MINING, METALLIFEROUS), Shaft Lining.—For rectangular vertical shafts under normal conditions the excavation through surface soil is commonly lined with cribbing, inside of which a concrete curb is sometimes built to dam out surface water. After reaching rock the lining is gen erally of horizontal sets of 8 by 8 in. to 12 by 12 in. squared timber, wedged against the walls by smaller pieces, with lagging, put behind the sets. In firm rock lagging may be omitted. Each set (fig. 2) consists of two long timbers (wall-plates) W, W, two shorter pieces (end-plates) E, E, and 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 4 to 6 ft. apart, with vertical posts (studdles) S, S, between them. At intervals of 8o to 120 ft., longer timbers (bearers) are notched into the walls under a set to prevent dis placement of the lining. A series of shaft sets, with their posts, are either built up from a bearing-set, or suspended from the latter by hanger-bolts. In firm rock, a con siderable depth of shaft may be sunk and then timbered; generally it is safer to put in a few sets at a time as sinking advances. Inclined shafts in solid ground are often timbered as described above, though some times longitudinal rows of posts support the roof and divide the shaft into compart ments.
A cylindrical shaft of any importance is lined with masonry, concrete or cast-iron "tubbing." Linings are generally built in sections, as sinking advances, each section being based on a walling-crib AB, CD (fig. 3). The thickness of the walling depends on the pressure anticipated; it is usually from 13 in. to 2 ft. Such linings shut out much of the water present in the surrounding rock. Iron tubbing is employed when the inflow of water is large. It consists of cast-iron flanged rings, cast in seg ments bolted together. To permit the rings to adjust themselves to the pressure, the horizontal joints are rarely bolted; they are best packed with sheet-lead or thin strips of dry pine, any leaks appearing subsequently being stopped with wedges. The space between the tubbing and the rock is filled with concrete or cement grouting. The lowermost tubbing ring is based upon a "wedging crib." This is a heavy ring of cast-iron segments bolted together, and set on a projecting shelf of rock, carefully dressed. The space behind the crib is driven full of wooden wedges, which expand on becoming water-soaked and make a water-tight joint.
By using this method very little pumping is necessary, so the cost is materially reduced.
The Kind-Chaudron process of sinking in rock, introduced in 1852, has been confined to Europe. Many shafts have been sunk thus, some to depths of i,000 ft. or more, without one instance of failure. It is applicable only to firm rock and was devised for cases where the quantity of water is too great to be pumped out while excavation is in progress; that is, for inflows greater than i,000 or 1,200 gal. per min. This 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. Instead of ordinary tools, massive trepans are employed, consisting of a heavy iron frame, in the lower edge of which are set a number of separate cutters. Shafts not ex ceeding about 8 ft. diameter are bored in one operation; for larger diameters an advance bore is usually made with a small trepan and afterwards enlarged to full size ; or the small and large trepans may be used alternately, the advance being kept 3o to 6o ft. ahead of enlargement. An 8-ft. trepan weighs about 12 tons; those of 14 or 15 ft., 25 to 3o tons. The trepan is attached to a heavy rod, suspended from a walking-beam operated by an engine on the surface. A derrick is erected over the shaft, with a hoisting engine for raising and lowering the tools. Average rock is bored at a speed of about 1 ft. per 24 hours. The ad vance bore is cleaned of debris at intervals by a bailer similar to that used for boreholes (q.v.). The enlarging trepan is so shaped that the cuttings, assisted by the agitation of the water, run into the advance bore. Owing to the difficulty of bailing, the advance bore is sometimes omitted even for large shafts, the debris being removed by a special dredger. For rather loose rock another somewhat similar system of drilling, the Pattberg, has been satisfactorily employed (see Coll. Guardian, Nov. 14, 1902).