Page 454 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

achieve the fill stiffnesses required if backfilling was to be effective in limiting
convergence and reducing energy release rates sufficiently (Heunis, 1980). As
the result of a major research and development programme undertaken in the
1980s, many of these problems have been overcome. Backfilling is now a well
established method of providing both regional and local support in a number of
deep-level South African gold mines (Ryder and Jager, 2002).
Computer simulations carried out by Jager et al. (1987) and others suggest that
backfill alone can provide the regional support required at depths of less than 3 km.
Below 3 km the major benefits of backfilling are to reduce the stresses acting on sta-
bilising pillars or to enable the spans between pillars, and hence the extraction ratios,
to be increased. Improvements recorded in those mines using backfilling include im-
proved access and hangingwall conditions, reductions in stoping width and hence in
dilution, reductions in rockburst damage to stope face areas when the backfill is kept
close to the stope face, decreases in the numbers of accidents caused by rock falls
and improved productivity. Backfilling should be used in conjunction with a good
temporary face support system and should be incorporated into the standard mining
cycle (Jager et al., 1987).

15.2.3 Support and reinforcement systems
Two different types of support and reinforcement system are required in hard rock
longwall mining and its variants. First, support is required for the hangingwall of the
mined-outvoidnearandbehindanadvancingface.Thissupportisusuallydescribedas
stope support. Second, support and reinforcement systems are required for the access
and transportation excavations, generally referred to as tunnels in South African
mines. Both types of support and reinforcement system may be required to stabilise
the rock mass under static loading conditions, thus reducing the risk of rock falls, and
to alleviate the rockburst hazard under dynamic loading conditions. The two types of
support and reinforcement system will be discussed separately.

Stope support. As illustrated in Figure 15.1, the rock surrounding stopes in the
deep-level gold mines of South Africa is intensely fractured. The mining-induced
extension and shear fractures generally strike approximately parallel to the face, may
be steeply dipping, and may extend several metres into the hangingwall and footwall.
Bedding planes are parallel to the reef and are of variable frequency above and below
the stope. Jointing, faults and dykes may also be present. As the stope face advances
and the stope span increases, the hangingwall and footwall converge in response to
the elastic and inelastic deformations of the rock mass. The support system must be
able to accommodate stope closure, support the fractured hangingwall, and ensure
a safe working environment in the vicinity of the face. In the event of a rockburst,
the support system will be subjected to large, rapid deformations. It must be able
to absorb energy rapidly in decelerating and limiting the displacements of blocks of
fractured rock (Roberts and Brummer, 1988).
Stope support systems typically consist of combinations of hydraulic or timber
props, timber or concrete or composite packs, tendons and backfill. The 20 to 40 tonne
capacity rapid yielding hydraulic props typically used under high stress conditions
are installed in rows as close as 1 m from the face. In order to be most effective,

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