Page 550 - Rock Mechanics For Underground Mining

BLASTING MECHANICS

It is to be noted that a high in situ state of stress, or a high local state of stress
around an excavation, promotes more effective smooth blasting. It is concluded that
smooth blasting is the preferred method of perimeter blasting at underground sites,
where high states of stress are common. In the design of a smoothing blast, however,
particular stress environments and excavation geometries may require that the evolv-
ing boundary stress around an excavation be taken into account to assure success of
the blast around the complete excavation periphery.

17.7 Transient ground motion

Blasts in an underground mine are conducted for two purposes: fragmentation and
comminution of the rock mass and excavation of access and service openings. Blast-
ing for fragmentation purposes is conducted on a large scale. For example, a major
stope blast, or a large pillar-wrecking blast, may involve the sequential detonation
of several hundred tonnes of explosive distributed through several hundred thousand
cubic metres of rock. In such cases, two objectives are to control transient general
motion in the far field, at the ground surface, and to prevent damage to mine access
and service openings.
Figure 17.11a is a schematic illustration of a blast site at an underground mine.
When the blast is executed, P waves are generated at the various blast sources. S
waves are generated in the rock medium by internal reflections and refractions. Thus
the blast site acts as an apparent source for both P and S body waves, which propagate
in all directions. Some of the waves travel to the ground surface, where they are partly
reflected as PP waves, PS waves etc. In addition, the waves are partly refracted in
the ground surface, and a surface wave is generated in the upper layers of the rock
medium. The point O directly above the blast site, on the ground surface, acts as the
Figure 17.11 Generation of surface
waves above a blast site, and the na- apparent source of the surface waves. The characteristic of these waves, of which
ture of particle motion associated with there are two types, is that they are generated and sustained only near the ground
Rayleigh and Love waves. surface.

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