Page 426 - Rock Mechanics For Underground Mining

14 Artificially supported mining

methods

14.1 Techniques of artificial support

An analysis was presented in Chapter 13 of the yield potential of an orebody when
mined with a naturally supported method. It showed that under certain circumstances,
the maximum extraction possible from the deposit may be unacceptably low. This
conclusion applies particularly when the compressive strength of the rock mass is
low relative to the local in situ state of stress. The discussion in section 13.4 implied
that a limited capacity of either orebody rock or of the host country rock to maintain
reasonable unsupported spans also restricts the economic yield from a deposit. Thus,
problems of low yield from naturally supported mine structures can be ascribed
directly to geomechanical limitations, either in maintaining the local stability of stope
wall rock, or in controlling displacements in the near-field domain.
Artificial support in a mine structure is intended to control both local, stope wall
behaviour and also mine near-field displacements. Two main ground control measures
are used. Potentially unstable rock near the boundary of mine excavations may be
reinforced with penetrative elements such as cable-bolts, grouted tendons, or rock
anchors. The principles of this method were introduced in section 11.4. This type
of ground control generates a locally sound stope boundary within which normal
production activity can proceed. The second, and most widely used, artificial support
medium is backfill, which is placed in stope voids in the mine structure. In this case,
a particular stoping geometry and sequence needs to be established to allow ore
extraction to proceed and the granular fill medium to fulfil its support potential.
Three mechanisms can be proposed to demonstrate the support potential of mine
backfill. They are illustrated in Figure 14.1. By imposing a kinematic constraint on
the displacement of key pieces in a stope boundary, backfill can prevent the spatially
progressive disintegration of the near-field rock mass, in low stress settings. Second,
both pseudo-continuous and rigid body displacements of stope wall rock, induced
by adjacent mining, mobilise the passive resistance of the fill. The support pressure
imposed at the fill–rock interface can allow high local stress gradients to be generated
in the stope periphery. This mechanism is discussed in section 7.6, where it is shown
that a small surface load can have a significant effect on the extent of the yield zone
in a frictional medium. Finally, if the fill mass is properly confined, it may act as a
global support element in the mine structure. That is, mining-induced displacements
at the fill–rock interface induce deformations in the body of the fill mass, and these
are reflected as reductions in the state of stress throughout the mine near-field domain.
These three mechanisms represent fill performance as superficial, local, and global
support components in the mine structure. The mode of support performance in any
instance can be assumed to be related to both the mode of rock mass deformation and
backfill properties.
Two mining methods may exploit techniques for reinforcement of stope periph-
eral rock. The crowns of underhand cut-and-fill slopes are frequently subject to

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