Conventional mine shaft sinking methods involve the performance of a cycle of different operations—drilling and blasting, removal of smoke and cleaning of fly-rock lodged on overhead timbers, mucking and hoisting of the broken rock, and timbering. The latter (a) may be the last operation of the cycle, (b) may be carried on during drilling and mucking, provided it is done from a substantial blasting shield or bulkhead so that the men below may be protected from injury by falling objects, or (c) may be done in sections while other work is stopped; that is, if the walls of the shaft stand well, several sets may be placed at one time following several drilling-blasting-mucking cycles during which no timbering is done.
Good organization of the work is required, especially where speed is desired. The crew is usually made up of miners in charge of a working shift leader, who are capable of performing the work of any part of the cycle so that each shift as it comes on duty takes up the work where the preceding shift left off. This applies particularly to continuous three-shift sinking. There are variations of this procedure, and often, especially if timbering is concurrent with drilling and mucking, a special timbering crew may be employed. In other instances three separate crews are employed—one for drilling and blasting, one for mucking and hoisting, and one for timbering. This system may be used to advantage where speed is not of prime importance and short enough rounds are drilled so that the mucking crew can clean out the entire round in one shift.
In any event, the crews are made up of shaftmen and surface or topmen. Best results are obtained with shaftmen trained especially in shaft work; many good stope, drift, or raise miners do not make good shaftmen. The top crew comprises the hoist engineers, top- landers, the mechanics in charge of equipment, drill sharpeners, blacksmiths and carpenters, and surface laborers. At an active mine the mechanical, drill-sharpening, and carpenter work usually can be handled by the regular mine staff.
Some shaft-sinking jobs entail starting from surface and involve clearing and preparing the site and installing a shaft collar and head-frame, whereas others are merely deepening operations. In either instance the routine at the shaft bottom is the same, though dumping and other top arrangements will be somewhat different.
Assuming that the shaft is to be sunk from the surface, the first essential is a substantial shaft collar adequate to support the shaft timbers hung below it until they are blocked or held by shaft-bearers below, to prevent surface water from running into the shaft or draining into it by seepage through overburden above bedrock, and in some instances to support the front legs of the headframe. A well-designed concrete collar usually will fulfill all requirements, and to provide room for it the shaft may be sunk to bedrock of a size such that the inside dimensions of the concrete correspond with the outside measurements of the regular shaft sets. The concrete collar should be carried into bedrock far enough to insure a solid footing and to seal off surface water. In flat terrane, the collar usually is carried some distance above the ground, thus giving dump room around the shaft and providing for surface drainage away from the collar in all directions.
In ordinary shaft sinking in rock formations, the shaft is deepened by drilling rounds of holes in accordance with a standard arrangement. The holes are loaded and blasted in a sequence designed to obtain maximum breakage with a given amount of explosive. Thus, cut-holes are employed as in driving horizontal headings, though the procedure is somewhat different and varies with local conditions and the preferences of the operator. In heading work, it is generally desirable to break the rock as coarse as possible but small enough so it can be handled conveniently by one man or by the loading machine. In shaft work, however, it is best to break the rock as small as economically possible because of the greater ease in digging it out of the bottom of the shaft with pick and shovel, since a mucking sheet cannot be employed to shovel from as in driving headings.
The cuts are commonly drilled in the form of a V (fig. 45) across the shaft if it is rectangular in cross section, or in the form of a diamond cut (fig. 46, A and B) if the shaft is square or circular, though the particular type of cut and details will depend upon the way the ground breaks and the size of the shaft. As in heading work, the relief and square-up holes are drilled and blasted to break in succession to the free faces provided by the cut holes and holes next preceding in the order of firing. Figure 46, D, shows a combination pyramid and V cut, in which the pyramid cut is shallow and reduces the burden on the first four holes of the deeper V cut. Figure 46, C, shows a benching round similar in principle to the slabbing round described under
Driving Adits. With this round, half of the shaft area is drilled and blasted at one time, first on one end and then on the other.
The cut-holes in shaft work may also be termed “sump holes;” by blasting them and cleaning out the muck before blasting, and in some instances before drilling the rest of the round, they provide a sump for collection of water. The round shown in figure 46, D, performs a similar function. Cut-holes in shaft sinking may thus serve a dual purpose.
