Page 442 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

for sudden, large displacements in the orebody near-field and in controlling the mine
internal environment. Of course, these functions are still served by any emplaced fill.
Mining under sensitive surface features is frequently permitted by close integration
of mining and backfilling. The control of a mine ventilation circuit is also simplified
considerably when mined voids are backfilled.
The modern structural function of backfill is to facilitate mining of pillar ore without
dilutionbywastematerialorlosingcontroloftheorebodynear-fieldrock.Thisimplies
that emplaced fill is capable of forming artificial pillars that will prevent the generation
of unstable spans of the orebody peripheral rock. Since the Young’s modulus of a
backfill is always low compared with that of a rock mass suitable for open stoping,
the operating mode of the fill is unlikely to involve the global mobilisation of support
forces in the fill mass. Instead, satisfactory performance of fill probably involves a
capacity to impose local restraint on the surface displacements of rock units in the
orebody periphery. This is inferred since the fill is likely to become effective when
the wall rock is in a state of incipient instability, and relatively small resisting forces
mobilisesignificantfrictionalresistancewithinthemassofwallrock.Thismodeloffill
performance suggests that it is necessary to understand local deformation mechanics,
in both fill and rock, at the low state of stress existing near the fill–rock interface, if
fill design is to be based on logical engineering principles.
In seeking to recover pillar ore under the ground control imposed by backfill, the
fill mass must satisfy a range of performance criteria. The primary one is the local
resilience of the medium, implied in the previous discussion of fill support mechanics.
Although relatively large strains may be imposed near the fill–rock interface, the local
resistance and integrity of the fill must be maintained.
In many mining applications an essential property of the fill mass is a capacity
to sustain the development of a large, unsupported fill surface. This is illustrated in
Figure 14.9, in which a pillar is to be recovered between fill masses emplaced in the
adjacent stope voids. Successful mining of the pillar ore might be achieved by the slot
and mass blast method illustrated in Figure 14.9b. Extraction of the ore from the stope
without dilution requires maintenance of the integrity of the complete fill mass. This
in turn demands that the fill has sufficient strength to sustain gravity loads, and also
any stresses imposed by displacement of the adjacent stope walls. Possible modes of
fill failure leading to dilution of pillar ore include surface spalls and the development
of deep-seated slip surfaces. The cohesive fills described in section 14.2 have been
formulated specifically to control these modes of fill response.
When the slot and mass blast method, or an alternative vertical retreat method,
is used for pillar mining, the result is a large bin filled with broken ore, which is
to be drawn empty. Depending on the layout of the drawpoints at the stope base,
plug flow of ore may occur past the fill surface. The requirement is that the fill
demonstrate sufficient resistance to attrition to prevent both excessive dilution of ore
and destruction of the fill pillar by progressive erosion.
The temporal rate of increase of strength may be an important factor in cemented
backfill applications, particularly when developed reserves of ore are limited. Strength
gain in the curing of cemented fill is determined by the kinetics of hydration and
subsequent crystal growth in the reactions of the chemical species in cement and
its silicate substitutes. It can be controlled in part by the chemical composition of
the cementing mixture (i.e. Portland cement and finely ground, reactive silicates),
but it is also related to the thermal and hydraulic setting in which mining occurs.
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