Page 424 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.24 THE WORK

hazard for electrical hookups. An entire blast can be prepared with detonating cord, and a fuse or elec-
tric cap attached at the last minute.

Low-Energy Detonating Cord. The noise made by the explosion of Primacord trunk lines above-
ground may be objectionable. This noise can be considerably reduced by covering it to depths
from a few inches up to a foot with sand, dirt, drill cuttings, or other stone-free material, or almost
eliminated by using low-energy cord.
LEDC, a low-energy cord that is relatively noisefree, was developed jointly by Ensign Bickford
Company and E. I. du Pont de Nemours Co. It contains 2 grains of PETN per foot, protected in a
tiny lead tube covered by wrapping of cotton and plastic. The noise made by exploding 150 feet
of it is equal to that caused by one blasting cap or 2 inches of reinforced Primacord.
Special connectors are available to fasten LEDC to Primacord and to itself. When the explosion
travels from LEDC to LEDC or to Primacord, a booster charge is needed. This may include a 0-,
10-, 15-, or 25-millisecond delay unit.
The preferred method of firing is to fasten an electric blasting cap to one of the down lines of
Primacord, and to fasten the LEDC to this down line with a nonexplosive connector called a trunk
line adapter. The LEDC is connected to other down lines by booster delay caps.

PRIMING

Primers. A primer may be a stick of dynamite that contains a blasting cap; or is any other heavy
explosive which has been fitted with a device for setting it off.
Since these primers combine the power of the dynamite with most of the sensitivity of the cap,
they must be handled with greater care than any other units of explosives.
They are ordinarily prepared at the borehole immediately before being placed, but may be made
in some central place and delivered to the loaders as required.
The essentials of a good primer are that the cap must be powerful enough to produce detonation,
there must be intimate contact between cap and explosive, they must be fastened together so that
they will not separate while being placed, the cap should be shielded from shock or friction, and
the wires or fuse should not be kinked or strained.

Black Powder. Black powder may be primed by placing a fuse in the hole and pouring the powder
around it. This method may be improved by tying a knot in the end of the fuse to anchor it, and
making several slits into the core above the knot where they will be in contact with the powder.
A paper cartridge may be prepared to hold powder closely around the knot and slits.
If the powder is to be exploded by an electric squib, a similar cartridge is made up to enclose
the squib with some powder.
Blasting caps may also be used to explode black powder.

Fuse Caps. Preparation of dynamite and fuse cap primers includes two jobs—attaching the fuse
to the cap and the cap to the powder. The fuse ends must be dry.
The fuse should be cut squarely with a clean, sharp blade, preferably a razor blade in a suitable
mounting, and pushed into the cap until it is seated against the explosive compound. The copper
shell is then crimped firmly onto the fuse with a hand or a bench crimper.
If the fuse is cut on a bevel, it may fail to make proper contact, Fig. 9.17, top, or the end of the
casing may curl over and prevent contact. A good contact is shown at the bottom.
The interior of the cap should be clean. Any foreign matter in it should be tapped out or
removed with a straw or toothpick. Blowing into it may dampen it and cause it to fail. If the cap
is suspected of being damp from any cause, it should not be used.
Figure 9.18 shows two ways to place and fasten a fuse cap in dynamite. In each case a hole or
slit is made in the cartridge and the cap inserted. The primer can be held together by lacing the
fuse through another hole, or by tying it with string.