TUNNELS AND SLUICES.
Tunnels.—Tunnels
are run for the purpose of opening gravel claims (where open cuts are
impossible on account of the formation of the ground), and also to
afford proper facilities for removing the washed material.
A tunnel should be driven well into the channel before any connection is made with the surface.
Shafts for Tunnels.—The
shaft which connects with the headings should be vertical, though in
some cases inclines have been used. Its size is determined by the
requirements of the work, and varies, for ordinary cases, from 3 by 3
feet to 4% by 9 feet in the clear. When raising from the tunnel
due precaution should be taken against accidents arising from the rush
of water, sand, and gravel, which is liable to occur on tapping the
bottom of a deposit. A shaft 4% by 9 feet should be divided
into two compartments, one of which will serve as a man-way. A
compartment 4 by 4 feet in the clear is ample for the water-way.
It
may be noted that a vertical shaft, when properly timbered, is the most
desirable and economical for opening hydraulic claims, and with drops
of 300 feet no trouble has been experienced. There is no difficulty in
connecting directly with the tunnel where the work is done well and
the mine properly opened. But where washing is going on through a shaft
into a tunnel in process of extension, it is convenient to have the
shaft located at one side and connected with the tunnel by a short
drift. By this means the work in the tunnel can progress while the
washing is carried on.
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