Civil Engineering Reference
In-Depth Information
11.3 Production blasting
The basic economics of rock excavation using
explosives are shown in Figure 11.2 (Harries
and Mercer, 1975). The production of a well-
fragmented and loosely packed muck pile that
has not been scattered around the excavation
area, facilitates loading and hauling operations.
This condition is at the minimum total cost point
on the graph. However, close to the final face,
drilling and blasting costs will increase because
more closely spaced and carefully loaded holes
will be required to limit damage to the rock. Con-
versely, the production of riprap will involve the
use of holes with a spacing greater than the largest
block size required.
In order to achieve the optimum results of blast-
ing under all conditions, a thorough understand-
ing of the following parameters is required:
of 10 ms. As the rock is unloaded due to radial
expansion and reflection of the compressive wave,
it is now possible for the expanding gases to
wedge open the strain wave-generated cracks and
begin to expel the rock mass (Figure 11.1(c)). This
stage is characterized by the formation of a dome
around the blast hole. As wedging action takes
place due to the heaving and pushing effect of
the expanding gases, considerably more fractur-
ing occurs due to shear failure as the rock mass is
expelled in the direction of the free face. In highly
fissured rocks, fragmentation and muck pile
looseness are caused mostly by expanding gases.
The fragmentation achieved by the process
described in the previous paragraph is highly
dependent upon the confinement of the explosive,
the coupling of charges within the blast holes, the
burden distance and the sequencing of the blast.
That is, if confinement of the charge by stemming
is inadequate, some energy will be lost from the
blast holes, and inadequate explosive/rock coup-
ling results in poor transmission of strain energy
to the rock mass. Also, excessive burdens res-
ult in choking and poor movement of the rock,
whereas inadequate burden results in waste of
explosive energy and excessive throw of blasted
rock. The best results are produced when effective
delaying of individual blast holes ensures max-
imum development and utilization of free faces
by reducing the effective burden. This provides
freedom for the rock to move toward the free face
and reduces damage to the surrounding rock (see
Section 11.3).
To prevent damage to rock behind the face, the
zone of crushed rock and radial cracking around
the holes in the final row is controlled by reducing
the explosive energy, and decoupling the charge
in the holes (see Section 11.4, Controlled blast-
ing). As the shock wave travels beyond the limit
of rock breakage and into the surrounding rock,
it sets up vibrations both within the rock and at
the ground surface. Structures located close to the
blast, and through which these vibration waves
pass, may be damaged by twisting and rocking
motion induced by the ground motion. Dam-
age can be controlled by reducing the explosive
weight detonated per delay (see Section 11.5).
1
type, weight, distribution of explosive;
2
nature of the rock;
3
bench height;
4
blast hole diameter;
Total
Loading and hauling
Drilling and blasting
Fragmentation
Finer
Coarser
Figure 11.2
Effect of fragmentation on the cost of
drilling, blasting, loading and hauling.
