Rock Mechanics.pdf - Mining and Blasting

Figure 17.13 Calculated peak surface
velocities and associated underground
excavation damage for earthquakes
(after Owen and Scholl, 1981).
DYNAMIC PERFORMANCE AND DESIGN OF UNDERGROUND EXCAVATIONS
for minor damage and substantial damage of 200 mm s −1 and 400 mm s −1 originally
proposed by Dowding and Rozen (1978).
According to St John and Zahrah (1987), the damage thresholds proposed above
are well below those observed in a major underground explosion test program, involving
the detonation of large charges adjacent to excavations with the purpose of
establishing dynamic design criteria. Damage associated with intermittent spalling
was observed at 900 mm s −1 , and continuous damage at 1800 mm s −1 . Because these
observations were made for single explosions, they are probably of limited relevance
to the performance of permanent mine excavations, which may be subject to many
episodes of explosive loading during their duty lives.
It is clear that a criterion for dynamic design of permanent underground excavations
based on the single parameter of peak tolerable velocity in the ground wave would
be subject to a substantial margin of uncertainty. Nevertheless, the threshold value of
200 mm s −1 may be suitably conservative to allow routine application. Site specific
empirical relations between peak velocity and explosive charge weight per delay, of
the type described by Siskind et al. (1980), but determined for transmission of body
waves, may be used to estimate probable peak velocity due to a particular blast.
Ultimately, more appropriate design for dynamic loading of an underground excavation
must be based on an analysis of rock displacements induced by a synthesised
history of likely ground motion, and take account of site conditions such as rock
structure. Dynamic analysis of jointed and fractured rock also requires description of
the dynamic properties of rough, dilatant joints under cyclic loading, which is a topic
about which there is comparatively little information.
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