Page 440 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.40 THE WORK

post, or a wire securely connected to it, is touched in succession to connections E, F, G, H, and C.
The bad reading will show up whenever the difficulty is inside the circuit being tested. As an
example, if normal readings are shown at E and F, but an abnormal one is found at G, the trouble
is in number 3 cap or its wiring.
After the blasting circuit tests properly, an additional test is made at the power end of the leading
wires.
Warning. Warning must be given of intention to blast. The type of warning may be determined
by either law or custom. For large blasts, particularly in pits employing numerous workers and
machines, blasting should be done at specified times, as at 12 and at quitting time, and should be
preceded by definite and well-understood signals, such as horns, sirens, whistles, or yelling, long
enough in advance to notify all persons and give them time to prepare.
A usual blast signal is a call of “Fire” or “Fire in the hole,” repeated two or three times at intervals
of 10 to 30 seconds. Signals should be arranged by which workers detailed to watch entrances to
the area can stop the blast if necessary.
It is the responsibility of the blasting crew to make sure that all personnel are out of the way,
machines are protected, and no visitors or trespassers are where they can get hurt.
The area, and particularly any roads or paths leading into it, should be marked with warning
signs. If any public roads are within the danger area, traffic should be stopped at a safe distance.
Firing. During the signaling, the wires are connected to the blasting machine or switch, or if a
battery is used, one wire is connected to a post. To fire, the blasting machine handle is slammed
to the bottom, or the switch is closed.
There is a wide divergence of opinion as to what constitutes a safe distance from which to fire
a blast. Some experienced people will take shelter behind a nearby rock or tree, whereas others
consider 500 feet a bare minimum. No one should stand in front of a rock face, at any distance.
Proper barricading may be as safe as distance and more convenient. Full protection requires
some sort of roof or overhang. A very safe spot is in the tucked-in bucket of a big front shovel
which is turned away from the blast. The shovel itself may be protected by wood lagging on the
rear of the cab.
The return to the blast should be slow for several reasons. The fumes, which dissipate in a few
minutes in the open, are toxic and may cause severe headaches and nausea. If more than one hole
has been shot, one of them may fire late. Rocks are occasionally thrown so high that they take a
long time returning. Rocks or debris may be lodged precariously in trees.

LIGHT BLASTING

Light blasting, Fig. 9.35, includes loosening up of shallow or small outcrops of rock and breaking
boulders. It may constitute the entire job, be done in connection with dirt excavation, or follow
heavy blasting which has failed to cut to grade or slope lines or has left chunks too large to load.
Chip Blasting. Shallow rock outcrops are most conveniently broken up by drilling and blasting.
Unless the rock breaks readily along planes more or less parallel with the surface desired, it will
be necessary either to drill much deeper than grade or to space the holes closely. It is often good
practice to blast each row before drilling the next.
Loading may be light, or very heavy, but in general it is necessary to use more powder per yard
of solid rock than in heavier work.
Laminated or jointed formations may be shaken apart by light charges. Fragments may be thrown
long distances, and mats used to confine them are more subject to damage than with deeper blasts.
The amount and direction of throw can often be controlled to a large extent by drilling and loading
procedures. A vertical hole causes scattering in all directions. A sloped hole tends to leave the lower
slope in place and to throw the upper one away from it. Throw is reduced by increasing the number of