The technical literature is replete with descriptions of and data on shaft sinking equipment, methods and costs. The subject of shaft sinking has been discussed at some length in an earlier Bureau of Mines bulletin. A number of shaft-sinking operations are described in detail in the series of information circulars on mining methods and costs, to some of which reference will be made later. Hence, the present discussion will be confined to the broader aspects of the subject. Small shallow shafts in isolated districts often are sunk by hand, hand windlasses or whims being used for hoisting the muck in small buckets, and such methods frequently are employed to depths of 10 to 25 feet or more in starting larger shafts.
During the past 20 or 30 years there have been decided improvements in the technique, equipment, and practices employed in shaft sinking, both as applied under normal hard-rock conditions and for sinking through wet, caving, or running ground.
Shaft Sinking Equipment
The usual equipment for sinking at metal mines may be grouped roughly under eight classifications—drilling, blasting, mucking, hoisting, pumping, ventilation, power plant, and miscellaneous equipment, not including general shops and necessary building structures. A great deal of this equipment, such as air compressors and power-plant and shop equipment, will be already available if the shaft is being sunk at an operating mine, and general equipment of this nature is not discussed here.
Drilling equipment comprises air compressors, rock drills and accessories such as hose and fittings, drill steel and steel-sharpening or grinding equipment, air and water pipes, and manifolds or headers. Rock drills for shaft work usually are of the hand-held or “plugger” type weighing 35 to 80 pounds, although heavier machines mounted on cross bars sometimes are employed for drilling very hard ground in long narrow shafts. Modern practice calls for hollow drill steel and wet drilling, using conventional types of forged bits or detachable bits.
In the larger shafts, where several drills are operated at a time, air hose, line-oilers, and water-hose connections to the drills usually are attached to a manifold or header. In the largest shafts there may be separate air and water headers. The header is connected to the air and drilling-water pipes in the shaft by heavy hose. It is usually raised and lowered quickly after and before drilling operations by a tugger hoist mounted on the shaft timbers 30 to 50 feet or more above the bottom of the shaft, or by chain blocks, or it may be hoisted to the surface when not in use. These headers may be made up of pipes and fittings, or of pipes, fittings, and special castings assembled in a variety of ways, one of which is shown in figure 34 and which is a combination air and water header. Some headers are also provided with hooks on which to hang the rock drills.
Although fuse and caps are still employed for blasting in small shafts, modern practice requires electric blasting for greater safety, for reducing the number of misfires, and for more precise timing of the order in which the holes will go. Electric blasting necessitates the use of either blasting machines or switches and wiring for connection to a power circuit. With blasting machines the leg wires of the detonators are connected in series, and with power-circuit firing they are connected in parallel.
In either instance leading or main wires usually are wound on a reel for convenience in lowering to the bottom of the shaft from the surface or from the end of the fixed wiring in the shaft or in an upper shaft station. The lead and fixed wiring should be of approved type, insulated to withstand water, and of ample capacity for the length of transmission and the current to be carried. Permanent wires should be attached to insulating spools or other standard suspension and should not be wound around nails or secured by other makeshifts.
Blasting shields for protection of the shaft timber against flying rock may be classed arbitrarily as part of the blasting equipment, although they serve other purposes of equal usefulness, such as protection for the men at the shaft bottom against falling rock or material and as staging on which they may work when installing timber. Figure 35 shows a common form of blasting shield. They may be constructed of standard structural-steel shapes and perforated plates, of timber, or of a combination of steel and timber, with openings for passage of the bucket, skip or cage, for ventilating and other pipes, and for men. For convenience in lowering, the set may be suspended by cables or from chain blocks at each corner, or both. The shield should be as light as possible, consistent with strength and rigidity. In small shafts it is common practice to use a false set bolted to the under side of the last permanent shaft set for protection of the latter instead of a more elaborate blasting set of the type described above.
Mucking usually is done by hand with ordinary pointed shovels, picks, and sledges for breaking large boulders, although at least one successful attempt to employ scraper loading has been reported. The conventional method of mucking is to shovel by hand into a sinking bucket or small skip. In large, deep shafts it is necessary to hoist in buckets or skips of relatively large capacity to reduce the hoisting time. To avoid the use of high buckets, which are difficult to shovel into, and to reduce mucking delays, a low loading “pan” or “scoop” has been used successfully in a number of shafts. The pan is filled by hand shoveling and is then lifted a few feet by means of a small hoist set on the permanent shaft timbers above and dumped into a skip (fig. 36) or into a car on a special sinking cage. This scheme has been credited with reducing mucking time by as much as 20 percent at some mines, although operators disagree as to its efficacy, some preferring direct mucking into skips or buckets.
