Page 452 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

encouraging results, rock-bursting continued as mining progressed to deeper levels.
The 3 m de-stressed zone did not provide a large enough buffer to prevent damage
resulting from large bursts occurring close to the face.
Toper et al. (1997) report a successful de-stress blasting or pre-conditioning ex-
ercise carried out in a series of shortwall panels at the Western Deep Levels Mine,
South Africa, at a depth of 2600 m. They found that although no new sets of fractures
were formed by the blasting, the high angle fracture sets typically found around deep
gold mine stopes as illustrated in Figure 15.1, were extended and sheared with gouge
forming. The technique is now used routinely to reduce face bursting in the deep
mines in this area. Based on the hypothesis that the rockbursts being considered here
result from unstable brittle fracture, Brummer and Andrieux (2002) suggest that the
realistic goals of de-stress blasting are:
to increase the degree of inhomogeniety in the rock mass through the formation

of micro-fracturing which will reduce the stiffness of (and hence the stress levels
in) the rock mass and dissipate energy through fracturing and frictional sliding on
internal surfaces; and
to promote increased shearing deformation on existing fracture surfaces which

will dissipate energy through gouge formation and heating.
De-stress or pre-conditioning blasting has long been used to alleviate the rockburst
hazard in several metalliferous mining districts using other than longwall mining
methods including the Sudbury district, Canada, the Coeur d’Alene district, Idaho,
USA and Sweden (e.g. Board and Fairhurst, 1983, Cook and Bruce, 1983, O’Donnell,
1992).
The energy release rate can be controlled most effectively by limiting the displace-
ment of the excavation peripheral rock in the mined-out area. This control may be
achieved in several ways.
(a) Provide active support for the hangingwall in the immediate vicinity of the
face using hydraulic props, with pack or stick support in the void behind
(Figure 12.11). As well as contributing to the limitation of the overall dis-
placements, these forms of support, if sufficiently closely spaced, will pre-
vent falls of rock blocks isolated by natural and mining-induced discontinuities.
Rapid yielding props can help minimise rockburst damage by absorbing released
energy.
(b) Practise partial extraction by leaving regularly spaced pillars along the entire
length of longwall stopes, generally oriented on strike. An elastic analysis by
Salamon (1974) shows that if a large area of a flat-lying, narrow, tabular orebody
is mined to a height, H, the difference in the quantity of energy released per unit
length of stope by extraction of a single panel of span, L, and that released by
partial extraction in a series of panels of span, l, spaced on centres of S (Figure
15.5a), is

W r = pL {H + [(1 − )Sp/ G] n (cos )} (15.1)

where p is the vertical in situ stress and = l/2S. It was shown previously that,
for a span greater than the critical, the quantity of energy released per unit length of
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