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An important operation in mining for reaching and working mineral deposits situated at a depth below the surface, whenever the topography does not admit of driving adits or tunnels. Shafts are often sunk also in connexion with certain civil engineering works, e.g. at intervals along the line of a railway tunnel, for starting intermediate headings, thus securing more points of attack than if the entire work were carried on from the end headings only. Sundry modifications of shaft-sinking are adopted in excavating for deep foundations of heavy buildings, bridge piers and other engineering structures.

If in solid rock, carrying but little water, shaft-sinking is a comparatively simple operation. But when much water is encountered or the formation penetrated comprises unstable, watery strata, special forms of lining become necessary and the work is slow and expensive. Mine shafts are often very deep; notably in the Witwatersrand, South Africa; the Michigan copper district; at Bendigo, Australia; and in certain parts of Europe. Many vertical shafts exceed 4000 ft. in depth, and at least two - the Whiting shaft, of the Calumet and Hecla mine and shaft No. 3 of the Tamarack mine (both in Michigan) - are over s000 ft. deep. The last named at the beginning of 1907 was about 5200 ft., and was then the deepest in the world. Several inclined shafts, in the same district, approximate 6000 ft. in length.

Shape of Shafts

In Europe shafts are generally cylindrical, sometimes of elliptical cross-section, and are lined with masonry, concrete, cast iron or steel; in the United States and elsewhere throughout the mining regions of the world, rectangular crosssections are the rule for sinking in rock, the shaft walls being supported by timbering, occasionally by steel lining. For sinking in loose, water-bearing soils, the cross-section is almost invariably cylindrical, as this form best resists pressure tending to cause crushing or caving of the shaft walls. The European practice of sinking cylindrical shafts even in rock is based mainly on four considerations: - (') custom; (2) high cost of timber; (3) apart from questions of first cost, a cylindrical shaft, lined with masonry or iron, is strong and permanent, and its cost of maintenance low; (4) more shafts in difficult formations have been sunk in Europe than elsewhere. The cheaper timber-lined, rectangular shaft, however, is generally appropriate under normal conditions in rocky strata, in view of the temporary character of mining operations. Vertical shafts may be either rectangular or cylindrical; when inclined they are always rectangular.

The primary purpose of mine shafts is to act as hoistingand travelling-ways; incidentally they serve for ventilation, for pumping and for transmitting power underground by steam, compressed air or other means. Rectangular shafts are usually divided longitudinally into compartments. One or more of these are for the cages or skips, which run in guides bolted to the shaft timbering (see Mining). Another is generally provided for a ladderand pipe-way and; for ventilation. When much water is encountered a separate pump compartment is desirable. Cylindrical shafts may be similarly divided by subsidiary timbering, though in many timbering is omitted and the hoisting cages are "guided by wire ropes stretched from top to bottom.

Dimensions

The cross-sectional area of shafts depends mainly on the size of the cages or skips - i.e. on the hoisting loads. Small rectangular shafts of one or two compartments measure inside of timbers, say 4 by 6 ft. up to 7 by 12 ft.; larger shafts of three compartments, from 5 by 12 ft. up to 8 or 10 ft. by 20 ft. For fouror five-compartment shafts, sometimes required for large scale work, as in the deep-level mines of the Witwatersrand, the inside dimensions range from 6 by 20 ft. to 6 or 8 by 30 ft., and for some of the Pennsylvania colliery shafts, up to 13 by 52 ft. Cylindrical shafts rarely have more than two hoisting compartments and are commonly from 10 to 16 ft., sometimes 20 or 21 ft. diameter, the segmental areas surrounding the hoisting-ways being utilized for ventilation, piping, &c.