Hoisting equipment includes, in addition to the auxiliary hoists already referred to, buckets, crossheads, skips and cages, hoisting rope, signal system between the hoistman and the shaft bottom and shaft landing, the hoist, and the head-frame structure, dumping arrangements, and head sheaves.
Sinking buckets usually range in capacity from 10 or 12 to 30 cubic feet, or from ½ to 1½ tons, though sometimes their capacity is as small as 750 pounds and rarely are they as large as 2 or 3 tons. They may be constructed as shown in figure 37, with the bail attached at the top and a ring or a chain and ball attached to the bottom for overturning
at the dump, or with the bail attached on the sides just below the center of gravity. The latter type is kept from overturning in the shaft by a ring on the bail, which slips over a lug extending above the rim of the bucket.
In sinking to depths of 200 or 300 feet, buckets without crossheads are sometimes used. Thus, in the Tri-State zinc and lead district it has been the practice for years to sink single-compartment shafts about 5 by 7 feet in cross section, using 1,200-pound buckets or “cans” attached to “nonspinning” ropes, and without a crosshead. However, except for very shallow depths, crossheads should be used to prevent spinning and swinging of the bucket against the sides of the
shaft. A single deep beam with a shoe on each end to fit over the guides sometimes is used but may cause accidents by jamming between the guides while the bucket is in motion. A simple truss structure, the depth of which is at least two-thirds the distance between guides, is often employed (fig. 38, A); or, for greater safety, a construction may be used that embodies a bonnet or hood and safety latches and safety dogs designed so that the bucket and crosshead will stay together and the dogs will grip the shaft guides and hold the crosshead if the rope breaks. Figure 38, B, shows a special type of crosshead used at some mines in Ontario, which prevents the bucket
from swinging and is equipped with a safety latch that prevents the bucket from dropping below the crosshead if the latter should catch and hang up in the shaft. Figure 39 shows a safety crosshead equipped with safety dogs to grip the guides if the rope should break.
When skips or mine cages are employed for shaft sinking, they are equipped with long extension runners or shoes that engage the wooden shaft guides or their extensions below the timbers when the car is in loading position at the shaft bottom (fig. 40).
In sinking inclined shafts, buckets may be employed if the inclination is steep, when they may either run on skids or on a trolley (fig. 41); for flat inclinations, skips or cars running on rails are used.
For signaling the hoistman, the usual practice is to employ a wire pull-cord, which operates a gong or knocker, although electric systems that signal direct from the shaft bottom are sometimes used. A combination of the two methods is not uncommon in deep shafts; the
shaft men signal by pull-cord to a landing or station higher up, whence the signal is relayed to the surface by an attendant by means of an electrical system.
Geared hoists usually are employed that will handle the desired load at a speed ranging from 300 feet per minute in small shafts and for shallow depths to 1,500 feet per minute for large deep shafts. Steam, compressed-air, gasoline-engine, or electric hoists may be employed for shaft sinking. Electric drive is usually preferable if
power is available at moderate cost. Slow-speed hoists sometimes are employed for starting the shaft and are replaced by higher-speed hoists after the shaft has attained considerable depth. For shallow depths and small shafts single-drum hoists usually serve the purpose, two buckets being used, one of which is loaded while the other is being hoisted, dumped, and lowered. As greater depths are attained, double-drum hoists sometimes are employed to speed mucking operations, three buckets being used, one of which is loaded while the other two are being hoisted and lowered in balance.
Head frames may be made of either timber or steel. Sometimes the permanent shaft head frame is erected before sinking is begun or as soon as the shaft has been sunk a few feet and the permanent collar has been installed. More often, a temporary sinking head frame is used. This is usually of such dimensions that the permanent head frame may be erected around and over it later without interruption of operations. Sinking head frames may be of either the A-frame or the four-post type and are equipped with suitable arrangements for dumping buckets or skips or for caging and uncaging cars, as the case may be. Figure 42 illustrates small sinking head frames for hoisting in buckets, and figure 43 shows dumping arrangements in a vertical head frame. These may be varied in different ways to suit conditions, personal preference, or legal requirements. Note the hinged doors over the shaft collar, which should always be provided as a safety measure. Sometimes the head frame is made higher to allow for a rock bin below the dump. The design of small wooden head frames has been discussed in an earlier Bureau of Mines publication.
