Blasting - Rock Slope Engineering: Civil and Mining - page 249

Table 11.1 Typical properties of explosives products

Explosive type Density

(g/cc)

VOD (m/s) Relative bulk

strength ∗
(ANFO

Water
resistance

=

100)

Packaged, detonator-sensitive
emulsions

1.12-1.2 4600-5200 115-170 Excellent

Packaged, booster-sensitive
emulsions

1.24-1.26 4300-5050 125-155 Excellent

Watergels 1.20 4785 129 Excellent

Dynamites 1.2-1.42 3350-5600 170-130 Good to excellent

Wall control dynamites 0.75-1.3 1650-2600 76-114 Good to poor

Boosters 1.34-1.6 5600-7900 167-280 Excellent

ANFO 0.84 4000 100 None

Bulk emulsions 1.25 5200-5500 120-150 Excellent

Note
∗

Relative of bulk strength (RBS)—is a comparison of theoretical chemical energy per unit volume of an explosive to ANFO that has been
assigned an RBS of 100.

by the specific gravity, and this value is important
in calculating the volume of blast hole required
to contain a given amount of explosive energy.
A higher bulk strength requires less blast hole
capacity to contain a required charge.
The sensitivity of an explosive is a characteristic
that determines the method by which a charge is
detonated, the minimum diameter of the charge
and the safety with which the explosive can be
handled. Highly sensitive explosives will deton-
ate when used in smaller diameter charges, and
can be detonated with a relatively low strength
detonator. As the sensitivity of the explosive is
decreased, the diameter of the charge and the
energy of the booster/detonator must be increased.
Table 11.1 provides information on typical
properties of the main classes of explosives. Each
class of explosive has a set of characteristics suit-
able to specific applications and conditions such
as the size of the blast hole, the presence of water
and the need to control flyrock and noise. For
example, ANFO and bulk emulsions are com-
monly used for large-scale blasts in open pits
and quarries, while watergels and dynamites are
used in smaller construction projects. Also, as
shown in Figure 11.3, it is necessary when using
ANFO as the main explosive, to use a higher
strength explosive in the lower end of the hole
(“toe load”). The function of the toe load is to

both ensure complete detonation of the ANFO,
and to break the rock in the floor of the bench
where it is most highly confined.

11.3.2 Bench height

Bench heights are usually determined by the geo-
metry of the site, with single benches being used
where the excavation depth is up to about 8 m.
On larger construction projects and in open pit
mines and quarries, multi-benches operations are
conducted. For these operations, the selection of
the optimum bench height to maximize the over-
all cost efficiency of drilling and blasting requires
the correct combination of drilling and loading
equipment. Furthermore, regulations may limit
the bench height, in relation to the maximum
reach of the excavating equipment, to minimize
the risk of damage or injury in the event of col-
lapse of the face. The following are some of the
factors that should be considered in the selection
of the bench height:

(a) Optimum blast hole diameter increases with

bench height. In general, an increase in blast
hole diameter results in a decrease in drilling
costs.

(b) For vertical blast holes and sloping bench

face, the front row toe burden may become