Page 453 - Rock Mechanics For Underground Mining

LONGWALL MINING IN HARD ROCK

Figure 15.5 (a) Mining plan, and stope by total extraction is given by equation 10.88; i.e.
(b) normalised reduction in energy
change, for partial extraction of a flat- W r = LHp
lying, narrow, tabular orebody (after
Salamon, 1974).
Dividing equation 15.1 by equation 10.88 and substituting the expression for the
critical span given by equation 10.86; i.e

L 0 = GH/[(1 − )p]

leads to the result
W r /W r = 1 + (S/ L 0 ) n (cos ) (15.2)

Figure 15.5b shows a plot of W r /W r against extraction ratio, l/S, for varying values
of S/L 0 . Note that, even for quite high extraction ratios, major reductions in energy
release rates are achieved by using partial extraction.
The use of stabilising pillars as they are known has been effective in reducing seis-
micity in a number of deep South African gold mines (e.g. Hagan, 1988, Vieira et al.,
2001). The major disadvantage of the method is that in deep mines with potentially
high stress conditions something like 15% of the ore reserves may be sterilised in
stabilising pillars. For high extraction ratios, the pillars may be subject to particularly
high stress concentrations at their edges and may even suffer bearing capacity failure.
(c) Backfilling the mined void with tailings, sand or waste rock has great poten-
tial for limiting convergence and for providing both regional and local support.
Backfill has two significant virtues. First, its presence reduces the permissible
convergence, limiting both the ultimate convergence volume and the displace-
ment induced at each increment of mining. Second, the rock mass must do work
on the fill to deform it; in the process large amounts of energy can be absorbed
(Salamon, 1983). Although the benefits likely to be associated with the use of
backfill were recognised in the 1960s (Cook, N.G.W. et al., 1966), the method
was not adopted in practice because of apparently high costs and perceived op-
erational problems. In particular, it was believed that it would be difficult to
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