Rock Mechanics.pdf - Mining and Blasting

STOPE-AND-PILLAR DESIGN IN IRREGULAR OREBODIES
An obvious mining objective in developing an extraction sequence is to assure
recovery of the highest grade blocks in the orebody. While this is not to suggest that
a strategy of ‘high grading’ should be pursued, the requirement is to extract the high
grade blocks completely, in order to maximise mineral yield.
Maximising mineral yield can be coupled to the geomechanics requirement to establish
a mining sequence which provides predictable behaviour of the mine layout, particularly
under conditions of an increasing volume of rock mass degradation as mining
proceeds. From a geomechanics perspective, the overall sequence should involve
the early extraction of blocks with little support potential;
avoiding leaving scattered pillar remnants;
where essential, leaving bracket pillars to control displacements on major structural
features; and
orderly retreat of mining towards stable or solid ground.
These guidelines were proposed from experience in mines achieving a high extraction
ratio or under challenging conditions of pre-mining states of stress. Some instructive
background is provided by South African mining practice in deep, tabular orebodies
(COMRO, 1998), where the use of bracket pillars at fault intersections with the orebody,
the use of mine-scale stabilising pillars and the necessity of orderly advance towards
the mine abutments is well demonstrated. However, it should be noted that high
pre-mining stresses are not the driver for a properly engineered extraction sequence.
Even for mines located in a benign stress field, it is possible to lose a considerable
proportion of the ore reserve through a poorly conceived extraction sequence.
Starting from the structural geology and a capacity to model the evolving stress
distribution in an extraction layout, an extraction sequence is defined by identification
of an initial point of attack in the orebody, a logical evolution of the stope and pillar
layout and an unambiguous order for pillar extraction. This is frequently possible in a
single orebody mined in a planar layout. For the case of multiple orebodies or a large
orebody mined in a three-dimensional structure of stopes and pillars, it is sometimes
difficult to identify the optimum extraction sequence. It is usually necessary to propose
several extraction sequences which satisfy production scheduling and other mine
engineering requirements such as access and ventilation, and also involve the maintenance
of an orderly direction of retreat of stoping towards stable ground. The preferred
sequence may be established by analysis of rock response for each extraction scheme,
comparing the evolution of states of stress and rock mass conditions for each of the
alternatives, using the techniques described earlier due to Diederichs et al. (2002).
Extraction sequencing is a key aspect of control of mine instability and seismicity.
As discussed by Morrison (1996), the stope and pillar layout illustrated in Figure
13.29a, based on primary stoping of each second ore block recovery of the pillars in
secondary stopes, has many advantages in terms of controlling ground displacements
in the initial stages of mining. However, under conditions of high stress and active
seismicity, pillar recovery in secondary stopes can present challenging ground control
problems. Many mines in such settings use the pillarless, centre-out stoping sequence
shown in Figure 13.29b, usually employing cemented backfill to limit the unsupported
spans of stope walls. Maintenance of the ‘chevron’ front for advance of stoping
has a particular advantage in that it promotes the development of a geometrically
regular stress abutment adjacent to the stoping front. The stress abutment is displaced
incrementally and uniformly with the advance of the stoping front and stope wall
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