Page 369 - Rock Mechanics For Underground Mining

ROCK MASS RESPONSE TO STOPING ACTIVITY

An alternative method of describing rock mass behaviour in the mining methods
represented by Figures 12.2 and 12.3 is in terms of the constitutive behaviour of the
host medium for mining. By restricting rock mass displacements, in both the near field
and far field, to elastic orders of magnitude, the supported method of working shown
in Figure 12.2 is intended to maintain pseudo-continuous behaviour of the host rock
medium. The caving method illustrated in Figure 12.3, by inducing pseudo-rigid body
displacements in units of the rock mass, exploits the discontinuous behaviour of a rock
medium when confining stresses are relaxed. The supported methods of working can
succeed only if compressive stresses, capable of maintaining the continuum properties
of a rock mass, can be sustained by the near-field rock. Caving methods can proceed
where low states of stress in the near field can induce discontinuous behaviour of
both the orebody and overlying country rock, by progressive displacement in the
medium. On the other hand, caving can also proceed under conditions in which the
stresses induced at and near the periphery of the rock mass above the undercut level
are sufficiently high to initiate fracturing and subsequent discontinuous behaviour of
the rock mass. Thus under ideal mining conditions, supported stoping methods would
impose fully continuous rock mass behaviour, while caving methods would induce
fully discontinuous behaviour.
The geomechanical differences between supported and caving methods of mining
may be described adequately by the different stress and displacement fields induced
in the orebody near-field and far-field domains. Added insight into the distinction
between the two general mining strategies may be obtained by considering the energy
concentration and redistribution accompanying mining.
In supported methods, mining increases the elastic strain energy stored in stress
concentrations in the support elements and the near-field rock. The mining objective is
to ensure that sudden release of the strain energy cannot occur. Such a sudden release
of energy might involve sudden rupture of support elements, rapid closure of stopes,
or rapid generation of penetrative fractures in the orebody peripheral rock. These
events present the possibility of catastrophic changes in stope geometry, damage to
adjacent mine openings, and immediate and persistent hazard to mine personnel.
For caving methods, the mining objective is the prevention of strain energy ac-
cumulation, and the continuous dissipation of pre-mining energy derived from the
prevailing gravitational, tectonic and residual stress fields. Prior to caving, rock in,
around and above an orebody possesses both elastic strain energy and gravitational
potential energy. Mining-induced relaxation of the stress field, and vertical displace-
ment of orebody and country rock, reduce the total potential energy of the rock mass.
The objective is to ensure that the rate of energy consumption in the caving mass, rep-
resented by slip, crushing and grinding of rock fragments, is proportional to the rate
of extraction of ore from the active mining zone. If this is achieved, the development
of unstable structures in the caving medium, such as arches, bridges and voids, is pre-
cluded. Volumetrically uniform dissipation of energy in the caving mass is important
in developing uniform comminution of product ore. The associated uniform displace-
ment field prevents impulsive loading of installations and rock elements underlying
the caving mass.
The contrasting mining strategies of full support and free displacement involve
conceptual and geomechanical extremes in the induced response of the host rock mass
to mining. In practice, a mining programme may be based on different geomechanical
concepts at different stages of orebody extraction. For example, the extraction of an
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