level intervals under a series of assumed conditions. These curves indicate broadly that for these methods of mining the cost per ton of ore mined decreases rapidly from intervals of 60 feet to intervals of 100 to 140 or 150 feet, but very slowly, if at all, for greater intervals.
It is obvious that, other things being equal, the greater the amount of ore developed per foot of development headings driven, the lower will be the cost of development per ton of ore mined. Were no other factors to be considered, it might be concluded that the greater the level interval, the less the development and, hence, the lower the cost of development per ton of ore.
Within variable limits this is broadly true, but other factors enter in to establish these limits. Thus, if the interval is too great, difficulties in ventilation may be experienced; the cost of driving connecting raises between levels usually increases rapidly beyond lengths of 100 to 150 feet; in irregular, erratic deposits a long level interval usually will not serve to explore the ore bodies adequately; or it may require so long a time to mine out the ore between two levels that costs of level maintenance will be excessive and the stope walls may become so heavy that stopes cannot be kept open long enough to permit extraction of all the ore.
A short level interval has the following advantages:
- More thorough exploration is possible, which is especially important in irregular, erratic deposits where the ore is not continuous;
- shorter connecting raises are required between levels;
- handling of steel, explosives, and timber into the stopes is facilitated;
- more points of attack by stoping can be made available;
- in heavy ground requiring the use of slow extraction methods the time required to work out a stope is less than with a high interval, and since the weight on a stope or sloughing of the walls often increases considerably the longer it is kept open, expense of maintenance and loss of ore may be decreased considerably with a short interval;
- good ventilation usually is maintained more easily.
Among the disadvantages of the shorter level interval are:
- Greater development cost due to increased number of stations, greater footage of drifts and crosscuts with correspondingly more trackage and pipelines;
- more levels to maintain;
- a greater proportion of the ore tied up in drift and floor pillars, which may have to be left permanently or extracted later at considerable extra expense.
The stoping method to be employed may have an important bearing on the level interval. Thus if the mining system requires grizzly levels above the haulage levels, or if floor pillars are required below the drifts, the effective stoping height will be reduced considerably; and if the level interval is short, the pillars will constitute a large proportion of the ore between levels.
Where block caving is the mining method employed haulage levels generally are located to come under or near the bottom of the ore body, and in modern practice high columns of ore up to 300 feet or more are undercut and caved in one lift. Opening of grizzly and undercutting levels above the main level is a part of stope development that will be discussed later.
In sublevel stoping a level interval of 200 feet or more usually is advantageous if the ore body is thick or deep enough.
If ground conditions require short stope lifts for rapid working out of the stopes before weight or sloughing becomes excessive, or if erratic ore occurrence demands a short level interval for prospecting purposes, it may be most economical to drive the haulage ways in the footwall and either to crosscut to the lode at frequent intervals (connecting the crosscuts with extraction drifts in the ore) or to drive inclined raises in rock from the haulageway to the bottom of the stopes for ore extraction. Footwall drifts, of course, add to the cost of dead work but may materially reduce the cost of maintaining the haulageways.
In tabular deposits dipping at flat angles, a given vertical interval between levels obviously will provide a longer stope lift measured on the dip; and the flatter the dip, the greater the relative length of the stope lift. Such deposits usually are mined through inclined shafts, and the level interval is measured along the dip rather than vertically and determined by the height of stope desired or by the requirements for prospecting the ore bodies.
In flat-lying deposits occurring in the form of disconnected ore bodies at different horizons the level interval may be variable, and levels may be established at elevations to match these horizons.
Where the ore is in widely separated shoots along a vein or fault or in the form of irregular disconnected bodies separated by barren rock the dead work required to get from one ore body to another may be reduced considerably if conditions are such that the ore can be mined from sublevels not connected to the shaft and the ore transferred to one main haulage level.
If a dipping vein deposit is worked through a vertical shaft in the footwall, crosscuts from it to the lode become progressively longer on each successive level, and dead work on the lower levels will be excessive, especially if the level interval is short or the dip flat. The choice is then between a high level interval or an inclined shaft, although a vertical shaft starting in the hanging wall and passing through the lode midway between the top and bottom of the ore will reduce the dead work considerably.
Sizes of Drifts & Crosscuts
The size of headings will be influenced by such considerations as strength of ground and requirements as to timbering for its support; type of haulage to be employed, whether hand or animal tramming, storage-battery or trolley locomotives, and size of cars to be used; type of loading chutes or platforms; economical size from the standpoint of cost of driving; stoping method to be followed; ventilation requirements; and amount of water to be drained through ditches.
The size of main openings with respect to requirements for ventilation, haulage, and drainage has been discussed already in connection with the driving of adits. The minimum size to meet these requirements is generally desirable from the standpoint of cost of driving and maintenance. When drifting in ore that stands well, it may be more economical to drive drifts larger than the minimum, especially if mechanical mucking is employed, since the cost per foot will not increase in proportion to the increase in size, whereas the cost per ton of ore broken in the drifts will decrease with increased size of heading. At one large gold mine in Canada it was found that by driving wide drifts the cost per ton of ore from the drifts was only 1.8 times the cost per ton of stope ore. In steeply dipping tabular deposits, development drifts in ore are often driven the full width of the ore if it is not too wide. If the ore is thick or the horizontal section of the ore body is wide by reason of its flat dip, the drift is often carried along the footwall and is the most economical size, first cost and maintenance cost considered.
The stoping method to be employed may influence the size of the drifts. Thus, if shrinkage or cut-and-fill stoping is to be done directly on the drift timbers, it may be desirable to drive high drifts rather than return later and take down backs preparatory to stoping. On the other hand, if pillars are to be left over the drifts and the stopes silled at a higher level, the drift would be kept low to take as little height from the stoping lift to the next level as possible.
