Page 451 - Rock Mechanics For Underground Mining

LONGWALL MINING IN HARD ROCK

on the assumption of elastic continuous rock mass deformation during advance of
mining.
It was shown in section 10.5 that for the crushing mode of rock bursting, there
is a well-developed correlation between rockburst damage and the spatial rate of
energy release or energy released per unit reef area or volume mined. The essential
principle used in this case in the design of mining layouts and in excavation scheduling
in rockburst-prone areas follows from this observation. This is that the spatial rate
of energy release associated with an increment of mining should be kept within
predetermined limits. It was also shown in section 10.8 that if a narrow orebody
could be mined as a partially closed, advancing single slot, mining would occur under
steady-state conditions with a uniform rate of energy release. Generally, it is not
practical to mine in this way, and more complex layouts with multiple longwall faces
must be used. Under these circumstances, energy release rates can reach critical levels,
particularly where the face approaches highly stressed dykes (Figure 3.4), faults or
other excavations, or where an attempt is made to mine highly stressed remnants
(Dempster et al., 1983). The general strategy of layout planning now used to control
rockburst incidence is to limit and to even out energy release rates in space and
time.
An early attempt was made to limit the incidence and severity of rockbursts around
longwall faces by application of the concept of de-stressing (Figure 15.4). Roux et al.
(1957) report the results obtained when de-stressing was used routinely in 32 stopes
at East Rand Proprietary Mines over a 19 month period. Holes 51 mm in diameter
and 3 m long were drilled into the face on 1.5 m centres. The bottom half of each
hole was charged and the remainder stemmed with sand tamping. As illustrated in
Figure 15.4, fracturing the rock ahead of the face in this way, successfully reduced the
stress level in the rock to be mined next. It was found that the incidence and severity of
the rockbursts associated with advance of the face were reduced. Despite these early

Figure 15.4 Stress concentration
ahead of a de-stressed face (after Roux
et al., 1957).

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