Questions and answers: excavation principles 255A75.5 (a) The key requirements for ensuring a good pre-split face2
(Langefors and Kihlstrom, 1963) are explained in ERM 1 as:
the blastholes must be parallel and locally coplanar in order to form a
plane;
the blastholes should be closely spaced (distance between them should
not be more than lox blasthole diameter) in order to allow fractures
driven by gas pressure to link the holes together;
the blastholes should be lightly charged and the charges decoupled
from the blasthole wall in order to avoid pulverizing the rock around
the boreholes and to maximize the gas pressure effect; and
the blastholes should be detonated simultaneously in order to maxim-
ize borehole interactions.
If these requirements are followed, the pre-split blasting will usually be
successful in fact, the technique is remarkably robust and a successful
pre-split face can be achieved even under difficult conditions.
(b) Photo 2 is an example where the guidelines have not been followed.
The holes are clearly not parallel even over a short distance, and it is
not certain that all the holes contained explosive. So, regardless of the
fact that the presence of rock fractures can adversely affect the pre-split
plane, insufficient care has been taken.
Photo 2 is an example of a rock face where the pre-existing fractures
have adversely affected the pre-splitting. The large pre-existing fracture
occurs at an orientation which has had an effect on the drilling of the
blastholes, in that the holes have deviated from their intended direction
to follow the fracture. Moreover, the pre-split plane has occurred along
the fracture. In this case, the guidelines were probably followed, but
the fractures in the rock mass have had a profound effect on the
outcome.
Photo 3 illustrates an acceptable face. The boreholes are parallel and
sufficiently closely spaced. The face has some irregularities, as there are
dominant fracture sets at this site which have affected the pre-splitting.
The purpose of the pre-splitting is to reduce damage behind the face
compared to bulk blasting, and this will have been achieved, despite the
rough final face.
Q15.6 In order to be able to estimate the blasting specific energy for
different rock masses (and hence the different amounts of explosive
required for blasting) a Blastability Index (BI) has been proposed
(Jimeno et al., 1995 3, using five rock properties. The BI is shown in"e pre-splitting technique has been used for many years and is comprehensively
described in the seminal rock blasting book, Langefors U. and Kihlstrom B. (1963) The
Modern Technique of Rock Blasting. Wiley, New York, 405pp. The authors mention that "in
conventional blasting the explosive is used in such a way as to destroy the quality of the
remaining rock and that with pre-splitting "the final contour appears almost as if it were
cut out of the rock with a knife leaving the remaining rock practically undamaged. This
final statement is, perhaps, an exaggeration, but it describes the technique perfectly.
p. 168 in Jimeno C. L., Jimeno E. L. and Carcedo E J. A. (1995) Drilling and Blasting
of Rocks. Bakema, Rotterdam, 391pp. (The example in the question also comes from this
reference.)