The excavation process 241A variety of considerations is involved, such as the brittleness of the
material, as expressed via the complete stress-strain curve, and matching
the explosive type and quantity to this for optimal fragmentation. In
such an excavation process, is it best to break the rock in compression,
tension or shear? Is there a choice? Because the tensile strength of rock is
about one-tenth the compressive strength and the energy beneath the
stress-strain curve is roughly related to the square of the peak strength
of the rock, breaking the rock in tension requires only about one-hundredth
of the energy required to break the rock in compression. So, not only
do we need to match the explosive to the rock type, but we need to
consider carefully how to use the energy in an optimal way to achieve the
objective.75.7.2 The in situ rock block and excuvuted frugrnent
size distributions
Rock is naturally fractured and consists of rock blocks of certain sizes, which
can be conveniently presented in an integrated way via a volumetric block
size distribution, the concept of which was introduced via a cumulative
block size distribution in Section 7.2.4. The fracturing of the rock during
excavation changes this natural block size distribution to the fragment size
distribution as illustrated in Fig. 15.2.
The engineer can consider how best to move from one curve to the
other in the excavation process. There is also the connection with the
basic objective mentioned earlier. If production of the resulting ‘hole in
the ground is the end product, the excavation is an end in itself, although
naturally coupled with the construction system. If, however, the excavated
material is the desired end product, then the primary mining excavation
process is the first of many stages of comminution. The first of these
problems would require the minimum of change between the pre- and
post-excavation block distributions, whereas the second may not involve
such a constraint.
Figure 15.2 The process of excavation interpreted as changing the pre-existing
natural rock block size distribution to the debris fragment size distribution.