cement gravel of ordinary tenacity, the following method has been found to give excellent results.
The
main drift should be run in a distance of two-thirds the height of the
bank to be blasted. The cross drifts from the end of the main drift
should be driven parallel with the face of the bank, and their lengths
determined by the extent of the ground which is to be moved. A single T
is all that is necessary.
The
minimum amount of powder required is from 10 to 20 pounds per 1,000
cubic feet of ground covered by the drifts. The quantity used
necessarily varies with the character of the gravel. When the banks are
strongly bound or the gravel is very tenacious the quantity must be
increased. Small blasts, everything else being equal, require a larger
amount in proportion to the ground than large ones, varying in practice
from 10 to 50 pounds for each 1,000 cubic feet. It is usually expected
that a blast will prepare nearly double the quantity of the ground
covered by the drifts.
The
annexed table is a record of all the large bank blasts fired on the
Milton Mining and Water Company's property at Manzanita Hill,
Svveetland, Nevada County, during a period of three years. These blasts
were made under the immediate direction of Richard Thomas, foreman.
The
top gravel had been previously washed off, leaving banks from 50 to
150 feet in height. The gravel is usually hard, and cemented for 50
feet (rarely higher) from the bottom. Above this cemented material the
gravel is comparatively soft and easily broken, and therefore the
amount of powder employed is proportionately lessened as the banks
increase in height.
From
the appearance of the ground subsequently washed it was estimated that
225 to 230 cubic feet were shattered per pound of powder exploded.*
*
" Report upon the Blasting Operations at Lime Point, California, by
Lieutenant-Colonel G. H. Mendell, Corps of Engineers, U. S. A.," gives
interesting details of large blasts in rock formation.
