Page 466 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

Figure 15.16 Ground-support inter- thismethodwerefoundtoagreewellwiththosefoundtogivesatisfactoryperformance
action analysis for longwall face sup-
in the UK coalfields.
port (after Everett and Medhurst,
Attempts to apply the “detached block” method to determine the required face
2003).
support loads in other mining environments have not always been successful, often
under-estimating the support requirements (Kelly et al., 2002). The method proposed
by Ashton et al. (1970) does not allow for the influence of strong, thick sandstone roof
strata, for thick seams or for the transmission to the supports of re-distributed stresses.
This latter effect becomes more important as mining depth increases. The inadequacy
of support loadings in early Australian longwalls, for example, led to the extensive
use of a large-scale physical model. This model demonstrated the complexity of the
loadings and the benefits of using higher setting and yield loads. More recently, nu-
merical models of increasing complexity have replaced physical and simple analytical
models in studies of this problem (Kelly et al., 2002).
Figure 15.16 illustrates the application to face support of the rock-support inter-
action principles introduced in Chapter 11. Ground characteristic lines or ground
response curves (GRC) for typical Australian longwall conditions are shown for a
depth of 300 m and allowing for a 10% additional loading contingency for a given
convergence. Support characteristics are shown for installed capacities of 100, 110
−2
and 120 t m . These characteristics are shown with a 90% ratio of setting load to
yield load to reflect optimal performance. In one case, a 80% setting to yield load
ratio is also shown. As shown by Figure 15.16, under-rated supports (in this case the
100tm −2 support) may allow excessive convergence before being set and may not be
able to accommodate the full load generated once deterioration of the roof develops.
Everett and Medhurst (2003) report the successful application of this ground response
curve method in a number of Australian longwalls.

15.3.5 Roadway formation and support
In the first edition of this text (Brady and Brown, 1985), attention was focused on the
formation and support of the gate roads used to service faces in the advancing longwall
method of coal mining then in use in the UK. In this case, the roadways were formed
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