Stoping

The foregoing discussion relates to general mine development and covers the work required to reach and penetrate the ore bodies and to provide the main arteries through which the ore is taken from the mine. These arteries provide mine drainage and avenues for travel and transportation of men and supplies and for installation of compressed air and drilling-water and electric transmission lines.

Little or no general or stope development may be required in some types of ore bodies that have been roughly delimited by drilling before development, and where strength of ore and wallrock, size of the ore bodies, and other physical conditions are such that the ore can be stoped virtually as it is developed. Typical of this type are the mines of the Tri-State lead and zinc district.

Other conditions may require varying amounts of stope preparation before actual extraction of ore by stoping can begin.

Many mining companies combine costs of general and stope development for accounting purposes, whereas others include stope-development costs as a part of the stoping cost. In this discussion, distinction is made between general development and stope preparation. Although stope development often is done simultaneously with general development and stoping and continues after stoping has begun, it is a distinct operation and has for its purpose actual blocking out of the ore and provision of facilities for carrying on stoping operations and removal of the ore. It consists principally of raising, installing ore passes and loading chutes, and driving sublevels.

The amount and nature of the stope development required depend on the stoping method selected, which, in turn, is determined by the physical characteristics of the deposit. Thus, in preparing for shrinkage stoping, chutes are installed at rather close intervals along the drifts, when stoping may begin immediately if the stopes are silled out on the level or on top of the drift timbers. If the stopes are to be started on top of drift pillars 10 to 15 feet or more above the back of the drift, raises are extended upward from the chutes the required distance, where they are connected by small subdrifts, which are slabbed off to the walls the full width of the ore to form the stope sill (fig. 60, A). In either case, it usually is necessary to run at least one raise through to the level above for each stope to provide adequate ventilation and means of egress to the upper level. These through raises, however, often are run after the stope has been in operation for some time and has advanced upward a considerable distance.

In preparation for cut-and-fill stoping, chutes are spaced considerably farther apart, and usually a timbered or cribbed manway is carried alongside each chute or alternate chutes for entrance to the stope and for handling timber and supplies into the stope. With this method of stoping, it is usually necessary to run a through-raise in each stope section to the level above before stoping begins, to provide a means for introducing waste filling and for ventilation of the stope. These raises serve also as a base from which to start cutting the successive stope floors. Likewise, a through raise generally is required in square-set stoping for ventilation and filling, and chutes must be provided at the level for drawing off the broken ore.

Where stopes are silled on the level or immediately over the drift timbers, the first step in stope preparation may be the slabbing of the sides of the drift to the walls of the vein or taking down the back to make room for sill timbering and starting the stope over the timber, or both.

The time required and amount of work to be done in preparation for shrinkage, cut-and-fill, and square-set stoping are relatively small, and stoping can be begun soon after the main level has advanced a hundred feet or so along the lode. Sublevel stoping, sublevel caving, top-slicing, and block-caving require more stope preparation before ore extraction can begin.

In sublevel stoping (a form of open stoping), ore chutes, manway, and “starting raises” must be put up and one or more drifts driven on each sublevel (fig. 60, B).

In sublevel caving and top-slicing, raises are put up at intervals of about 20 to 50 feet or more to the top of the ore body to serve as ore

passes, manways, and timber slides, and these usually are connected by a so-called timber drift on the top sublevel before slicing is begun. If the bottom of the ore is some distance above the main haulageway, so that raises from that level would require a large amount of barren rock work, it may be desirable to put up a rock raise at one or both ends of a block of ore and drive a transfer drift in ore at or just above the bottom of the ore body, from which raises are then driven in ore at regular intervals to the top of the ore. In recent years, transfer-drift systems have come to be employed in some mines, even where the raises would be in ore, to cut down the cost of driving and maintaining raises.

In preparation for block caving on a large scale, considerable time is required and relatively large expenditures are involved before ore breaking by caving begins. As usually practiced, it involves installation of a series of loading chutes along each haulage drift, from each of which long, inclined, main extraction or transfer raises and branch raises are driven; the driving of grizzly drifts, cutting of grizzly chambers, and installation of grizzlies; the driving of boundary cut-off raises, subdrifts, and, in some cases, boundary shrinks; the driv

ing of finger raises and installation of draw sets; and work on the undercutting level. (Fig. 61, B and C.) There may be some difference of opinion as to which of these operations should be included with stope development and which with stoping. Thus, there may be some question as to whether the excavation of boundary shrinks is a part of stope development or stoping, and as to whether the work on the undercutting level is stoping or preparation. The exact classification may reasonably depend upon the particular details of practice employed in specific instances. Boundary shrinks are certainly, strictly speaking, stopes, and the operation involved is stoping. The driving of drifts on, or connecting finger raises to, the undercutting level might logically be considered as development; whereas the slabbing of the drifts or the belling out of the tops of the finger raises to meet each other might be considered as either stoping or development.

The size and shape of the ore body and the stoping method employed obviously will have an important influence on the tonnage of ore developed by a given amount of development work. The tons of ore developed per foot of stope development have been estimated closely for a number of mines using various stoping methods and have been approximated for other methods. These figures are given in tables 19 to 22, inclusive. The examples selected for tabulation represent a wide range of conditions and of variations in widths and sizes of ore bodies.

Stope-Development Methods

The foregoing section indicates in a general way the nature of work comprised in what is termed stope development. Installation of chutes for drawing off ore on the haulage levels is in reality a part of stope preparation. However, chutes are employed for loading ore into the mine cars (the first operation in transportation of ore), and details of chute construction and operation therefore will be discussed later under the caption “Handling and Transportation of Ore and Waste.”

