Engineering Rock Mechanics

Rock blasting 245

I Free face

Gas vents along discontinuities

behind borehole

discontinuities

Blasthole

Figure 15.6 Complex effect of discontinuities on stress waves and gas pressure.

75.2.2 Blasting rounds

One of the basic principles of designing all blasting rounds, i.e. the
configuration and sequential detonation of blastholes in one blast, is the
presence of a free face parallel or sub-parallel to the blast holes, as detonation
occurs. In some cases, these free faces may be automatically present
(benches in a quarry), but in other cases may need to be created by the blast
itself (a tunnel face).
It is interesting to consider what might be the 'most ideal' blasting round.
When excavating a circular tunnel, a circular free face is required in the
form of the final cylindrical tunnel outline, as illustrated in Fig. 15.7. Such
a free face, or kerf, could be cut-in a weak rock-by a long, tungsten
carbide-tipped chainsaw (Fig. 15.7(a)). Given the basic process of
fragmentation which we have described, it might then be sufficient to
detonate an explosive charge in a single blasthole at the centre of the free
cylinder created by the kerfing. The compressive stress wave would radiate
outwards, in a cylindrical form, and be reflected back towards the centre

All blasting energy retained

within blasted cylinder

Annular kerf formed by

(a) rock-

• damage
  (a) Before blasting (b) After blasting

Excaval

cross-sec..-..

Figure 15.7 Idealized blasting geometry using a circular kerf. (a) Before blasting.

@) After blasting.