Sinking in Rock

If the rock be overlaid by loose soil carrying little water, excavation is begun by pick and shovel, and after the rock is reached it is continued by drilling and blasting (see Blasting). The sinking plant, usually temporary, comprises a small hoist and boiler, several buckets or sometimes a skip, one or more sinking pumps, according to the quantity of water, occasionally a small ventilating fan, and a timber derrick or head-frame over the shaft mouth, with appliances for dumping the buckets, handling the rock and safe-guarding the men in the shaft against falling objects. In some circumstances a portion of the permanent mine plant is erected for sinking. The choice between hand and machine drilling depends chiefly on the kind of rock and the size and depth of shaft. For very hard rock or when rapid work is desired, machine drilling is advisable, a compressor and additional boiler capacity being then required. Remarkable speeds, however, have been made by hand-sinking in some of the deep vertical shafts on the Rand, the world's record being that of the Howard shaft, sunk by hand labour 203 ft. in one month. But such speeds are attainable only in dry, or nearly dry, ground, at a high cost per foot and by crowding as many men into the shaft as possible, both for drilling and loading away the blasted rock. The conditions being the same, inclined shafts closely approaching the vertical can be put down about as fast -as vertical shafts; but for inclinations between say 75° and 30 to the horizontal, inclines are generally slower on account of the greater inconvenience of carrying on the work, both of excavation and timbering. Very flat shafts, on the other hand, can be sunk at speeds little less than for driving tunnels, unless there is much water. The highest speed on record for a very flat incline (10) is 267 ft. in one month.

As a rule, the speed attained in sinking depends less on the drilling time per round of holes than on the time required to handle and hoist out the rock; hence the speed generally diminishes with increase of depth. Furthermore, omitting shafts of small area, the cost per foot of depth does not increase greatly with the cross-sectional dimensions. For the same rock the rate of advance in wet formations is always much slower than in dry and the cost greater.

The work of sinking in rock is carried on as follows. A round of holes is drilled, usually from 3 to 4 ft. deep if by hand, or from 5 to 8 or 9 ft. if by machine drilling (see Blasting). A common mode of arranging machine drill holes is shown in plan and section in fig. 1. The holes are charged with dynamite and fired by fuze or electricity - in deep shafts preferably by electricity, as the men may have to be hoisted a long distance to reach a place of safety. After the smoke has cleared away (which may be hastened by sprays or by turning on the compressed air if machine drills are used), the work of hoisting out the broken rock is begun and drilling resumed as soon as possible. For shafts not over 6 or 8 ft. wide, machine drills are usually mounted on horizontal bars stretching across from wall to wall, or, in wider or cylindrical shafts, on tripods or special sinking-frames. In shafts of small area, or deep shafts which are timbered during sinking, the hoisting buckets must be guided to prevent them from striking against the sides. Small quantities of water are bailed into the buckets; when the inflow is too great to be so disposed of, a sinking pump is employed (see Mining).

Shaft Timbering

In sinking rectangular vertical shafts under normal conditions the excavation through the surface soil is commonly lined with cribbing, inside of which a concrete curb is sometimes built to dam out the surface water. After reaching rock the lining is generally composed of horizontal sets of 8 by 8 in. to 12 by 12 in. squared timber wedged against the walls, with smaller pieces, or planking, called " lagging," placed behind them, to prevent portions of the walls from falling away. In firm rock lagging may be omitted. Each set consists of (fig. 2) two long timbers (wall w Longitudinal Section Fig. 1.

plates) W, W, two shorter pieces (end plates) E,E, and usually one or more cross pieces (dividers or buntons) D,D, to form the compartments, strengthen the sets and support the cage guides, G,G. The sets are from 4 to 6 ft. apart, with vertical posts (studdles) S,S, between them. At intervals of say 80 to 120 ft., longer timbers bearers ") are notched into the walls, under a set, to prevent displacement of the lining as a whole. A series of shaft sets, with their posts, are either built up from a bearing-set, or suspended from the latter by hanger-bolts. When the rock is firm, a considerable depth of shaft may be sunk and then timbered; generally, however, it is safer to put in a few sets at a time as sinking advances, the lowermost set being always far enough from the bottom to prevent it from being injured by the blasting. Inclined shafts in solid ground are often timbered as described above, though sometimes merely by setting longitudinal rows of posts, for supporting the roof and dividing the shaft into compartments.

Lining for Cylindrical Shafts in Rock. - Wooden linings are occasionally put in small shafts, or for temporary support, before the permanent lining is built, but a cylindrical shaft of any importance is lined with masonry or iron. Masonry linings are generally built in sections, as the sinking advances, each section being based on a walling-crib AB, CD, (fig. 3). Specially moulded tapered bricks are convenient, shaped to conform with the radius of the shaft. Concrete may be similarly moulded into large blocks, often weighing 1200 to 1600 lb each. The thickness of the walling depends on the depth of shaft and pressure anticipated; it is usually from 13 in. to 2 ft., laid in cement mortar. Such linings, while not entirely water-tight, will shut out much of the water present in the surrounding rock.

