Shaft Sinking |
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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).
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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.
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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.
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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. |
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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.
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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.
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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.
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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.
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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.
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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.
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