248 Excavation principles
(e) detonators: electric detonators consist of an aluminium tube with an
electrically activated fusehead which initiates a priming charge and
then a base charge of high explosive. Generally, delay detonators are
produced as either ‘short delay’, measured in milliseconds, or ’half-
second delay’, measured in seconds.
These are the basic principles of blasting and we have illustrated, with
idealized and practical cases, how blasting rounds are designed. There has
been much practical experience accumulated in this subject area, and all
major manufacturers of explosives have produced Blasters’ Handbooks
which give excellent guidance on details of all technical matters, including
safety, associated with the use of explosives.
With the knowledge and understanding of the principles explained here,
all this information is readily assimilated, and the reader can create a new
type of blasting round for a new purpose. Without such understanding,
these handbooks are simply series of instructions for technicians.
In the military area, explosives have been developed for a variety of
specialized techniques. The power of these explosives is specified by the
heat generated in the explosion (Q-value, units of J/g) and the volume of
gas produced (P-value, units of cm3/g). The standard, taken as 100, is that
of picric acid, which has a Q-value of 3745 J/g and a P-value of 790 cm3/g.
On this scale, the power of other military explosives is:
Picric acid 100
Gunpowder 20
TNT (trinitrotoluene) 110Thermonuclear explosives > 1,000,000.RDX (Research Department Explosive) 160The explosives currently available for civil purposes are adequate for rock
fragmentation, and it is more important to consider the blasting technique
itself than the explosive for optimal engineering.1 5.3 Specialized blasting techniques
As illustrated in Fig. 15.6, the explosive damage may not only occur
according to the blasting round design, but there may also be extra rock
damage behind the borehole wall, particularly if there are major discon-
tinuities present which reflect and refract the stress waves and provide
paths for the gas pressure. When blasting to produce a final rock surface,
such damage is malignant because it is out of sight and alters the rock in
the very regon where we require optimal quality (cf. Fig. 15.1).
For permanent rock faces at or near the ground surface, an elegant form
of blasting has been devised which takes advantage of the principles we
have outlined to minimize the damage to the rock. This technique is known
as pre-splitting, and its fundamental function is to ’create the final plane first’.
For permanent rock faces at depth, pre-splitting cannot generally be used,
and because of the alteration of the local stress field during initial blasting,
another technique known as smooth-wall blasting is used for the final