workmen
before the water is turned on; and if the blockade is not broken in a
reasonable time the water is shut off, men go down and extend the pipes
nearer the blockade, and again the water is turned on, and the
operation is continued until the blockade is broken. If the shaft or
tunnel is closed by gravel mixed with heavy boulders it is necessary
often to employ powder.
First Washing.—The
first washings through a shaft should be done with care, and the
surface within as great a radius as can be conveniently washed and
drawn should be cleared on all sides before taking off the top timbers.
Attempts to push this preliminary work have frequently caused an over
crowding of the shaft, resulting in its tilling up or caving. It is therefore essential that the gravel should be run so as to avoid the rush of material from caves.
Size of Tunnel.—The size of the tunnel is generally dependent on the size of the sluice. It is usually driven 2 to 3 feet wider than the inside width of the sluice, and 7-1/2 to
8 feet high. These proportions permit the proper construction of the
sluice and give sufficient room for the blocks and for the workmen when
cleaning up. The grade depends on the topography of the country.
Location of Tunnels.—In
locating the mouth of a drainage tunnel (or of an open cut) that point
is to be selected from which the sluices, running on the most direct
practicable line, with a given grade, can bottom the maximum extent of
the " pay channel" at the smallest expense. Due regard should be had to
the dump, and allowances made for contingencies arising from changes,
such as depressions and holes in the bed-rock.
Where
the bed-rock disintegrates on exposure to the air an extra allowance
for depth is advisable. This additional depth is a matter of judgment,
and is regulated by the character and peculiarities of the bed-rock,
extent of ground to be worked, and the position of the shaft. It is
always possible to " ease up " the grade ; but if the main