Iron lining, or " tubbing," is employed when the inflow of water is rather large. It is usually composed of cast iron flanged rings, each cast in a single piece for shafts of small diameter, or in segments bolted together for large diameters. To permit the rings to adjust themselves to the pressure, the horizontal joints are rarely bolted; they are packed with sheet-lead or thin strips of dry pine, any leaks appearing subsequently being stopped with wedges. Though preferably of cast iron, tubbing is occasionally built of steel plate rings, stiffened by angles or channels riveted to them. The irregular annular space between the tubbing and rock-walls is afterwards filled with concrete or cement grouting. The lowermost tubbing ring is based upon a " wedging-crib." This is a heavy cast iron ring, composed of segments bolted together, and set on a projecting shelf of rock, carefully dressed down. The space behind the crib is driven full of wooden wedges, which expand on becoming water-soaked and thus make a tight joint at the bottom of the tubbing with the rock just above the mineral deposit. By this means most of the water may be permanently shut out of the shaft, and the cost of pumping materially reduced.

Kind-Chaudron System of Sinking

This ingenious method, introduced in 1852, has thus far been confined to Europe. Up to 1904, 79 shafts had been sunk by its use, some of them to depths of moo ft. or more, without a single instance of failure. It is applicable only to firm rock and was devised to deal with cases where the quantity of water is too great to be pumped out while excavation is in progress; that is, for inflows greater than woo or 1200 gallons per minute. In its after results the system is most successful when the water-bearing rocks rest on an impervious stratum, overlying the mineral deposit. The entire excavation is carried on under water; then a lining of special design is lowered into place and the shaft unwatered. The shaft is sunk by boring on an immense scale, by apparatus resembling the rod and drop-drill (see Boring). Instead of ordinary drills, massive tools called " trepans " are employed, consisting of a heavy iron frame, in the lower edge of which are set a number of separate cutters (fig. 4). Shafts not exceeding 8 ft. diameter are bored in one operation; for larger diameters an advance bore is usually made with a small trepan and after wards enlarged to full size. The advance bore may be completed to the required depth of shaft before beginning enlargement, or the small and large trepans used alternately, the advance being kept 30 to 60 ft. ahead of the enlargement. An 8 ft. trepan weighs about 12 tons, those of 14 or 15 ft. 25 to 30 tons. The trepan is attached to a heavy rod, suspended from a walking-beam operated by an engine on the surface, as in ordinary boring. A derrick is erected over the Elevation FIG. 2.

30. ---- - i?mmmim?mm??n?imin - %).< FIG. 3.

FIG. 4. - Large and Small Trepans for shaft sinking, Haniel & Lueg, Dusseldorf, makers.

shaft, with a hoisting engine for raising and lowering the tools. Average rock is bored at a speed of about I z ft. per 24 hours. The advance bore is cleaned of debris at intervals by a bailer similar to that used for bore-holes. The enlarging trepan is so shaped that the bottom of the enlargement slopes to the centre, whereby the cuttings, assisted by the agitation of the water, run into the advance bore and are bailed out. Owing to the difficulty of this latter procedure the advance bore is sometimes omitted even for large shafts, the debris being removed by a special dredger (Coll. Guard., Dec. 22, 18 99, p. 1181). For rather loose rock another somewhat similar system of drilling, the Pattberg, has been satisfactorily employed.

When the shaft has passed through the watery strata the lining is installed. This is composed of cast iron rings, like tubbing (cc, dd ), bolted together at the shaft mouth and gradually lowered through the water (fig. 5). The first two rings, called the " moss-box " (aa, bb ) are designed to telescope together and have a quantity of dry moss packed between their outer flanges. When the lowermost ring reaches a the bottom, the weight of the lining compresses the moss and forces it against the surrounding rock, making a tight joint. The lining is suspended from the surface by threaded rods, and to regulate and reduce its weight while it is being lowered the bottom is closed by a diaphragm (ff), from the centre of which rises an open pipe (g). This pipe is provided with cocks for admitting inside the lining from time to time enough water to overcome buoyancy. Finally, concrete is filled in behind the lining, the diaphragm removed and the completed shaft pumped out. In some FIG. 5

formations the moss-box is omitted, the concreting being relied on to make the lining water-tight. The cost of this method of sinking and lining (generally £35 to £60 per foot), as well as the speed, compare favourably with results obtainable under the same conditions by other means; in many cases it is the only practicable method.

