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SHAFTESBURY
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SHAFTESBURY , a See also:market See also:town and municipal See also:borough in the See also:northern See also:parliamentary See also:division of See also:Dorsetshire, See also:England, 103 m. W.S.W. from See also:London by the London & See also:South-Western railway (Semley station). Pop. (1901) 2027. It lies high on a See also: Shaftesbury was a borough containing 104 houses in the king's See also:demesne during the reign of Edward the See also:Confessor; in ro86, 38 houses had been destroyed, hut it was still the seat of a See also:mint with three mint-masters. In the See also:manor of the abbess of Shaftesbury were 111 houses and 151 burgesses; here 42 houses had been totally destroyed since St Edward's reign. In 128o the abbess obtained the royal manor at an See also:annual See also:fee-See also:farm See also:rent of £12 and remained the See also:sole See also:mistress of the borough until it passed at the See also:dissolution of the monasteries to See also:Sir See also: Shaftesbury returned two members to See also:parliament from 1294 to 1832, when the See also:representation was reduced to one, and it was lost in 1885. See also:Leland speaks of Shaftesbury as a See also:great market town, and it possessed a market in the time of Edward I. The Martinmas See also:fair was granted in 1604. In the 17th See also:century worsted, buttons and See also:leather were manufactured, but these See also:industries have disappeared. See Charles See also:Hubert See also:Mayo, The Municipal Records of the Borough of Shaftesbury (See also:Sherborne, 1889). See also:SHAFT-SINKING, an important operation in See also:mining for reaching and working See also:mineral deposits situated at a See also:depth below the See also:surface, whenever the See also:topography does not admit ofdriving adits or tunnels. Shafts are often sunk also in connexion with certain See also:civil See also:engineering See also:works, e.g. at intervals along the See also:line of a railway See also:tunnel, for starting intermediate headings, thus securing more points of attack than if the entire See also:work were carried on from the end headings only. Sundry modifications of shaft-sinking are adopted in excavating for deep See also:foundations of heavy buildings, See also:bridge piers and other engineering structures. If in solid See also:rock, carrying but little See also:water, shaft-sinking is a comparatively See also:simple operation. But when much water is encountered or the formation penetrated comprises unstable, watery strata, See also:special forms of lining become necessary and the work is slow and expensive. Mine shafts are often very deep; notably in the Witwatersrand, South See also:Africa; the See also:Michigan See also:copper district; at See also:Bendigo, See also:Australia; and in certain parts of See also:Europe. Many See also:vertical shafts exceed 4000 ft. in depth, and at least two—the See also:Whiting shaft, of the See also:Calumet and Hecla mine and shaft No. 3 of the Tamarack mine (both in Michigan)—are over 5000 ft. deep. The last named at the beginning of 1907 was about 5200 ft., and was then the deepest in the See also: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 See also:cross-See also:section, and are lined with See also:masonry, See also:concrete, See also:cast See also:iron or See also:steel; in the See also:United States and elsewhere throughout the mining regions of the world, rectangular cross-sections are the See also: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 See also:form best resists pressure tending to cause crushing or caving of the shaft walls. The See also:European practice of sinking cylindrical shafts even in rock is based mainly on four considerations: (r) See also:custom; (2) high cost of See also:timber; (3) apart from questions of first cost, a cylindrical shaft, lined with masonry or iron, is strong and permanent, and its cost of See also:maintenance See also: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 See also:character of mining operations. Vertical shafts may be either rectangular or cylindrical; when inclined they are always rectangular. The See also:primary purpose of mine shafts is to See also:act as hoisting-and travelling-ways; incidentally they serve for See also:ventilation, for pumping and for transmitting power underground by See also:steam, compressed See also: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 See also:ladder- and See also:pipe-way and for ventilation. When much water is encountered a See also:separate See also:pump compartment is desirable. Cylindrical shafts may be similarly divided by subsidiary timbering, though in many timbering is omitted and the hoisting cages areJguided by See also:wire See also:ropes stretched from See also:top to bottom. Dimensions.—The cross-sectional area of shafts depends mainly on the See also: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 four- or five-compartment shafts, sometimes required for large See also: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 See also:Pennsylvania colliery shafts, up to 13 by 52 ft. Cylindrical shafts rarely have more than two hoisting compartments and are commonly from io to 16 ft., sometimes 20 or 21 ft. See also:diameter, the segmental areas surrounding the hoisting-ways being utilized for ventilation, piping, &c. Sinking in Rock.