Pumping equipment includes the pumps proper, suction and discharge hose, pipe, pipe fittings, and pipe hangers or supports. Formerly, steam-driven pumps were quite common in shaft-sinking work, but they are seldom employed now. Their use caused great inconvenience by reason of the heat from the steam and exhaust pipes; wet shafts were sometimes crowded with hot pipes and water columns, and the hazard of a breaking steam line was always present. The use of compressed-air-driven pumps is much to be preferred and is now common practice. Electrically driven pumps of both the multistage centrifugal and plunger types that are suitable for sinking have come into use and offer certain advantages due to greater flexibility and ease of handling.
In small shafts, sinker pumps that are suspended by a bridle in a vertical position may be required; but where there is enough room, ordinary horizontal pumps are preferable, as they are lighter, cheaper, and, in the smaller sizes, easier to handle. In shaft-sinking the water contains a large amount of coarse and fine grit, and for this reason plunger pumps are better suited to the work than are standard centrifugal pumps. In recent years new rubber compositions and rubber cements have been developed that have been utilized for centrifugal sand-pump impellers in ore-dressing plants, and they might, perhaps, be applied to centrifugal pumps in shaft sinking.
A suction hose wrapped with rope for protection and a section of hose connecting the pump discharge and the lower end of the water column in the shaft provide flexibility in moving the pumps out of the way when blasting and make it unnecessary to add short sections of pipe to the bottom of the column with every cycle of the sinking operations. The water column is usually standard pipe, which should have flanged couplings, Victaulic connections, or other couplings that do not have to be screwed on and off. When a large flow of water must be handled, heavy pumps may be mounted on a framework provided with shoes to run on guides extending to the shaft bottom, the pump and framework being lowered and raised by an auxiliary hoist.
It frequently happens in shaft sinking that the inflow of water is concentrated at one or more comparatively short sections. In this instance it is economical to excavate water rings around the shaft below the heavy flow to catch the water. A pump may then be installed permanently at this point to lift the water to the surface. Water rings may also be convenient to collect water pumped from the bottom of the shaft and thus reduce the discharge head on the sinking pump.
Ventilation during shaft sinking is furnished by rock-drill exhaust, compressed-air-driven blowers, air nozzles or injectors similar to those mentioned in the discussion of adits, or fans. In continuous three-shift work it is desirable to remove the smoke quickly from the shaft after blasting, and a centrifugal fan on the surface is the most efficient equipment for this purpose. The fan may deliver air to the bottom of the shaft through pipe or flexible tubing 8 to 12 inches in diameter or may exhaust smoke and gases from the bottom of the shaft through ventilating pipe extending close to the shaft bottom. Pipe is preferable for several reasons; flexible tubing can only be used for blowing, whereas an exhaust system is often more efficient; and when pipe is used, the fan, if reversible, may be operated either blowing or exhausting, as circumstances dictate. Moreover, tubing tends to kink or collapse; in long lines considerable power is expended in keeping the tube open, and the frictional loss is high. With pipe and a reversible fan, smoke may be removed quickly by exhausting followed by reversal of the fan for blowing fresh air to the bottom of the shaft while men are working. When blowing, a few lengths of flexible tubing may be attached to the lower end of the pipe to permit directing the air where it will be most effective. Ventilating pipe usually is galvanized iron, plain or spiral-riveted, of 16- to 20-gage iron, depending on the diameter of the pipe. Wood-stave pipe has been employed in tunnel work, where, in at least one instance, it was found to be more serviceable than iron pipe and easier to keep tight at the joints.
Under miscellaneous equipment may be listed that required for framing shaft timber, for pouring concrete linings (concrete mixer, steel or wooden forms, and pipe, hose, or buckets for lowering concrete), and equipment for special shaft-sinking jobs. The latter category includes equipment for sinking through quicksand by the shield method or by the caisson method under air pressure, equipment for sealing off heavy flows of water in fractured rock by the cementation process, and that required for employing the freezing process. Contracting companies making a specialty of the caisson method or the freezing process are usually employed for such work.
The shield method has been used for sinking short distances through wet sand. The equipment is illustrated by figure 44, which is a cross section through one side or one end of the shaft. The shoe with trailing plates extending upward and overlapping the last regular shaft set is jacked ahead and the sand is excavated inside. This shoe extends around the shaft on all four sides, the members being rigidly bolted together.
In the cementation process, quick-setting cement or cement and sand are forced by means of a plunger pump through pipes inserted in holes drilled outward from the shaft into the fractured ground.
Occasionally a stiff-leg derrick with hoist and clamshell or orange-peel bucket has been used for sinking through overburden to bedrock.