The standard V-cut may be drilled across the center or at one end, depending upon details of practice. By making the cut under the hoisting compartment and mucking it out first, the bucket may be lowered into it, making mucking from the benches less arduous. Sometimes a V-cut is drilled across one end of the shaft, blasted, and
mucked out, and then the rest of the round is blasted progressively in a series of benches retreating toward the other end (fig. 45, B).
As stated under Shaft-Sinking Equipment, electric blasting is advocated for shaft work, and the holes are wired in series when fired from a blasting battery and in parallel when a power circuit is used. Large rounds of 60 to 80 holes are best fired from a power circuit with connections in series parallel. In series wiring, one leg wire of a detonator is connected to a leg wire of the second one, the other leg of which is connected, in turn, to the next one, and so on around until all holes are connected. This leaves two free ends, which are later connected to the lead wires after the circuit has been tested with a galvanometer and found not to be grounded. In parallel connection, two bus wires are laid along the shaft, one leg wire of each detonator being connected to one bus and the other leg to the opposite bus. Tests for grounding are made, and the round is then ready for connection to the lead wires. The latter are not connected to the source of power until everyone is out of the shaft and everything is in readiness for the blast. In battery firing, this source of power is the blasting machine, and the final connection is made at the binding posts of the machine. In firing from a power circuit, the final connection is made through a switch or, better, two switches, the first of which is open, the second being the firing switch. The latter should be in a locked box, the key to which is in the possession of the foreman or shift leader and which is so constructed that the switch cannot be closed unless the door is open, and then only by manual operation.
Mucking is usually the most time-consuming operation in conventional sinking in rock, and in some instances may take more time than all other operations combined. Thus, in sinking the No. 3 Frood shaft, which was 28 by 16 feet in cross section, 55.52 percent of the time was consumed in mucking operations.
At the 20- by 6-foot Pim shaft, 30.4 percent of the time was employed in mucking. In sinking the 9- by 25-foot Guadalupe shaft, 50.9 percent of the time was spent in mucking, 24.6 percent in drilling and blasting, and 24.5 percent in timbering.
In sinking the McIntyre No. 11 shaft, which was 17 by 24 feet in cross section, 49.2 percent of the time was spent in mucking, 15.5 percent in timbering, 11.3 percent in blasting and clearing gas from the shaft, 9 percent in drilling, 8.5 percent in scaling, 2.7 percent in hitch cutting, and 3.8 percent was consumed in delays. Twelve to 14 machines were used to drill about 66 holes aggregating 770 feet of hole per round in a double or triple V-cut arrangement. The rounds were blasted in three steps; first, the short cut holes and short easers, which were then mucked out, and then the long cut and bench holes, and last, light end holes and two or three center sump holes, which were drilled while the bench was being cleaned.
In sinking the four-compartment No. 5 Magma shaft at the Magma mine, 56.3 percent of the time was employed in mucking, 6.2 percent in blasting and clearing smoke, 25.0 percent in drilling, and 12.5 percent in timbering.
At the Argonaut mine, sinking a 7- by 17-foot shaft required one shift for drilling and blasting, three shifts for shoveling, and one shift for placing a set of timbers.
Table 14 gives man-hour distribution at a number of typical shaft-sinking jobs.
As already indicated, the entire round may be blasted and mucked out in one stage or, more commonly, the cuts are blasted and mucked out first, and the remaining holes are then blasted and mucked out in one or more additional stages.
From data already presented it is apparent that speed of sinking will depend largely on the speed with which the muck can be loaded and hoisted. The amount of water in the shaft also has an important influence upon the speed of sinking, and where considerable quantities must be handled progress is greatly retarded. Drilling water and small seepages may be bailed with the sinking buckets. Not only do raising and lowering pumps and extending water lines consume considerable time, but water retards drilling and mucking operations. If the inflow occurs only in a restricted section or horizon in the shaft, the problem is not so serious, because, as previously stated, it can be caught and kept from interfering with work at the shaft bottom or may be sealed off by grouting. If water comes in through fissured ground along considerable lengths of the shaft, it may be necessary to resort to grouting at numerous points, which involves frequent stopping of sinking, drilling of grout holes (fig. 47), rigging up for and pumping grout into the holes, and then waiting for the grout to set and for accumulated water to be pumped out. Conditions must be met as they arise, and a routine procedure ordinarily cannot be followed.
