Page 423 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.23

(A) (B)

(C) (D)
FIGURE 9.16 Primacord connections. (A) Trunkline spliced with square knot. (B) Double
wrap, half-hitch knot connection between a reinforced trunkline and downline. (C) Trunkline
clove hitched over a downline. (D) Special clove hitched trunkline to prevent slippage from
subsidence of the charge.

important that the lines be at right angles to each other. (See Fig. 9.16.) When very stiff cord is
used, it may be necessary or convenient to use plastic connectors instead of knots.
Electric caps may be used to combine quiet surface wiring with Primacord downlines. Connection
is made by taping a cap (or preferably two caps) tightly to the Primacord. This may be a short
piece, called a tail, which is tied to the down line with a square knot just before firing. The caps
are wired into a conventional blasting circuit.
There are also nonelectric delay caps, Primadets, that are made up attached to a light detonating cord,
Primaline. They are strong enough to explode AN-FO and most slurries directly, without a primer.
The cap is placed near the bottom of the hole, with or without a primer, before pouring in the main
charge of explosive. The Primaline is knotted to the Primacord trunk line. It does not have sufficient
strength to detonate AN-FO or slurry, so timing is regulated by the delay feature in the cap. It is not
used with dynamite or other explosives that are sensitive enough to be set off by the cord.
On all large blasts it is customary to arrange Primacord so that each hole may be reached by
the explosion from two directions. Large blast holes, or holes of any size with deck loading, may
have two cords strung on opposite sides of the hole to ensure firing.
Primacord has become the standard method of setting off large blasts, because of its excep-
tional safety. As an explosive it is quite inert, and is less likely to be detonated by accident than
the main charge of explosives. Particularly, it cannot be exploded by stray electric currents, a serious