Page 366 - Rock Mechanics For Underground Mining

MINING METHODS AND METHOD SELECTION

the exploitation of the deposit. The stoping operation, i.e. ore mobilisation from its
in situ setting and its subsequent extraction from the mine void, is the core of the mine
production process. Clearly, control of rock performance within the orebody, and in
the rock mass adjacent to the orebody, is critical in assuring the efficient geomechan-
ical and economic performance of the individual stopes, and of the mine as a whole.
The size of stopes means that their zone of influence is large relative to virtually
all other mine excavations. Stope design therefore exercises a dominant rˆole in the
location, design and operational performance of other excavations which sustain
mining activity. The principles of stope layout and design are integrated with the set
of engineering concepts and physical operations which together compose the mining
method for an orebody. This chapter considers in an introductory way the relation
between the geomechanical properties of an orebody (and its host rock mass) and the
appropriate method of mining it.
The second type of mine excavation identified in Figure 12.1 is represented by
stope access and service openings. These consist of in-stope development such as
drill headings and slot raises, horizontal and vertical openings for personnel access
to stope work places, and ore production and transport openings such as drawpoints,
tramming drives and ore passes. These excavations are developed within the orebody
rock, or within the orebody peripheral rock. Their operational life approximates that
of adjacent stoping activity, and in some cases, such as drill headings, the excavations
are consumed in the stoping process.
The location of stope development in the zone of geomechanical influence of
the stope, and, typically, the immediate proximity of a stope and its related service
openings, may impose severe and adverse local conditions in the rock medium. The
procedures discussed in preceding chapters may be applied to the design of these
openings, provided account is taken of the local stress field generated by stoping
activity, or of rock mass disturbance caused by stoping-induced relaxation in the
medium. In all cases, design of a stoping layout requires detailed attention to the issues
of position, shape and, possibly, support and reinforcement of stope development
openings, to assure their function is maintained while adjacent extraction proceeds.
These excavation design issues are obviously related closely to the geomechanical
principles on which the mining method is based.
Permanent access and service openings represent the third class of mining exca-
vations illustrated in Figure 12.1. This class consists of openings which must meet
rigorous performance specifications over a time span approaching or exceeding the
duration of mining activity for the complete orebody. For example, service and ore
hoisting shafts must be capable of supporting high speed operation of cages and
skips continuously. Ventilation shafts and airways must conduct air to and from stope
blocks and service areas. Haulage drives must permit the safe, high speed operation of
loaders, trucks, ore trains and personnel transport vehicles. In these cases, the excava-
tions are designed and equipped to tolerances comparable with those in other areas of
engineering practice. The practical mining requirement is to ensure that the designed
performance of the permanent openings can be maintained throughout the mine life.
In rock mechanics terms, this requirement is expressed as a necessity to locate the
relevant excavations (and associated structures) in areas where rock mass displace-
ments, strains and tilts are always tolerable. The magnitudes of these mining-induced
perturbations at any point in the rock medium surrounding and overlying an orebody
are determined, in part, by the nature and magnitude of the displacements induced by
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