Page 427 - Rock Mechanics For Underground Mining

TECHNIQUES OF ARTIFICIAL SUPPORT

Figure 14.1 Modes of support of mining-induced fractures, or unfavourable structural geology with associated local
mine backfill: (a) kinematic constraint
instability. The technique of ground control under these conditions is discussed in de-
on surface blocks in de-stressed rock;
tail later. The walls of large open stopes are also candidates for local reinforcement,
(b) support forces mobilised locally in
when required by the state of stress and rock mass strength or the structural geology
fractured and jointed rock; (c) global
support due to compression of the fill of the stope boundary rock. There are now many examples of the use of long cable
mass by wall closure. reinforcement of the boundaries of large stopes.
Backfill can be used as a support medium in mining practice in two ways. In
conventional cut-and-fill stoping (by overhand or underhand methods), fill is intro-
duced periodically, during the progressive extension of the stope. The operational
effectiveness of the fill is related to its capacity to produce a stable working surface
soon after its emplacement in the stope. Where backfill is used in an open stoping
operation, fill placement in a particular stope is delayed until production from it
is complete. Successful performance of the fill mass requires that during pillar re-
covery, free-standing walls of fill, capable of withstanding static and transient loads
associated with adjacent mining activity, can be sustained by the medium. In both
cases, the function and duty of the fill mass can be prescribed quantitatively. It is
necessary to design the backfill to meet prescribed operational functions and safety
requirements.
It has already been observed that mine backfill is frequently a granular cohesionless
medium. The height of a fill mass in a stope can exceed several hundred metres. It is
also well known that the shear strength of a granular medium is determined directly
by the pore-water pressure according to the effective stress law. Therefore, great care
must be exercised in fill design and in mining practice to ensure that significant pore
pressure cannot develop in a body of backfill. The particular problem is the potential
for catastrophic flow of fill under high hydrostatic head, should high pore pressure lead
to complete loss of shear resistance and subsequent liquefaction of the medium. The
practical requirement is to ensure that, as far as possible, any in situ static or dynamic
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