—If the rock be overlaid by loose See also:soil carrying little water, excavation is begun by pick and See also:shovel, and after the rock is reached it is continued by drilling and See also:blasting (see BLASTING). The sinking plant, usually temporary, comprises a small hoist and See also:boiler, several buckets or sometimes a skip, one or more sinking pumps, according to the quantity of water; occasionally a small ventilating See also:fan, and a timber See also:derrick or See also:head-See also:frame over the shaft mouth, with appliances for dumping the buckets, handling the rock and safe-guarding the men in the shaft against falling See also:objects. In some circumstances a portion of the permanent mine plant is erected for sinking. The choice between See also:hand and See also:machine drilling depends chiefly on the See also: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 See also: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 See also:Rand, the world's See also:record being that of the See also:Howard shaft, sunk by hand labour 203 ft. in one See also:month. But such speeds are attainable only in dry, or nearly dry, ground, at a high cost per See also: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 750 and 30° to the See also:horizontal, inclines are generally slower on See also:account of the greater inconvenience of carrying on the work, both of excavation and timbering. Very See also:flat shafts, on the other hand, can be sunk at speeds little less than for See also:driving tunnels, unless there is much water. The highest See also:speed on record for a very flat incline (ro°°) is 267 ft. in one month. As a rule, the speed attained in sinking depends less on the drilling time per See also: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 See also: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 See also:drill holes is shown in See also:plan and section in fig. 1. The holes are charged with See also:dynamite and fired by See also:fuze or See also:electricity—in deep shafts preferably by electricity, as the men may have to be hoisted a See also:long distance to reach a See also:place of safety. After the See also: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 See also: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 some-times built to See also: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 re'' rtions of the walls from falling away. In See also:firm rock lagging may omitted. Each set consists of (fig. 2) two long timbers (wall- Plan See also:Longitudinal Section Fig. I. plates) \V, 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 12o ft., longer timbers (" bearers ") are notched into the walls, under a set, to prevent displacement of the lining as a whole. A See also: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-See also:crib AB, CD, (fig. 3). Specially moulded tapered bricks are convenient, shaped to conform with the See also: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 See also:cement See also:mortar. Such linings, while not entirely water-tight, will shut out much of the water See also: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 See also:joints are rarely bolted; they are packed with See also:sheet-See also:lead or thin strips of dry See also:pine, any leaks appearing subsequently being stopped with wedges. Though preferably of cast iron, tubbing is occasionally built of steel See also:plate rings, stiffened by angles or channels riveted to them. The irregular See also:annular space between the tubbing and rock-walls is afterwards filled with concrete or cement grouting. The lowermost tubbing See also: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 See also:joint at the bottom of the tubbing with the rock just above the mineral See also: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 See also: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 See also: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 See also:I000 or 1200 gallons per See also:minute. In its after results the system is most successful when the water-bearing rocks See also:rest on an impervious stratum, overlying the mineral deposit. The entire e-excavation is carried on under water; then a lining of special See also:design is lowered into place and the shaft unwatered. The shaft is sunk by See also:boring on an immense scale, by apparatus resembling the See also:rod and drop-drill (see BORING). Instead of See also:ordinary drills, massive tools called " trepans " are employed, consisting of a heavy iron frame, in the See also: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 See also:bore is usually made FIG. 4.