Page 421 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.21

FIGURE 9.14 Detonating caps.

tools or machines. The fuse should be cut off square, preferably by a razor blade or other very sharp
edge, which will not pinch the wrappings together.
The electric firing device consists of a very thin wire lying in a highly combustible mixture.
Passage of electricity causes the wire to become white hot and ignite the mixture, which explodes
the primer and high explosive.

Delay Blasting. This is used to improve rock fragmentation and rock movement. Rock frag-
mentation is dependent on the stress wave set up in the rock by the explosion. That may be between
10,000 and 20,000 feet per second—faster in harder rock. The rock fractures radiating out from
the borehole travel from 0.15 to 0.4 times the stress wave in the rock, so they travel from 1,500 to
1
8,000 feet per second. In other words, the radial cracks travel from 1 ⁄2 to 8 feet per millisecond.
As a guide to when rock movement starts, the common expression of 1 millisecond per foot of
burden is reasonably correct. The rate of movement is used as a guide so that rock broken by the
first row of holes will move out of the way before the second holes fire, or material from the sec-
ond holes has no place to go. The delay firing may be obtained by use of delay electric blasting
caps. Time interval averages about 1 second in “standard” caps. A more effective delay system
will be obtained by using MS connectors with 5- to 25-millisecond delays. The very short intervals
give the finest breakage but give reduced efficiency and increase throw, making mucking of the
round difficult due to widely scattered rock.
Wires may be obtained in almost any desired length, and should be long enough to reach the
wires from adjacent charges or to connect with the lead wire to the electrical source. They may be
copper or iron, and are protected by a plastic insulation. The ends of the wires are fastened together
into a bridge or shunt, to prevent premature firing through contact with stray electric currents.
Instantaneous and delay caps may be used on the same round. If they are in the same series, it is
good to increase firing current by one-third. Caps made by different manufacturers should never be
fired together. They are almost certain to have different current requirements, so that one brand of cap
will fire and break the circuit before the bridge wires in the other brand are heated enough to fire.
Caps contain explosives which are more sensitive than dynamite, and they must be handled with great
care. Heat, friction, and shock must be avoided. In the original package, electric caps are usually cush-
ioned in their own folded wires, and are often protected by a cardboard or paper wrapper in addition.