BLASTING GRAVEL BANKS.
Where the deposits are very strongly cemented blasting is necessary.
The
ordinary method of blasting gravel banks is as follows : A drift is run
in from the face on the bottom of the deposit a distance proportionate
to the height of the bank (as a general rule not over three-quarters of
this for high banks) and the character of the ground to be moved. From
the end of this drift a cross drift is driven each way (forming a T).
The cross drift is charged with kegs of powder, the main drift is
securely tamped by filling it up solid with the material which has been
extracted, and the powder is exploded by means of a time fuse or an
electric-battery. In some instances when the ground is " heavy and
bound " several cross drifts are used The amount of powder used is
determined by the position, character, and height of the bank, a
quantity sufficient only to shatter the ground being employed.
Blast at Smartsville.—The
following details of several large blasts are given as illustrating the
general facts. A blast of 450 kegs of black powder was made at
Smartsville in hard cement with an 80-foot bank, the ground being
ordinarily bound {i.e., with two sides free). The main powder
drift was run in from the face of the bank 85 feet, cross drifts being
opened each side 40 feet and 85 feet from the mouth. Each cross drift
was 45 feet long, and from its ends and centres two " lifters " were
driven at right angles to it, extending respectively half way to the
next cross drifts and to the face of the bank. After charging the cross
drifts the main drift was tamped and the powder exploded by means of an
electric battery.
