Page 441 - Rock Mechanics For Underground Mining

BACKFILL APPLICATIONS IN OPEN AND BENCH STOPING

The preceding discussion of the geomechanics of cut-and-fill stoping, and of as-
sociated ground control practice, took no account of the structural geology of the
orebody. When mining in jointed rock, the design of the active mining zone should
follow the rules established in Chapter 9, relating to a single excavation in a jointed
medium. The particular requirement is that the stope boundary be mined to a shape
conformable with the dominant structural features in the medium. Maintaining the
natural shape for a stope, with the excavation boundary defined by joint surfaces,
restricts the potential for generating unstable wedges in the crown and sidewalls of
the active domain.
It was noted in Chapter 12 that shrink stoping can be regarded as a variant of cut-
and-fill stoping. At any stage in the upward advance of mining, the broken remnant
ore in a stope performs the same role as backfill in cut-and-fill stoping. The perfor-
mances of crown and sidewalls of cut-and-fill and shrink slopes during mining are
also directly comparable. The additional geomechanical aspect of shrink stoping is
expressed during the final draw from the stope. Since the stope sidewalls are under
low confining stress, or de-stressed, removal of the superficial support applied by the
resident, fragmented ore allows local, rigid body displacements to develop in the stope
wall rock. If the zone of de-stressing is extensive, or the rock mass highly fractured,
draw from the stope can be accompanied by dilution of the ore by caved hangingwall
rock.

14.5 Backfill applications in open and bench stoping

Open stoping is a naturally supported mining method, in which control of rock mass
displacement is achieved by the generation of ore remnants to form support elements
in the orebody. As was observed in Chapter 13, any mining setting in which field
stresses are high relative to rock mass strength requires the commitment of a high
proportion of the proven mineral reserve to pillar support. In metalliferous mining,
where reserves are always limited, the life of a mine may be linked directly to efficient
and economical recovery of a high proportion of pillar ore. Because the location of
pillars in an orebody is in some way related to the maximum stable stope spans
that can be sustained by the orebody boundary rock, it follows that pillar extraction
may introduce the possibility of orebody wall rock or crown collapse. Under these
conditions, the need is apparent for artificial support elements distributed in the mine
structure during pillar mining, and operating on a scale comparable with that of the
natural pillar system. The current position in technically advanced countries is that
very little metalliferous mining, undertaken using pillar support, is not accompanied
by subsequent stope filling and pillar mining. In general, the stope filling operation
in this method of mining is not as closely integrated in mine production activity as
it is in cut-and-fill stoping. However, in both cut-and-fill stoping and open stoping
with delayed filling and pillar recovery, the support potential of the fill is exploited to
achieve a high proportional extraction of the ore reserve.
Although the modern use of backfill is as a structural component in pillar recovery,
its application in underground mining evolved from a need for achieving regional
ground control above a mining area. According to Dickhout (1973), backfill was first
used to control surface displacements above a mining domain in 1864. Much of its
subsequent use until recently appears to have been in this rˆole, in restricting the scope
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