Sinking in unstable, watery soils, which often cause serious engineering difficulties, is accomplished by: (I) spiling, vertical or inclined; (2) drop-shafts; (3) caisson and compressed air; (4) the freezing process.

Vertical spiling consists in driving one or more series of spiles around the sides of the excavation, supported by horizontal timber cribs. When the first spiles have been driven, and the enclosed soil removed, a second set follows inside, and so on. As a result of the successive reductions in cross-section of the shaft, vertical spiling is inapplicable to depths much greater than say 75 ft.

Inclined spiling is also limited to small depths. Cribs are put in every few feet and around them, driven ahead of the excavation, are short, heavy planks, sharpened to a chisel edge. The spiles incline outward, being driven inside of one crib and outside of that next below (fig. 6).

4), ro"x ro" ?!! The shaft bottom also is usually sheathed with planking, braced 1"a° is against the lowest crib supported by a framework on the surface. The rods are lengthened as required for lowering the lining. For deep shafts the lining must be of iron or steel, as wood is too weak and masonry too heavy. When the inflow of water can be met by a reasonable amount of pumping, the material is excavated by hand; otherwise, the water is allowed to stand at its natural level and the excavation carried on by dredging. This saves the cost of pumping during sinking, and the pressure of the unstable soil is largely counteracted by the weight of the column of water within the shaft. After the lining has come to rest on the solid sub-stratum, the shaft is pumped out, inflow underneath the shoe stopped as far as possible and sinking resumed by ordinary means. The dredging appliance commonly employed is the " sackborer." This consists of an iron or wooden rod, suspended vertically in the shaft, at the lower end of which on each side is attached a heavy hoop-like wing. The wings carry two large sacks of canvas and leather, opening in opposite directions. By rotating the rod by machinery at the surface, the sacks are swept round horizontally like the cutting edges of an auger, and partly filling after a few revolutions are then raised and emptied. The leakage under the shoe may be stopped in several ways, e.g. by concreting the shaft bottom, then pumping out the water and sinking through the concrete by drilling and blasting; by unwatering the shaft and calking below the shoe; or by inserting a wedging crib. There are various modifications of the drop-shaft which cannot here be detailed.

Sinking with caisson and compressed air is rarely adopted except in civil engineering operations, for deep foundations of bridge piers, &c. (see Caisson).

Freezing Process

This useful process was introduced in Germany in 1883, by F. H. Poetsch. The soil in which the shaft is to be sunk is artificially frozen and then excavated like solid rock. A number of drive-pipes are put down (see Boring), usually 4 to 6 in. diameter and about 3 ft. apart, in a circle whose radius is, say, 3 ft. greater than that of the shaft, and reaching to bed-rock or other firm formation. Each pipe is plugged at the lower end and within it is placed an open pipe, I z in. in diameter, extending nearly to the bottom. Or, preferably, after the drive-pipes are down, a slightly smaller pipe, closed at its lower end, is inserted in each drive-pipe, the latter being afterwards pulled out. The inner 12 in. open pipes are then put in place. At the surface, the outer and inner pipes are connected respectively to two horizontal distributing rings, which in turn are connected with a pump and ice-machine. A circulatory system is thus established. The freezing fluid, a nearly saturated solution of calcium or magnesium chloride (freezing point about-29°F.), is pumped through the ice-machine, where it is cooled to at least o°F., and goes thence to the freezing pipes. It passes down the inner pipes, up through the outer pipes, and returns to the ice-machine. The cold solution rising in the large pipes absorbs the heat from the surrounding watery soil, which freezes concentrically round each pipe. As the process goes on the frozen masses finally join (in from 3 to 4 weeks), forming an unbroken wall. The enclosed soft soil may then be excavated by dredging; or the freezing may be continued (total time usually from 5 to 10 weeks according to the depth), until the solidification reaches the centre and to some distance beyond the circle of pipes, after which the ground is drilled and blasted. This process has been successfully employed to depths of over 700 ft., and is applicable not only to the most unstable soils but also to heavily water-bearing rocks. It is questionable whether it will prove to be practicable for great depths, largely because of the difficulty of maintaining verticality of the boreholes for the freezing pipes. Even a slight angular divergence would leave breaks in the wall of frozen soil and cause danger. In a modification of the Poetsch process, introduced by A. Gobert in 1891, the calcium chloride solution is replaced by anhydrous liquid ammonia, which on vaporizing in the freezing pipes produces a temperature of - 25° to - 30° F. Sixty-four shafts had been sunk by the freezing process up to 1904.