Table 16 gives data on dimensions of drifts and crosscuts that are typical of American practice.
Drifting and crosscutting practices have been discussed by Gardner in an earlier Bureau of Mines publication.
In general, the same types of rounds, methods of drilling, blasting, mucking, and ground support are employed as those previously discussed under the caption Methods, Equipment, and Costs of Driving Adits.
Cost of Driving Drifts & Crosscuts
Costs of lateral development vary between rather wide limits and depend on the size of the headings; nature of the ore and rock, its drillability, breakability, and ground support needed; rate of driving; type of equipment, whether hand or mechanical loading is employed; wage rates and efficiency of labor; and management. These points have been discussed already under Costs of Driving Adits. It has also been pointed out that driving costs will depend on the amount of overhead they must carry and thus that at a producing mine where such costs are distributed between production and development, the unit cost may be considerably lower than where development work only is being done.
Table 17 presents typical drifting and crosscutting costs.
The figures are not necessarily directly comparable, even where they cover costs under similar conditions because of variations in the methods employed by different companies in distributing overhead charges, power costs, etc.
Table 18 gives typical performance, man-hour, and supply-consumption data on drifting and crosscutting
Plan Of Level Lay-Out
The level lay-out will depend on the size, shape, dip, and distribution of the ore bodies, the strength of the ore and wall rocks, the mining method to be employed, the anticipated rate of production, and the type of haulage.
In the Tri-State and Southeast Missouri districts, wide, flat, bedded deposits of firm ore with strong capping are mined by breasting out
the full width and height of the ore but leaving pillars for support of the back. The level plan thus becomes the ore outline. If the shaft is sunk outside the ore body, a short connecting crosscut from the shaft, and perhaps others to separate ore bodies, constitute all the development work required, the ore being mined as the workings advance. (Fig. 55.)
Figure 56, A, shows a partial plan of a level at the Morning mine in Idaho, where the ore is in a vein 6 to 30 feet wide, dipping at 80° to nearly vertical. The deep levels are developed by a shaft in the foot-wall sunk from an adit level. Levels are established at 200-foot vertical intervals, and level development comprises the cutting of shaft stations and skip pockets, crosscuts to the vein with extensions to provide room for tail track, pump, and motor-charging stations near the shaft and drifts on the vein. Drifts are 13 by 13 feet in cross section and are timbered with standard drift sets.
Where the vein is much wider or flatter, as at the Bunker Hill and Sullivan mine, crosscuts are driven from the shaft or incline to and through the ore, but more than one drift may be required to facilitate
rapid removal of broken ore from the stopes. (Fig. 56, B and C.) Here the dip is 40° to 50° and the footwall drift (fig. 56, C) is used as a main haulageway and base from which to drive rock raises to the upper part of the stopes. If the ore were the same width and vertical, a single extraction drift through the center might be employed; or, where very wide, two or more parallel drifts might be required.
Figure 57, A, is a plan of the 550-foot level of the Hollinger mine in Ontario as it was in 1924 and where a network of parallel and cross lenses or veins of ore (shown in solid black) is developed from a main haulage drift in the footwall of the ore zone, from which crosscuts are driven approximately at right angles and about 300 feet apart to intersect the various veins. Extraction drifts are then driven in the veins from these crosscuts.
Figure 57, B, is part of a level plan at the Homestake mine (1930). The ore body is wide and the ore was mined in transverse shrinkage stopes between pillars, which were mined later by square-set stoping.
Two main haulage drifts were driven, one in the hangingwall and one in the footwall, which were connected at 100-foot intervals by extraction crosscuts through the ore as shown.
In tabular, dipping ore bodies where the ground is heavy, the drifts may be driven in waste in the footwall and parallel to the lode to reduce the cost of drift maintenance. From the footwall drift, rock raises may then be driven to the sills of the stopes, or crosscuts may be driven to the vein and connected by short drifts for ore-extraction purposes; the latter may be abandoned as soon as the stope section above has been mined out. If, in addition, the dip is fairly flat and the stopes must be filled (cut-and-fill or square-set-and-fill stopes)
one drift may be driven in the ore or in the footwall for extraction of ore from the stopes above the level and another in the hanging wall, through which waste may be hauled and dumped into the stopes below the level.
Figure 58 shows the level-development plan (1929) of the Inspiration mine, where large bodies of low-grade copper ore of the “porphyry” type are mined by block caving. Note the equal spacing of drifts and standard curves to facilitate rapid movement of ore trains.
Other level plans or patterns differ in detail, but those mentioned are typical.
Shaft-station plans or “plats” are of two general kinds— (1) those in which a connecting crosscut or station is driven straight out from the hoisting compartments (fig. 59, A) and (2) those in which the drift or connecting crosscut is driven parallel to the shaft (fig. 59, B).
In general, the first plan is employed where the loaded mine cars are hoisted to surface on cages or where they are dumped directly into skips without intervening storage pockets, and occasionally where pockets are employed. With this arrangement, trains of cars must be uncoupled for hoisting or for dumping one at a time.
With the second plan, trains of cars may be run past the shaft and dumped, without uncoupling, into skip pockets or through short
chutes directly into the skips. Where large tonnages are handled, this plan usually will facilitate more rapid and efficient ore handling.
The subject of shaft stations and skip pockets will be discussed in greater detail under “Handling and Haulage of Ore.”
Pump rooms and sumps (fig. 59, B), car-repair shops, battery-charging stations, and locomotive barns often are situated near the shaft, and the details of the level plan will vary with their size, shape, and position.