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Shaft Sinking

The construction of large and deep shafts is operated by specialist firms, not by the mining company itself. In stable dry ground the excavation work is done by manual methods, with the aid of pneumatic picks and spades, and blasting where hard rock is encountered. A multiblade grab or other mechanical device may be used for loading the loosened soil or rock into a heavy steel bucket, which is then winched to the surface (Fig.1).

Shaft Sinking

Normally, shaft sinking and shaft lining occurs together, sinking being interrupted at the intervals in order to line the newly sunk portion. Brick or concrete is used as a lining material, brickwork being more particularly used for round shafts in Britain and on the Continent.

Shaft sinking, excavation from the surface of an opening in the earth. Shafts, which are generally vertical, are usually distinguished from tunnels, which are horizontal. Little difficulty is experienced in shaft sinking through solid rock, which contains little water. When loose, water-bearing strata have to be contended with, careful shoring and sealing of the shaft lining become necessary, and pumping facilities are needed. Shafts are usually circular or rectangular and are generally lined with wood, masonry, concrete, steel, or cast iron.

In water-bearing strata the shaft lining is usually built up from tubing which consist of cast-steel segments (Fig.3). The latter are about 1.3m (4 1/2ft.) high and provided with ribs and flanges for strengthening and interconnecting them. They are bolted together to form rings which are installed one above the other, with lead gaskets in the joints, so that a closed watertight lining is obtained which is able to withstand pressure acting on it from the outside.

The space between the lining and the wall of the excavation is filled with concrete. In loose water-bearing ground a shaft-boring technique may be employed, whereby it is possible to construct deep and wide shafts, e.g., up to about 500m (1600ft.) depth and 5 m (16ft) diameter.

Boring is carried out with the aid of drilling mud, which helps to withstand the pressures (from the ground and from the underground water) which tend to collapse the shaft during construction. The cuttings from the drills are removed from the shaft bottom by suction through the hollow drill rod by means of special pneumatic pumps (mammoth pumps). The shaft lining, a cylinder fabricated from steel plate, is lowered into the shaft as excavation proceeds and is cemented in.

Shaft Sinking

In the drop-shaft method of construction (Fig.2), the brickwork or concrete lining is built up at the surface and sinks into the shaft under its own weight, assisted by ballast loading as excavation at the bottom proceeds. The bottom of the lining is provided with a cutting edge of steel to assist its penetration.

Another technique that may be used in water-bearing ground is cementation, which consists in sealing the cavities and fissures with grout (cement slurry-a fluid mixture of cement and water) which is injected under pressure through holes drilled into the strata concerned. The grout solidifies and stops the inflow of water, so that shaft sinking can be carried out in the ordinary way.

Shaft Sinking

In soft waterlogged ground the freezing process is sometimes may be in use. It is an expensive method because of the fairly elaborate equipment it requires refrigerating platn, cooling tank etc. Pipes spaced about 1yd apart are sunk vertically into the ground at a distance of 2 or 3 yds, from the edge of the shaft to be excavated. Pipes of smaller diameter are installed concentrically inside these vertical pipes, and a freezing liquid (brine) is circulated through the inner pipes and flows back to the surface through the outer ones (Fig.4). The temperature of the brine is about –20oC. Ammonia is used as the refrigerating agent.

A solid cylinder of frozen ground is gradually can found around each pipe, and when these frozen cylinders unite to form a solid ring round the ground to be excavated, shaft sinking can proceed in the usual way within the protection of this encircling wall of solidified ground. Sometimes the freezing is preceded by cement grouting if the ground contains wide fissures or cavities. When the shaft has been completed, freezing is stopped and the ground allowed thawing. The pipes are then withdrawn. The freezing process has been used successfully for shafts up to about 600 m (2000 ft.) deep.