244 Excavation principles
consider this phenomenon as being composed of a ’stress wave effect’ and
a ‘gas pressure effect’. The exact form of the curve in Fig. 15.4 will depend
on the type of explosive, the charge size, the coupling between the
explosive and the rock and the degree of fracturing of the rock around the
blasthole, among other things. Although there are many different types of
proprietary explosive, by far the most widely used explosive in the civil and
mining industries is ’ANFO-Ammonium Nitrate and Fuel Oil. This
material was found to be explosive when a ship, which had originally
transported a consignment of fuel oil, was subsequently loaded with
ammonium nitrate fertilizer and, unexpectedly, it exploded. An example
of the maximum pressure generated in a blasthole after detonation of an
ANFO charge density of 820 kg/m3 is 2490 MPa.
A compressive stress wave is reflected at a free face as a tensile stress
wave, see Fig. 13.4. Thus, after detonation of a borehole charge, there tends
to be local pulverization due to the high compressive stresses caused by
the blasthole pressure, followed by a dissipation of the stress wave.
However, if there is a proximate free face, as illustrated in Fig. 15.5, the
compressive stress wave is reflected at such a free face as a tensile stress
wave, and surface spalling occurs due to the low tensile strength of the
intact rock. This is because the rock is able to sustain a compressive wave
at a given stress magnitude, but unable to sustain a tensile wave with
stresses of the same magnitude. The energy remaining in the spalled
fragment of rock is then liberated as kinetic energy-the fragment flies off
the rock face.
Following the local fracturing at the blasthole wall and the spalling at the
free face, the subsequent gas pressure then provides the necessary energy
to disaggregate the broken rock. It is emphazised that no one understands
the precise way in which the stress wave effect and the gas pressure effect
combine to break the rock, but our understanding of the phenomenon
associated with the proximity of a free face is sufficient to allow us to
design all blasting rounds. This has been well proven in practice, despite
the difficulties incurred by the presence of discontinuities, illustrated
in Fig. 15.6.
%Wedge blown out by- Explosive detonated
in blasthole
Figure 15.5 Effect of stress waves and gas pressure adjacent to a free face.