Engineering Rock Mechanics

Specialized blasting techniques 249

surface. Both of these techniques tend to be used only where it is essential
to produce a high-quality, high-strength and low-maintenance final surface.
There is considerable history in the modern development of blasting
techniques, and indeed the cover of the 1963 book Rock Blasting by
Langefors and Kihlstrom shows a perfect example of pre-splitting at a
conduit wall in the Niagara project. Their book provides many examples
of the application of blasting technology and, as we will be emphasizing
later, the on-site problem is more one of blasting management rather than
the requirement for new technology.

75.3. I Pre-split blasting

In Fig. 15.10, we illustrate the excavation of a cutting through a rock
outcrop. The primary purpose of the blasting is to remove the rock to form
the cutting, but there are several operational reasons why a high-quality
final slope profile may be required. This is the perfect circumstance for
utilizing pre-split blasting.
As shown in the figure, a series of small-diameter, parallel boreholes are
first drilled in the planes of the required final slopes. The principle is then
to tailor the explosive parameters such that detonation of the explosive in
these initial holes will primarily create a plane intersecting the holes. Under
these circumstances, no provision has been made for dilation, but a
complete new fracture is formed in the rock. When, subsequently, the main
body of rock is bulk blasted to form the cutting, the pre-split plane reflects
the stress waves back into the rock being excavated and dissipates excess
gas pressure, such that the bulk blast has little effect on the rock behind
the pre-split plane.
As Fig. 15.10 indicates, we have now used engineering knowledge to
separate the two concepts, shown in Fig. 15.1, of excavation and support:

Calculated distance according to dip

of design slope and surface topography

4 Surveyed in line marking the

/

Floor of excavation

Figure 15.10 Use of the pre-splitting technique to create high-quality final surfaces

(from Matheson, 1983).