At the Blue Tent Mine, Nevada County, in 1880, a bank 200 feet high was thrown down with 43,000 pounds of powder.
Blasting- Powder.—Common
blasting powder was almost universally used up to 1876. Since that time
Judson powder has been introduced, and combinations of black blasting
powder and Giant powder also have been experimented with. Giant powder
is extensively used for breaking up lava, pipe-clay, boulders, trunks
and stumps of trees, for all of which purposes it is found to be very
efficient.
Methods of Blasting.—In
certain districts it is customary to wash off the top or lighter gravel
and subsequently blast the bottom cement. For this purpose shafts 15
to 20 feet deep are sunk to the bed-rock, and a small chamber is
excavated at the bottom. This chamber is charged with a few kegs of
powder and tamped, and a blast is fired by means of a fuse.
The
want of proper information concerning the use and application of powder
to bank-blasting has undoubtedly caused a great waste of explosives,
and the subject is well worthy of investigation with a view to future
improvement.
In
blasting gravel banks it is desirable to thoroughly shatter the
material. To accomplish this purpose one must be governed by the
character of the ground in the selection of the powder. In hard
cemented deposits quick powders like the Judson (a low-grade
nitroglycerine powder) and the Vulcan B B are found to work better
than black powder; while the latter does fully as much work in softer
ground, a slow-lifting powder is in such cases all that is requisite.
With
very high banks it is more economical to blow out the bottom and not
attempt to raise the superincumbent mass. The charge should be placed
so that the line of least resistance is horizontal.
With banks from 50 to 150 feet high, and likewise in
