Page 449 - Rock Mechanics For Underground Mining

LONGWALL MINING IN HARD ROCK

Figure 15.1 Vertical section illus-
trating fracturing around deep tabu-
lar stopes in strong brittle quartzites in
South African gold mines (after Ryder
and Jager, 2002).

1978). Second, rockburst phenomena can be associated with slip on discontinuities
or with the stress-induced fracturing that is commonly observed to occur around
longwall faces in high-stress settings.
Arockbursthasbeendefinedastheuncontrolleddisruptionofrockassociatedwitha
violent release of energy additional to that derived from falling rock fragments (Cook,
N.G.W. et al., 1966). Rockbursts are a sub-set of a broader range of seismic events,
and are associated with conditions of unstable equilibrium as defined in sections
10.6–10.9. Mining gives rise to seismic events ranging in energies in the approximate
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range 10 to 10 J. Sudden, violent events which might cause considerable damage
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to workings will radiate not less than about 10 J (Salamon, 1983). Rockbursts may
have damaging effects on the rock surrounding other mine openings as well as on
the rock in the vicinity of a longwall face. The reinforcement system used to limit
the effects of rockbursts in a haulage in a South African gold mine was described in
section 11.6.5.
The theory of elasticity has been used with outstanding success to develop an
understanding of the causes of rockbursts in longwall mining in hard rock, and to
develop mining strategies which limit the incidence and effects of rockbursts. As
shown in Figures 10.22 and 10.23, the longwall stope is represented as a narrow slot
in a stressed elastic medium. The stresses and displacements induced by the creation
of a new excavation, or by the extension of a longwall stope, may be calculated
most conveniently using one of the forms of the boundary element method outlined
in section 6.5. Closed-form solutions may be obtained for some simple problem
configurations (Salamon, 1974).
As observed in Chapter 10, it is now generally recognised that there are two basic
modesofrockmassinstabilityleadingtorockbursts.Fault-slipeventsresemblenatural
earthquakes,andusuallyoccuronaminepanelorminescale.Stressinducedfracturing
or crushing of pillars, or fracturing at or near the mining face, can lead to local
instabilities, sometimes called strain bursts, which occur on a stope or excavation
scale. There is clearly a potential for much greater release of energy in a fault-slip
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