Page 432 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.32 THE WORK

If the hole is ragged or partially caved so that it is not practical to load it with cartridges, a free-
running explosive may be poured down it. If it blocks the hole and starts to build over an obstruc-
tion, it should be poked down with a long jointed pole or a dislodging block.
If such a hole is wet, it may be necessary to use a water-resistant slurry or water gel, either
placed or poured.
A tamping block used for unwrapped dynamite should be kept clean by resting it on a box or
some sacks when it is not in use.
String. If the borehole is wet enough that slit or unwrapped dynamite would be spoiled, or if
solid loading would make too heavy a charge, cartridges somewhat smaller than the borehole, but
not small enough to fit side by side, may be dropped in one after the other without tamping, or
after having tamped the bottom cartridge or two.
This is the easiest way to load, and is satisfactory for small or occasional blasts. However, it
is inefficient. More rock must be drilled than is necessary to hold the charge that is used. Part of
the strength of the explosive is wasted in the air cushion around it.
Spaced. Spacers may be used to string cartridges out along a hole that is not to be fully loaded.
These may be square, round, or hollow pieces of wood, tile, lean concrete, or rolled cardboard.
They are usually made up ahead of time, in lengths of 8 to 10 inches. There should be sufficient
airspace around them to allow for detonating cord or wires, without squeezing or rubbing.
Spacers may be alternated with cartridges or pairs of cartridges in the parts of the borehole that are
not to be fully loaded. The primer cartridge should have at least one additional stick in contact with it.
Decking. In large boreholes, charges which are to be strung out are usually separated by solid
plugs of sand or other stemming material, and each section of the charge primed separately, unless
fired with Primacord or other detonating fuse.

Stemming. Stemming is inert material such as dirt, sand, or finely crushed rock that is used to
fill parts of a borehole that do not contain explosive.
Its primary use is in filling vertical holes from the top of the powder to the surface. Its use
improves breakage by confining the force of the explosion, and adds to safety by preventing acci-
dental igniting of the charge before it is fired.
Stemming is also used to space out charges, as in Fig. 9.25.
1
The minimum depth of top stemming should be about 1 foot in a 1 ⁄ 2 -inch hole, and 12 feet in
a 12-inch hole. Deeper stemming is used where the top will be shattered simply by having its base
blown out from under it. About two-thirds of a very high face may be broken in this way, with
explosive in only the bottom third.
Carrying the load too high in a hole is at best a waste of powder, and at worst can mean exces-
sive and damaging rock throw and noise. On the other hand, too little explosive and too much
stemming will give poor top breakage.
Drill cuttings are often the best source of stemming, particularly from rotary drills or air drills equipped
with dust collectors. It is important that stemming contain no sharp pieces or sizable stones that might
cut wires or fly in bullet fashion. Moist material usually is more effective than loose dry sand.
Mines and quarries that have rock-crushing plants and that use large-diameter holes often use
fine crushed rock (screenings) for stemming. This may be hauled in Dumpcrete trucks that can
dump directly in the hole through chutes. Ordinary dump trucks may leave a pile of screenings
near each hole or group of holes, for distribution by hand shovel or wheelbarrow.

LIGHTING FUSES

The length of fuse determines the time which will elapse between lighting it and the explosion.
Regular sequences of firing can be obtained by varying the lengths of fuse in different holes, and
lighting them at the same time.