Metalliferous Mining

When the shaft has passed through the watery strata the cast-iron lining (cc, dd) is bolted together at the shaft mouth and lowered through the water (fig.

4). Sometimes, the first two rings (aa, bb) are designed to telescope together with dry moss packed between their flanges.

When the lowermost ring reaches 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 sur face by threaded rods, and to reduce its weight while it is be ing lowered the bottom may be closed by a diaphragm (if), in the centre of which is an open pipe (g). This pipe is provided with cocks for admitting enough water to overcome buoyancy. Finally, concrete is filled in behind the lining, the diaphragm removed and the completed shaft pumped out. The moss-box is now usually omitted, the concreting being relied on to make the lining water tight. The cost of this method (generally £50 to £8o per ft.), as well as the speed, compares favourably with results obtainable un der the same conditions by other means; in many cases it is the only practicable method.

Sinking in Unstable Soils.

Unstable or watery soils often cause serious engineering difficulties. Shaft sinking is then ac complished by: (I) spiling, vertical or inclined; (2) drop-shaft; (3) caisson and compressed air; (4) the freezing process. Vertical spiling consists in driving series of spiles around the sides of the excavation, each series 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. Steel sheet spiling is also used for small depths in silt, quicksand and other very watery soil. Inclined spiling (fig. 5) is limited to small depths. Cribs are put in every few feet; around them, driven ahead of the excavation, are short planks, sharpened to a chisel edge. The spiles incline outward, being driven inside of one crib and outside of that next below. The shaft bottom also may have to be sheathed with planking, which is braced against the lowest crib and advanced to new positions as sinking of the shaft progresses.

Drop-shafts have been used for depths of nearly Soo ft. A heavy timber, iron or concrete lining (usually cylindrical) is sunk through the soil, new sections being successively added at the surface, while excavation goes on inside. In quite soft soil the drop-shaft sinks with its own weight ; when necessary, added weights of pig-iron, rails, etc., are applied at the top. If the first lining refuses to sink farther a second is lowered inside of it.

The drop-shaft rests 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 the drop-shaft is built of concrete, its great weight may become unmanageable in very soft soil, and it is hung from a frame work on the surface. For deep shafts the lining must be of iron or steel, as wood is too weak and concrete too heavy. When only a reasonable amount of pumping is required, the soil is excavated by hand ; otherwise dredging is resorted to. The leakage of water under the shoe may be stopped by concreting the shaft bottom or by various other methods. There are many modifications of the drop-shaft which cannot here be detailed. Sinking with caisson. (q.v.) and compressed air is rarely adopted except in civil engi neering operations where deep foundations are necessary; e.g., bridge piers.

Watery soil in which a shaft is to be sunk is sometimes arti ficially frozen and excavated like rock. A number of drive pipes are put down (see BORING), usually 6 to 8 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 then plugged at the lower end and within it is placed an open pipe, II- in. in diameter, extending nearly to the bottom. Or, preferably, after the drive-pipes are down and cleaned out, a slightly smaller pipe, closed at its lower end, is inserted in each drive-pipe, the latter being afterwards pulled out. The inner If-in. open pipes are then inserted. The outer and inner pipes are connected with a refrigerating machine (see REFRIGERATION) and brine at a temperature of o° is passed down the inner pipes and up through the outer pipes. The cold solution rising in the large pipes absorbs the heat from the sur rounding watery soil, freezing concentrically round each pipe. The frozen masses finally join (in say 3 to 4 weeks), forming an unbroken wall. The enclosed soft soil may then be excavated by dredging. Generally freezing is continued until the solidifica tion reaches the centre and to some distance beyond the circle of pipes, after which the frozen ground is drilled and blasted and the shaft is lined. This process has been successfully employed in stages to depths of about 2,000 ft. Nearly ioo shafts have been sunk by this process. In 1924 two shafts were sunk in Belgium by freezing to 2,025 ft. at a cost, including lining, of £230 per foot.