Page 289 - Rock Mechanics For Underground Mining

10 Energy, mine stability, mine

seismicity and rockbursts

10.1 Mechanical relevance of energy changes

The discussion in preceding chapters was concerned with the design of single, or
mechanically isolated, excavations in different types of rock media. In all cases, a
design objective was to achieve a static stress distribution, or a set of static forces on
discontinuity-defined internal surfaces, which could be sustained by the constituent
elements of the rock mass. This approach would be completely satisfactory if rock-
bursts and similar dynamic events did not need to be considered in underground
excavation design, or if the stress concentrations which occur around openings were
achieved in a pseudo-static way. Seismic events and rockbursts are a pervasive prob-
lem in mines which operate at high extraction ratios, and involve release and trans-
mission of energy from the zone of influence of mining. Furthermore, in metallifer-
ous mining, the development of mine excavations, for both access and ore produc-
tion, frequently involves near-instantaneous generation of segments of the excavation
surface. As observed in Chapter 1, the development of an underground opening is
mechanically equivalent to application of a set of tractions over a surface repre-
senting the excavation boundary. Thus, typical excavation development practice is
represented mechanically by the impulsive application of these surface forces in the
rock medium.
Seismicity is the sound of rock slipping or cracking. A seismic event is a sudden
episode of radiation of acoustic energy in ground waves induced by discontinuity
slip or rock material fracture. A rockburst is the sudden displacement of rock, under
seismic impulse, in the boundary of an excavation, causing substantial damage to
it. Both seismic events and rockbursts involve unstable energy changes in the host
rock mass, suggesting that an examination of energy changes during mining is fun-
damental to understanding of these phenomena. In conventional solid mechanics, it
is well known that impulsive loading of a structural member or component results
in transient stresses greater than the final static stresses, and that the most effective
means of determining transient stresses and deformations under impulsive loading
is by consideration of the energy changes to which the component is subjected. In
fact, the amount of energy that a component can store or dissipate is frequently an
important criterion in mechanical design. A component which is operationally subject
to rapid loading must be constructed to a specification which reflects its duty as a
transient energy absorber. In the mining context, it is reasonable to propose that rock
around mine excavations will be subject, during development, to transient stresses
exceeding the equilibrium static stresses, due to rapid application of surface tractions.
It is thus inferred that both rockbursts and these transient effects may be best studied
through methods which account for energy changes in the system.
As observed later, energy changes in a mine domain arise from generation and
displacement of excavation surfaces and energy redistribution accompanying seis-
mic events. Because mine development practice frequently employs drill and blast

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