In good ground, timbering need not appreciably reduce the rate of progress, since it may be conducted concurrently with other operations from a suspended bulkhead or blasting set. In loose, sloughing, or swelling ground timbering must be kept close to the bottom and a set will usually have to be installed after each round is removed, which obviously adds to the time of the sinking cycle. If the ground is bad enough to require bridge or jacket sets (fig. 48), cribbed sets, or spiling, progress is necessarily slow, due not only to the additional timber that must be installed but to the care that must be exercised in removing the loose ground without causing runs. This may require the use of breast boards and bracing to hold back the ground while the sets are placed.
Shaft sets may be of timber, steel, or precast concrete beams. Timber is employed most commonly, although in recent years steel sets have been coming into favor owing to the availability of standard structural shapes suitable for this purpose and to reduction of fire hazards by their use. Precast concrete beams have not found general favor largely because of their weight. Timber sets have certain advantages in heavy ground since squeezing and crushing give warning of development of dangerous pressures, usually quite a while before actual failure occurs, and they can be removed by sawing and chopping with ordinary hand tools.
In recent years the practice of lining the entire shaft with concrete has grown. Not only does this provide a fireproof shaft, but if the shaft is to be used for ventilation purposes, the sides will be smooth and offer minimum resistance to the passage of the air current. In some instances concrete walls are poured after sinking has been completed, beginning at the bottom and removing the timbers installed during sinking as the concrete advances. In other instances concreting is done in sections of 50 to 100 feet or more as the shaft is deepened.
Timber sets are framed on the surface; and the separate members are lowered, one at a time, and hung in approximate position from the bearers or from the sets above by means of hangar bolts. When the set has been assembled in this manner, it is leveled and alined with plumb bobs, blocking and the threaded hanger bolts being used to hold it in place, and it is finally wedged tightly against the walls to prevent movement.
At intervals of 50 to 100 feet or more bearing sets are installed to hold any weight not sustained by the blocking, which may work loose owing to disintegration of the wall rock. These bearers also facilitate shaft-repair work that may be required later and that may involve removal and replacement of the original sets. The bearers consist of long, heavy beams placed across the shaft under the end plates of a regular shaft set and sometimes also under the dividers. Their ends rest in hitches cut into the wall rock.
Figure 49 shows methods of framing and assembling timber sets. Framing details vary somewhat at different mines, but those shown are typical. When framed as shown in figure 49, A, the wall plates are hung first, the end plates are then lowered and placed on top of the wall plates, corner posts or studdles are set, and usually after the set has been blocked and wedged at the corners the dividers and center posts are put in.
Figure 50 shows details of a steel shaft set. H-beams are well adapted to shaft sets because they give a wide bearing against the ground and at the same time provide a convenient method of supporting the lagging.
In firm ground that does not slack and slough on exposure to the air it may be unnecessary to lag the sets. In such instances the chief function of the timbers is to support the shaft guides, pipe lines, electric cables, and sheathing separating manway and hoisting compartments. Some ground will stand well without lagging if the air can be kept from it. This may be accomplished by guniting the walls with a coating of cement and sand.
When lagging is required, it may be of plank or of concrete slabs. Plank is lighter and therefore easier to handle and install and throws less weight on the sets and bearers. However, concrete slabs with steel sets provide a fireproof shaft.
For concreting shafts, various kinds of forms and methods of pouring are employed. Forms may be of wood or steel, or partly of wood and partly of steel, and may be built-in or of the removable
sectional type. Built-in forms are of plank nailed to regular shaft sets used for sinking or to false sets of smaller outside dimensions, the regular sets being removed as the concrete walls are raised. After the concrete has set properly the plank is stripped off, and the false sets are removed. Removable forms of wood or steel are made in sections of a size convenient for handling, placing, and lowering through the shaft compartments. Figure 51 shows a form employed for concreting the Denn shaft in Arizona. As a rule, only inside forms are used, the rock walls of the shaft furnishing the back support.
Thin concrete walls should be reinforced with steel, but walls thicker than about 10 or 12 inches usually will not require reinforcement. Concrete mixes are generally 1:2:4, 1:2:5, or 1:3:5, the latter
probably being the one most commonly employed. The concrete is mixed on the surface and may be lowered in the shaft by means of buckets, in a car on a cage, or through a pipe. Concrete has been delivered successfully through pipe to depths of 3,000 feet or more.
