250 Excavation principles
the bulk blasting will be optimized through the use of blasting rounds; and
the natural stability of the rock slopes will be optimized by minimizing the
damage at and behind the slope surface through use of the pre-splitting
technique. This latter point is particularly significant in a highway or
railway cutting, where the lack of either any dilation of pre-existing
fractures in the rock face or the creation of new fractures leads to a
maintenance free slope. Bulk blasting alone could leave a highly damaged
surface leading to potentially expensive post-excavation maintenance. This
concept also applies to the final slopes of quarries and open pit mines.
Mechanism of pre-split blasting. As with regular blasting, the mechanisms
by which the pre-split plane is created and the way in which the stress
wave and gas pressure individually contribute to the process are not
completely understood. Application of the principles of blasting does,
however, enable us to produce an outline design of blasthole geometry and
detonation sequence, such that the generation of a single pre-split plane is
favoured.
In Fig. 15.11, there are two stages in the detonation sequence of a series
of coplanar blastholes: a few microseconds after detonation; and a few
milliseconds after detonation.
Initially, the stress wave effect generates radial fractures with some bias
towards the plane of the blastholes, due to reflection from the nearest point
Medium-polyester resin 0 5 Hole diam.-3/16" (4.8 mm)
Explosive4 grainlfoot (0.8 gm/m) cm Hole spacing-5" (12.7 cm)
PETN cord\
I\Medium-polyester resin 0 5 Hole diam.-3/16" (4.8 mm)
Explosive-4 grainlfoot (0.8 gm/m) cm Hole spacing-3" (7.6 cm)
PETN cord
Figure 15.11 The progressive creation of a fracture plane during pre-split blasting
(from Matheson, 1983).