The methods of driving sublevel or sill drifts a short distance above the haulage levels are similar to those employed in driving main-level headings. They are often, but not necessarily, smaller in cross section, and, when driven to connect a line of raises, direct shoveling into raises or handling of broken ore and rock for short distances in wheelbarrows may be employed. Types of rounds similar to those already described are used.

Raises

Raises are driven in stope development for ore passes from the stopes to the loading chutes, to provide manways for entrance to the stopes, for handling timber and supplies into and from the stopes, for ventilation, for introduction of waste filling, as bases from which to start stoping, and for installation of air and drilling-water lines and electric cables. In flat-dipping seams they may be laid with track for handling ore, timber, and supplies in cars or skips, and in some instances may actually constitute part of the general mine development.

They may be single-compartment or divided into two or more compartments, depending on their use. Likewise, the function of the raise will determine to a considerable extent its size and shape. The nature of the ground, the inclination, and the use to which it is to be put will determine whether or not it must be timbered.

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Single-compartment raises sometimes are driven for ore passes or for prospecting purposes, but two-compartment raises in which one compartment serves as an ore pass and the other as a manway are more generally employed. If ore hangs up in a long single-compartment raise, great difficulty and danger often are experienced in clearing it; whereas, if there are two compartments, the trouble can be attacked from the manway at the point where the hang-up occurs by making an opening in the side. In some mines, described later, three-compartment raises are standard for certain purposes.

Raise timbering may consist of stulls, cribbing, sets constructed similarly to shaft sets, or square sets with lagging and lining between the compartments as required.

In starting a raise, it is customary to carry it up several rounds before installing chutes or permanent timber if the ground will stand, to avoid damage to the timber from blasting and to keep the top of the timbered section some distance below the face as the raise is advanced upward. On the other hand, it is advantageous in a two-compartment raise to install the loading chute as soon as possible to eliminate shoveling of the broken rock.

Whether or not the raise is to be timbered, a temporary staging is necessary in steep raises, from which to reach and drill the back, and means of access thereto must be provided. If the raise inclination is less than about 55° or 60°, the miners can walk up on the footwall, with the aid of a rope, if necessary. For steeper inclinations, chain or rope ladders often are employed, but it is usually best to install the regular timbering with fixed ladders to within a short distance of the back as the raise advances. In some localities the mining laws require that raises steeper than 55° and more than a given maximum length shall have at least two compartments and shall have a ladderway equipped with suitable ladders. Such regulations usually specify the maximum allowable distance from the top of the last timber to the face. To avoid damage to the timbers from blasting, the ore pass or chute compartment is usually kept nearly full of broken ore, and a bulkhead (fig. 62, A) is constructed over the manway or it is covered with pieces of short timber before blasting (fig. 62, B).

Figure 63 shows two- and three-compartment stull raises, figure 64 a two-compartment cribbed raise, and figure 65 a two-compartment square-set raise.

The type of drill rounds employed in drilling raises are in principle the same as those used in drifts, crosscuts, and shafts. Thus, center V-cut, side-V or draw cut, and burnt-cut rounds are all employed. The pyramid-type cut is sometimes used in large raises. Figure 66 illustrates typical raise rounds. At A there is shown a stoper drill

set up on staging in an inclined raise. It is apparent that in small square or short rectangular raises, pyramid or center V cuts are difficult to drill because the foot of the stoper leg or the back end of a jackhammer or drifter-type machine cannot be swung back far enough to give the required inclination to the holes. The burnt cut (fig. 66, B) has an advantage in this respect in small square raises because the holes are drilled straight in, normal to the face. In a rectangular raise 7 feet or more in length, fairly deep V-cut rounds may be drilled readily but cutting the V at one end (draw cut; fig. 66, C). Figure 66, D, shows a center V-cut round in a 5 by 7 foot raise.

Except in very hard ground, stoper machines usually are employed in vertical or steeply inclined raises. In exceptionally hard ground heavy drifter-type machines mounted on a cross bar often are employed.
Raises, especially long ones, often are difficult to ventilate, and unless some form of forced ventilation is employed, considerable time may be lost after blasting waiting for smoke to clear before the miners can ascend to the face in safety. In the past, gassy raises have too frequently caused fatal accidents. In two-compartment raises a fan pipe may be carried in the manway if it is large enough and a fan be employed to blow air to or exhaust gas from the face.

Sometimes advantage may be taken of exploratory drill holes and raises may be driven along them, a compressed-air ejector being employed at the top of the hole on surface or on the level above to draw out blasting fumes and provide ventilation. In some instances, it may even pay to drill a long hole along the proposed course of the raise for the sole purpose of providing ventilation. It is surprising

how much air can be drawn through even a 1¼- or 1½-inch hole in a short time by this method, which has been employed successfully at a number of mines. Figure 67 shows an ejector employed at the Bussieres mine in Quebec and the method of installing it on an upper level. In this instance several raises were run simultaneously on drill holes put down from the same drift and the ejectors discharged into a common ventilating pipe connected to an exhaust fan.

Cost of Driving Raises

Table 23 presents typical raising costs and includes data on the sizes of raises, types of rounds employed, and methods of timbering.

The cost of raising increases with length or height of raise after the first 50 feet or so, and when the work is done on contract a sliding scale of rates usually is established, which provides one price for the first 25 or 50 feet, the rates increasing by increments with increased distances driven.

Table 24 gives costs of raising in terms of labor and unit-supply consumption.