For great depths the pipe delivers first to a bucket provided with a spout, which in turn delivers behind the forms.
Figure 52 shows a method of supporting a shaft with concrete rings employed at the United Verde Mine.
In lagged or concreted shafts, care should be taken to fill all open spaces behind the timber or concrete to prevent breakage by large slabs sloughing into the opening.
The foregoing discussion refers to the conventional methods of sinking in rock formations. Caisson and freezing methods have been referred to already, and it was pointed out that when such methods are employed contracts for the work usually are let to specialists who have the required equipment and are experienced in such work.
New shafts at operating mines are often raised instead of sunk. Raising eliminates the expense of mucking and pumping and the delays incident to hoisting the muck through the shaft. In raising, the shaft usually is carried from one level of the mine to the next in lifts of 100 to 200 or, in rare instances, 300 to 600 feet. It may be raised full size; or, more commonly, a small pilot raise is put up first, which is then enlarged from the top down to full size of the shaft, timbers being installed as in conventional sinking operations, the muck being drawn off through the bottom.
One objection to raising is the difficulty of ventilating, which is enhanced as the raise becomes longer. To overcome this objection it has been found feasible to put down a drill hole first along the axis of the proposed shaft and then raise on the drill hole. By placing an injector-type ventilator in the drill hole at the top of the raise or an ejector-type ventilator at the top of the drill hole, good ventilation can be obtained. The ejector type is preferable as it is outside the raise, so that the compressed-air line can be connected to it and the raise thus cleared of smoke before the miners re-enter after blasting. In very long raises the handling of tools and supplies likewise becomes laborious and time-consuming, and workmen expend an undue amount of time and energy in getting to and from the working place. Hence, shorter lifts usually are more economical.
A shaft may be carried up full size in the form of a small shrinkage stope, as shown in figure 53. In this instance a cribbed manway was carried up in each of two corners of the shaft, and the miners worked on top of broken rock supported on a timber bulkhead at the level as shown. Blasting was done in three stages—A, B, and C. After each part of the round had been blasted, entrance to the face was gained through one of the cribbed manways by pushing back the inclined 8-foot timbers. The particular job illustrated would have been greatly accelerated and the cost lessened considerably had the shaft been raised on a borehole with a compressed-air ejector at the surface or had a fan been installed at the level for blowing up one manway and down the other. As it was, the cost was only about half the estimated cost of sinking. The broken rock supported the shaft walls during raising, and timber was put in after the raise broke through to surface by drawing off the rock from below to make room for successive sets from the top downward.
A recent innovation in shaft-sinking methods, for which wider use in the future is predicted, was drilling with a shot drill the full diameter of the shaft. A 5-foot circular shaft was drilled to a depth of 1,125 feet through gabbro, serpentine, and ankerite by means of a shot drill close-connected to a motor-driven driving mechanism installed in a cage or cab (fig. 54), the whole being lowered to the bottom of the shaft on cables wound on the drum of a hoist. The drill consists of a circular steel core barrel 15 feet long with a removable cutting shoe 1½ inches thick by 12 inches high attached to the lower end. This drill is rotated at about 60 r. p. m. on chilled-steel shot, which grinds an annular ring around a central core of rock. A tapered ring with a number of fluted dogs around the inside and at the bottom of the core lifter served to grip, break off, and raise the core in lengths up to 8 or 10 feet.
Although the operation was an experimental one and mechanical and other difficulties had to be overcome, suprisingly low costs were obtained. The actual cost of the drilling operation was $23.57 per foot, whereas the equipment, including head frame, transformers, and hoist, cost $15,588 or $13.86 per foot. Most of this equipment, of course, could be used again on another job. Three 60-inch holes drilled close together would be at least the equivalent in capacity of a three- or four-compartment shaft. By drilling the shaft, the walls were not shattered and weakened by blasting as in ordinary shaft-sinking operations, and only short sections required artificial support, which was obtained by concreting. A three-deck cage that would accommodate 10 men to a deck comfortably was installed later in the shaft.
As this is written, another shaft, 66 inches in diameter, has been sunk by this method to a depth of 694 feet in greenstone; the ultimate depth is to be 1,200 feet.