Engineering Rock Mechanics

Specialized blasting techniques 25 1

of the adjacent blastholes. Second, there are dynamic and quasi-static
effects of the gas pressure reinforcing this tendency. These are:

(a) the gas travels along the fractures and causes further cracking in a

manner similar to hydraulic fracturing; and

(b) the pressurized blastholes and radial fractures induce high ‘elastic’ stress

concentrations which contribute to failure of the rock along the plane.

Thus, as illustrated in the lower diagram of Fig. 15.11, the fractures lying
in the plane of the blastholes coalesce to form a continuous plane. Because
the resultant plane is formed by the coalescence of these radial fractures,
it will tend to be rather rough on the scale of the blastholes (a few centime-
tres) but will be smooth on the engineering scale (a few metres). Note that
if the creation of the pre-split plane is successful, the half cylinders of the
blastholes should be visible on the final slope, as shown in Fig. 15.12.

Guidelines for successful pre-split blasting. From the discussion so far, it is
possible to directly deduce the guidelines that govern the successful man-
agement of pre-split blasting. These are as follows.

(a) The blastholes must be parallel and locally coplanar-so that the blast-

ing mechanisms occurring in each blasthole can interact to successfully

form the desired final surface.

(b) Use closely spaced boreholes-the distance between the blastholes should

not exceed about 10 times the blasthole diameter. This is important to

ensure that blastholes are, indeed, close enough to allow interaction.

(c) The blastholes should be lightly charged and the charges decoupled from

the blasthole wall-this is to ensure that local pulverization is minimized

whilst maximizing the gas pressure effect (in the margin sketch we show

the effect of decoupling an explosive charge on the stresstime curve).

(d) Detonate simultaneously-this maximizes the interaction between

adjacent holes, such that the preferred plane for fracturing is the pre-

split plane.

Decoupled

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Figure 15.12 A successful pre-split face.