Engineering Rock Mechanics

252 Excavation principles

(e) Ensure good site management-accurate surveying and setting out of
blastholes followed by accurate drilling is essential if truly parallel and
coplanar blastholes are to be achieved.

If these guidelines are followed, successful pre-splitting will generally
occur. The method is forgving and robust. The authors have seen examples
of successful pre-splitting under remarkably adverse conditions.
There are three main factors which can mitigate against pre-splitting.

To avoid the pre-split blasting degenerating into bulk blasting (as

shown in Fig. 15.5), it is important that the blast occurs sufficiently

far from a free face parallel to the blasthole plane, so that the

blasting energy is preferentially guided into forming the pre-split

plane.

Discontinuities within the rock mass can act as free faces such that, if

they are at a low enough angle, as shown in Fig. 15.13(a), a ragged pre-

split could result. Conversely, discontinuities which are almost perpen-

dicular to the pre-split plane have little effect on the outcome, as shown

in Fig. 15.1303).

In situ stresses can induce an effect analogous to the proximity of

discontinuities, because the rock will tend to fracture perpendicular to

the least principal stress, as illustrated in Fig. 15.13(c). If the principal

stresses are adversely orientated and of sufficient magnitudes, the pre-

splitting mechanisms may be rendered ineffective.

Intended presplit

/

Blast-induced fractures

parallel to major

principal stress

I

Figure 15.13 The effects of discontinuities and in situ stress on the creation of

the pre-split plane. (a) Low-angle discontinuities. (b) High-angle discontinuities.

(c) In situ stress.