Rock blasting 247Figure 15.9 Use of the free-face concept with delay detonators and the wedge cut.
75.2.3 Explosives
It is also important to tailor the type of explosive to the overall objective
and the type of blasting round. In considering the type of explosive to be
used for a particular purpose, we should take into account their various
characteristics. These include:
(a) strength: a function of the energy content (J/g) and the energy release rate;
(b) density: varies from 800 to 1500 kg/m3;
(c) velocity of detonation: speed at which the detonation wave passes
through a column of explosive (the higher the velocity the greater the
shattering effect), typically in the range of 3000-5500 m/s with extremes
of 1500 m/s for ANFO and 6700 m/s for detonating cord;
(d) sensitivity: ease of initiation;
(e) sensitiveness: ability to maintain the detonation wave;
(f) water resistance: capacity for detonation in wet conditions;
(g) fume characteristics: applicability in zones of poor ventilation;
(h) gas volume: gas liberated on detonation (l/kg); and
(i) stability: includes chemical and storage stability.Some of these properties are related to one another and we take advan-
tage of the best combination for our particular circumstances.
There are many types of explosive now available, the main types being:
(a) gelatine explosives: nitroglycerine thickened with nitrocellulose to give a
gelatinous consistency. These are the most widely used nitroglycerine-
based explosives;
(b) semi-gelatine explosives: which have a consistency between a gelatine and
a powder. These are special-purpose explosives for use in wet condi-
tions and small-diameter boreholes;
(c) nitroglycerine powder explosives: these are made from ammonium nitrate
with nitroglycerine as a sensitizer, have a moderate bulk strength and
are economical;
(d) ammonium nitrate-fuel oil mixtures: these are effective because ammo-
nium nitrate is a cheap chemical source of oxygen for the explosive
reaction. They have a low bulk strength;