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TUNNELS AND SLUICES.
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Shaft Timbering.—Where
a shaft is in hard rock, and no man-way is needed, timbering is
unnecessary ; but in soft rock or gravel, to avoid any accident or
delay the shafts should be strongly timbered, closely lagged, and lined
on the inside with blocks (6 to 10 inches thick) to within 8 to 30 feet
of the surface, the depth being dependent on the softness of the
gravel. This top, being the first washed off, thereby gives the initial
grade for the ground sluices. As washing proceeds the upper lining and
timbers are removed to enable the material to be drawn into the shaft.
A shaft in hard rock can be partitioned for a man-way with
stoll-timbers firmly wedged and blocked.
No
extraordinary precaution is required for the protection of the bottom
of the shaft, the material washed being allowed to drop directly on the
bed-rock, where it soon wears a hole, in which the large stones from
the mine lodge and form a pavement. At the junction of the shaft and
the tunnel the latter should be increased in height at least 50 or 75
per cent.
Second Shaft.—With
long tunnels it is advisable to sink a second shaft at a convenient
distance from the heading. Formerly, as a precautionary measure, a man
was placed in the tunnel to watch the washings, and in such cases a
second shaft was indispensable. It is now customary, when washing into
a shaft, to provide a swinging door over the sluice, about 75 feet
below its head, and connected by chain and ropes to a signal on top of
the shaft which gives the pipe-men notice in case of overflow.
Should
an accident occur at the main shaft by its caving or closing up, the
second shaft might afford the necessary facilities for continuing the
work. When a line of pipe is carried down the second shaft for the
purpose of assisting in opening the closed one, great precaution must
be used in piping, particularly if the closed shaft is filled with
water. When this expedient has to be resorted to it is usual to place
the pipes in position and withdraw the
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