Page 386 - Rock Mechanics For Underground Mining

MINING METHODS AND METHOD SELECTION

high recovery of ore immediately above the undercut horizon is virtually certain. In
general, a fairly uniform distribution of values throughout the orebody is required to
assure realisation of the maximum ore potential of the deposit.
It has been observed that initial and induced geomechanical conditions in an ore-
body determine the success of block caving. Productive caving in an orebody is
prevented if the advancing cave boundary can achieve spontaneously a mechanically
stable configuration, such as an arched crown, or if caved fragment sizes are too large
to be drawn through the raises and drawpoints of the extraction system. Details of
the mechanics of caving will be discussed in Chapter 15. At this stage it is noted that
factors to be considered in evaluating the caving potential of an orebody include
the pre-mining state of stress, the frequency of joints and other fractures in the rock
medium, the mechanical properties of these features, and the mechanical properties
of the rock material. It also appears that the orientations of the natural fractures are
important. Kendorski (1978) suggested that initiation and propagation of caving re-
◦
quire a well-developed, low-dip joint set (dip less than 30 ). The most favourable
rock structural condition for caving is represented by a rock mass containing at least
two prominent subvertical joint sets, plus the subhorizontal set.

12.5 Mining method selection

The mining principles and methods which have been described have evolved to meet
the geomechanical and operational problems posed in the recovery of ore deposits
characterised by a broad set of geological and geometric parameters. A common
industrial requirement is to establish the mining method most appropriate for an
orebody, or segment of an orebody, and to adapt it to the specific conditions applying in
the prospective mining domain. In addition to orebody characteristics which influence
methodselection,thevariousminingmethodshave,themselves,particularoperational
characteristics which directly affect their scope for application. These operational
characteristics include mining scale, production rate, selectivity, personal ingress
requirements and extraction flexibility. The final choice of mining method will reflect
both the engineering properties of the orebody and its setting, and the engineering
attributes of the various methods. For example, a non-selective method such as block
caving would not be applied in a deposit where selective recovery of mineralised
lenses is required, even if the deposit were otherwise suitable for caving.
It sometimes appears that method selection for a particular mining prospect can
present acute technical difficulty. With the exception discussed below, this is not usu-
ally the case. In fact, the choice of potential methods of working a deposit is quickly
circumscribed, as candidate methods are disqualified on the basis of specific proper-
ties of the orebody and its surroundings. It follows that the development of various
selection schemes, based on determination of a ‘score’ which purports to reflect the
gross mining characteristics of an orebody, is unnecessary. Such an approach implies
that, for an orebody, any mining method is a candidate method. This is clearly at vari-
ance with the philosophy and historical development of mining engineering. Mining
methods were developed to accommodate and exploit particular mining conditions.
A more appropriate procedure to be developed for method selection might involve
the formal application of the eliminative logic invoked in computer-based expert
systems.
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