Page 420 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.20 THE WORK

Characteristics. Consistency is regulated by the amount of thickener or jelling agent (often guar
gum) that is used. It varies from that of pancake syrup to soft (flows if jiggled) jelly at room temper-
ature. Stiffening occurs at low temperatures, but most formulas are resistant to damage by freezing.
Water resistance varies from good to excellent, unless water flow is sufficient to wash it away.
Loading water gel in its sealed plastic packages is recommended for severe water conditions.
Packaged slurries are generally jelled in cylindrical shapes slightly smaller in diameter than
the boreholes in which they are to be used. They are shipped in polyethylene bags, protected by
burlap or by cardboard containers.
The bags are usually soft enough to allow slump to fill the borehole almost completely, and
can be obtained in even softer, expandable types. However, it is normal to slit the top 6 inches of
each bag, and drop it into the hole. Some water gels have unique gellant systems which are
designed to permit them to be poured into the hole.
It does not mix readily with water, and its weight (specific gravity is 1.1 to 1.5) causes it to
flow to the bottom, displacing any water and filling all borehole space. But if it is very thick, it
may bridge over and retain water pockets.
For large-scale use, slurries may be shipped and even mixed in tank trucks and pumped into
holes. If the hose can be extended to the bottom of the hole, danger of bridging is eliminated. Most
slurries are insensitive. They cannot be detonated by blasting caps or Primacord, and require spe-
cial booster-primers.
Recently developed small- and intermediate-diameter grades of water gel are cap-sensitive, so
that they require shipment and storage as a class A explosive. This type is finding wide accep-
tance as a replacement for dynamite in bottom loads.
Applications. Slurries are less expensive than dynamite, but cost more than ammonium
nitrate–fuel oil mixtures (AN-FO). They are now used chiefly in open-pit mines where rock is
hard and/or holes are wet.
The high density of slurry (1.37 to 1.68) compared with AN-FO permits use of smaller-diam-
eter boreholes, or wider spacing, to obtain the same explosive power and fragmentation. The high-
er price of the slurry may be more than offset by reduced drilling cost.
In wet holes AN-FO is not practical, so the choice is between dynamites and slurries. Here the
slurries have a smaller advantage in density, but have a price advantage as well.
1
Some slurries have been employed successfully in boreholes down to 1 ⁄ 2 -inch diameter, using
a special small-diameter booster.

INITIATORS

Safety Fuse. The original timing device is a fuse made up of a black powder core, surrounded
by layers of protective wrappings. Two speed ranges are available, with burning speeds of 90 sec-
onds and 120 seconds to the yard. These speeds must be considered approximate, as they are
affected by altitude, weather, storage conditions, and possible damage to the fuse.
Fuse is water-resistant except at the cut ends, which are immediately spoiled by contact with
moisture. It should not be used unless it can be shot the same day it is loaded.
It is manufactured in lengths of 50 feet or more, and wound in coils or on spools to be cut to
the desired length on the job. As short an interval as possible should be allowed between cutting
and using.
Squibs. Electric squibs are devices for igniting charges of black powder, and may be used
instead of fuses. They consist of copper or aluminum tubes with powder, an electric firing ele-
ment, and wires sealed into them. They are embedded in the powder charges, and when sufficient
current passes through them, they catch fire and ignite the charge.

Blasting Caps. Figure 9.14 shows construction of blasting caps for both fuse and electric firing.
Fuses are inserted in the hollow shell of the cap, and fastened in by crimping the metal with special