-Large and Small Trepans for shaft sink- with a small See also:ing, Haniel & Lueg, See also:Dusseldorf, makers. trepan and after- wards enlarged to full size. The advance bore maybe completed to the required depth of shaft before beginning enlargement, or the small and large trepans used alternately, the advance being kept 3o to 6o 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-See also:beam operated by an See also:engine on the surface, as in ordinary boring. A derrick is erected over the D E Jo cC Jo c[ E w w 768 shaft, with a hoisting engine for raising and lowering the tools. See also:Average rock is bored at a speed of about i z ft. per 24 See also: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 See also:procedure the advance bore is sometimes omitted even for large shafts, the debris being removed by a special dredger (See also:Coll. Guard., Dec. 22, 1899, 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 " See also:moss-See also:box " (aa, bb) are designed to See also:telescope together and have a quantity of dry moss packed between their See also:outer flanges. When the lowermost ring reaches the bottom, the See also: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 See also: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 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 X35 to 06o per foot), as well as the speed, compare favour-ably 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: (1) spiling, vertical or inclined; (2) drop-shafts; (3) See also:caisson and compressed air; (4) the freezing See also: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 spiting is also limited to small depths. Cribs are put in every few feet and around them, driven ahead of the excavation, are See also:short, heavy planks, sharpened to a See also:chisel edge. The spiles in-cline outward, being driven inside of one crib and outside of that next below (fig. 6). The shaft bottom also is usually sheathed with planking, braced against the lowest crib 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 masonry lining (usu- ally cylindrical), is sunk through the soil, new sections being succes- sively added at the sur- See also:face, while the excava- tion goes on inside. In quite soft soil the lining or drop-shaft sinks with its own weight; when necessary, additional weights of See also:pig-iron, rails, &c., are applied at the top. If, from excessive See also: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 See also:shoe, generally of triangular cross-section, which cuts into the soil as the weight of the structure increases and the excavation See also:pro- ceeds. 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 See also:sand or mud at the bottom. It is then suspended by iron rods, fastened to the shoe and threaded for passing through large nuts 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 See also: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 See also: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 See also:side is attached a heavy hoop-like wing. The wings carry two large sacks of See also: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 See also: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.—T his useful process was introduced in See also: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 See also:bed-rock or other firm formation. Each pipe is plugged at the lower end and within it is placed an open pipe, 11 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 after-wards pulled out. The inner 11 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 See also:ice-machine. A circulatory system is thus established. The freezing fluid, a nearly saturated See also:solution of See also:calcium or See also: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 See also:cold solution rising in the large pipes absorbs the See also: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 See also:weeks), forming an unbroken wall. The enclosed soft soil may then be excavated by dredging; or the freezing may be continued (See also: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 bore-holes 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 See also: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 See also: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. Sinking in Rock: Engineering (London, 2nd Feb. 1894) ; Coll. See also:Guardian (7th See also:April 1898) p. 631, (loth April 1906, and loth May 1898) ; Coll. Engineer (Oct. 1898) p. 135, (Dec. 1895) p. too, and (See also:Jan. 1896) p. 103; Mines and Minerals (See also:June 1900) p. 481, (Dec. 1905) p. 225, and (Feb. 1906), p. 311; Eng. and See also:Min. Journ, (13th April 1901) p. 461, and (16th Sep. 1905) p. 483; Min. and Sci. See also:Press (3rd April 1904) p. 299; Australian Min. See also:Standard (1st Feb. 1900) ; Trans. Instn. Min. and Met. xv. 333; Jour. South See also:African Assoc. Engs. (3rd Feb. 1906) ; Rev. Univ. See also:des mines (Oct. 1899); Glackauf (8th Oct. 1904 and 4th See also: 178; Rev. Univ. des Mines (Oct. 1902). Sinking in Soft Ground:—Das Schachtabteufen in schwierigen Fallen, J. Riemer (1905), translated into See also:English in 1907 by C. R. See also:Corning and See also:Robert See also:Peele; Coll. Guard. (6th April 1894, 14th Nov. 1902, 3rd Jan. 1903 and 29th Dec. 1905) ; Mines and Minerals *W !^,Iurn=' 1111M; 1 /ld'ougumomms X 11' x (Nov. 1904), p. 188; Trans. Amer. Inst. M.E., xx. 188; Gluckauf (14th June 1902); School of Mines Quart. iii. 277; Rev. univ. des mines (See also:July 1902); See also:Bull. See also:Soc. de l'Ind. Min. (1903), No. 1; See also:Ann. des mines de Belgique, x. pt. i; Mining dour. (21st April 1906). Freezing Process: Gluckauf (12th May 1906, 2nd June 1906); Ostrr. Zeitschr. f. See also:Berg- u. Huttenwesen (14th, 21st and 28th July 1906, 14th, 21st and 28th April, and 5th May, Igoo); 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. Additional information and CommentsIf you wish to find out more about Shaftesbury please visit its community and tourism home page at www.shaftesburytown.co.uk
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