Rock Mechanics.pdf - Mining and Blasting

Figure 15.1 Vertical section illustrating
fracturing around deep tabular
stopes in strong brittle quartzites in
South African gold mines (after Ryder
and Jager, 2002).
LONGWALL MINING IN HARD ROCK
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.
A rockburst has been defined as the uncontrolled disruption of rock associated with a
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
range 10 5 to 10 9 J. Sudden, violent events which might cause considerable damage
to workings will radiate not less than about 10 4 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
modes of rock mass instability leading to rockbursts. Fault-slip events resemble natural
earthquakes, and usually occur on a mine panel or mine scale. Stress induced fracturing
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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