Shaft-Sinking

SHAFT-SINKING is an important mining operation for reaching and working mineral deposits situated at a depth below the surface, whenever the topography does not admit of driving adits or tunnels (q.v.). Shafts are also often sunk in con nection with certain civil engineering works, e.g., at intervals along the line of a railway tunnel for starting intermediate head ings, 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 com paratively simple operation. But when the formation penetrated comprises unstable, watery strata, special forms of lining are necessary and the work is slow and expensive. Mine shafts are often very deep, notably in the Witwatersrand, South Africa ; at Bendigo, Australia; and in some of the coal fields of Europe. Several inclined shafts in Michigan approximate 8,600 ft. at an inclination of 39° to the horizontal.

Shape of Shafts.

Vertical shafts may be either rectangular or cylindrical; when inclined they are always rectangular. In Europe shafts are generally cylindrical, sometimes of elliptical cross-section, lined with masonry, concrete, cast iron or steel. In the United States and elsewhere rectangular sections are the rule for sinking in rock, the shaft walls being supported by timbering, occasionally by steel framing. For sinking in loose, water-bearing soils, the cross-section is almost invariably circular to resist crushing or caving of the shaft walls. The cheaper timber or steel lined, rectangular shaft, however, is generally appropriate in rocky strata, in view of the temporary character of mining oper ations. 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 or by wire ropes stretched from top to bottom (see MINING, METALLIFER OvS) ; another is provided for a ladder- and pipe-way and for ven tilation. When much water is encountered a separate pump com partment is desirable. Cylindrical shafts may be similarly divided. The cross-sectional area of shafts depends mainly on the size of the cages or skips. Rectangular shafts of one or two com partments measure, inside of timbers, say 4 by 6 ft. to 7 by 12 ft.; larger shafts of three compartments, from 5 by i2 ft. to 8 or 10 ft. by 20 ft. Shafts of four to seven compartments, as in the deep level mines of the Witwatersrand, range from 6 by 20 ft. to 6 or

8 by 3o 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 vary from 10 to 21 ft. diameter. Sinking in Rock.—If the rock be overlaid by loose soil carry ing little water, excavation is begun by pick and shovel, and after the rock is reached is continued by drilling and blasting. The plant usually comprises a small hoist and boiler, several buckets or a skip, one or more sinking pumps according to the quantity of water, occasionally a ventilating fan and a timber derrick or head-frame over the shaft mouth. In some cases a portion of the permanent hoisting plant is erected for sinking. For very hard rock or for rapid work, machine drilling is advisable, a compressor and additional boiler capacity being then required. Many shafts in South Africa and the United States have been sunk at a speed of from 200 to 250 ft. per month. Inclined shafts when nearly vertical can be sunk about as fast as vertical shafts; but for inclinations between 75° and 30° to the horizontal, in clines are generally slower, due to the greater inconvenience of carrying on excavation and timbering. Very flat shafts can be sunk faster, unless there is much water. The highest speed on record for a flat incline (10°) is 267 ft. in one month (Howard shaft, South Africa). The speed of sinking depends mainly on the time required to hoist out the broken rock and therefore generally diminishes with increase of depth. Furthermore, omitting shafts of small area, the cost per foot of depth does not increase pro portionately to 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.

A round of holes is drilled, usually 3 to 4 ft. deep if by hand, or 5 to 8 ft. if by machine (see BLASTING) . A common mode of placing machine drill holes in shown in fig. I. The holes are charged with dynamite and fired preferably by electricity, as the men may have to be hoisted a long distance to a place of safety. After the smoke has cleared away, hastened by sprays or by compressed air if available, the broken rock is hoisted out and drilling resumed. In rectangular shafts not over 6 or 8 ft. wide, machine drills are usually mounted on horizontal bars stretching across from wall to wall ; in wider or cylindrical shafts, on tripods or special sinking-frames. Small quantities of water are bailed but when the inflow is large, a sinking pump is employed (see