Page 372 - Rock Mechanics For Underground Mining

MINING METHODS AND METHOD SELECTION

12.3.5 Orebody value and spatial distribution of value
The monetary value of an orebody, and the variation of mineral grade through the
volume of the orebody, determine both mining strategy and operating practice. The
critical parameters are average grade, given various cut-off grades, and grade dis-
tribution. The former parameter defines the size and monetary value of the deposit
as the market price for the product mineral changes. It also indicates the degree of
flexibility required in the selected method of mining the orebody, since it is necessary
that marginal ore be capable of exclusion from the production operation, in response
to changing market conditions. The significance of dilutions of the ore stream, aris-
ing, for example, from local failure of stope wall rock and its incorporation in the
extracted ore, is related to the value per unit weight of ore. In particular, some mining
methods are prone to dilution, and marginal ore may become uneconomic if mined
by these methods.
Grade distribution in an orebody may be uniform, uniformly varying (where a spa-
tial trend in grade is observed), or irregular (characterised by high local concentrations
of minerals, in lenses, veins or nuggets). The concern here is with the applicability
of mass mining methods, such as caving or sublevel stoping, or the need for com-
plete and highly selective recovery of high-grade domains within a mineralised zone.
Where grade varies in some regular way in an orebody, the obvious requirement is
to devise a mining strategy which assures recovery of higher-grade domains, and yet
allows flexible exploitation of the lower-grade domains.

12.3.6 Engineering environment
A mining operation must be designed to be compatible with the external domain and
to maintain acceptable conditions in the internal mining domain. Mine interaction
with the external environment involves effects on local groundwater flow patterns,
changes in the chemical composition of groundwater, and possible changes in surface
topography through subsidence. Different mining methods interact differently with
the external environment, due to the disparate displacement fields induced in the far-
fieldrock.Ingeneral,cavingmethodsofmininghaveamorepronouncedimpactonthe
mine external environment, through subsidence effects, than supported methods. In
the latter case, it is frequently possible to cause no visible disturbance or rupture of the
ground surface, and to mitigate the surface waste disposal problem by emplacement
of mined waste in stope voids. In fact, stope backfill generated from mill tailings is
an essential component in many mining operations.
Specific mining methods and operating strategies are required to accommodate the
factors which influence the mine internal environment. Mine gases such as methane,
hydrogen sulphide, sulphur dioxide, carbon dioxide or radon may occur naturally
in a rock mass, or be generated from the rock mass during mining activity. Pre-
mining rock temperatures are related to both rock thermal properties such as thermal
conductivity, and regional geophysical conditions. The thermal condition of mine
air is subject to local climatic influences. Supported and caving methods require
different layouts for ventilation circuits, and present diverse opportunities for gas
generation and liberation in a ventilation air stream. In all cases, the requirement is to
understand the interaction between the rock domain, active at any stage of mining, and
the thermodynamic process of ventilation which operates in that domain and sustains
the operation. As a general rule, supported and caving methods may impose grossly
dissimilar loads on a ventilation air stream, due to the different opportunities offered
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