Another method proposed for dealing with quicksand or similar watery ground is to inject through pipes a mixture of cement and water. The entire mass of soil would be solidified by the setting of the cement, and the shaft sunk by drilling and blasting, with no trouble from water.

Bibliography. - The following partial list of references may be useful Sinking in Rock: Engineering (London, 2nd Feb. 1894); Coll. Guardian (7th April 1898) p. 631, (loth April 1906, and 20th May 1898); Coll. Engineer (Oct. 1898) p. 135, (Dec. 1895) p. 100, and (Jan. 1896) p. 103; Mines and Minerals (June 1900) p. 481, (Dec. 1905) p. 225, and (Feb. 1906), p. 311; Eng. and Min. Journ, (13th April 1901) p. 461, and (16th Sep. 1905) P. 483; Min. and Sci. Press (3rd April 1904) p. 2 99; Australian Min. Standard (Ist Feb. 1900); Trans. Instn. Min. and Met. xv. 333; Jour. South African Assoc. Engs. (3rd Feb. 1906); Rev. univ. des mines (Oct. 1899) Gliickauf (8th Oct. 1904 and 4th March 1905).

Kind-Chaudron System: Engineer (London, Aug. 1904); Coll. Guardian (23rd March 1900), p. 541; North of Eng. Inst., M.E. xx. 187; Proc. Instn.. C.E. lxxi. 178; Rev. Univ. des Mines (Oct. 1902).

Sinking in Soft Ground: - Das Schachtabteufen in schwierigen Fallen, J. Riemer (1905), translated into English in 1907 by C. R. Corning and Robert Peele; Coll. Guard. (6th April 1894, 14th Nov. 1902, 3rd Jan. 1903 and 29th Dec. 1905); Mines and Minerals and advanced to new positions as sinking progresses.

Drop - Shafts

This important method has been used for depths of nearly 500 ft. A heavy timber, iron or ro' l? masonry lining (usually cylindrical), is sunk through the soil, new sections being successively added at the surface, while the excavation goes on inside. In quite soft soil the lining or drop-shaft sinks with its own weight; when necessary, additional weights of pig-iron, rails, &c., are applied at the top. If, from excessive friction or other cause, the first lining refuses to sink farther, a second is lowered telescopically inside, followed by others if required. The drop-shaft, which must be strongly built to resist collapse, distortion or rupture, is based on a massive wooden or iron shoe, generally of triangular cross-section, which cuts into the soil as the weight of the structure increases and the excavation proceeds. When built of masonry the great weight of the drop-shaft may become unmanageable in very soft soil, either sinking too fast or settling irregularly and spasmodically, accompanied by inrushes of sand or mud at the bottom. It is then suspended by iron rods, fastened to the shoe and threaded for passing through large nuts FIG. 6.

(Nov. 1904), p. 188; Trans. Amer. Inst. M.E., 'xx.' 188; Gliickauf (14th June 1902); School of Mines Quart. iii. 277; Rev. Univ. des mines (July 1902); Bull. Soc. de l'Ind. Min. (1903), No. i; Ann. des mines de Belgique, x. pt. 1; Mining Jour. (21st April 1906).

Freezing Process: Gliickauf (12th May 1906, 2nd June 1906); Ostrr. Zeitschr. f. Bergu. Hiittenwesen (14th, 21st and 28th July 1906, 14th, 21st and 28th April, and 5th May, 1900); Ann. des mines, xviii. 379; Genie civil (18th and 25th Jan. " and 1st Feb. 1902); Mines and Minerals (July 1898), p. 565; Trans. Fed. Inst. M.E. xi. 297; Coll. Guard. (1st Dec. 1893) p. 960, and (12th June 1896) p. 1108; Eng. and Min. Jour. (12th and 26th Oct. 1907). (R. P.*)