Page 370 - Rock Mechanics For Underground Mining

MINING METHODS AND METHOD SELECTION

orebody may exploit completely natural support in the initial stoping phase, using
orebody remnants as pillar elements. In the early stages of pillar recovery, various
types of artificial support may be emplaced in the mined voids, with the objective of
controlling local and regional rock mass displacements. In the final stages of pillar
recovery, pillar wrecking and ore extraction may be accompanied by caving of the
adjacent country rock. It is clear that the transition from one geomechanical basis
to another can have important consequences for the integrity and performance of
permanent openings and other components of a mine structure. This indicates that
the key elements of a complete mining strategy for an orebody should be established
before any significant and irrevocable commitments are made in the pre-production
development of an orebody.

12.3 Orebody properties influencing mining method

A mining method consists of a sequence of production unit operations, which are
executed repetitively in and around the production blocks into which an orebody is
divided. The operations of ore mobilisation, extraction and transport are common to
all mining methods, while other operations may be specific to a particular method.
Differences between mining methods involve different techniques of performing the
unit operations. The different operating techniques employed in the various methods
are the result of the different geometric, geomechanical and geologic properties of
the orebody and the host rock medium. Other more general engineering and social
questions may also be involved. In the following discussion, only the former issues,
i.e. readily definable physicomechanical orebody properties, are considered.

12.3.1 Geometric configuration of orebody
This property defines the relative dimensions and shape of an orebody. It is related
to the deposit’s geological origin. Orebodies described as seam, placer or stratiform
(stratabound) deposits are of sedimentary origin and always extensive in two dimen-
sions. Veins, lenses and lodes are also generally extensive in two dimensions, and
usually formed by hydrothermal emplacement or metamorphic processes. In massive
deposits, the shape of the orebody is more regular, with no geologically imposed
major and minor dimensions. Porphyry copper orebodies typify this category.
Both the orebody configuration and its related geological origin influence rock
mass response to mining, most obviously by direct geometric effects. Other effects,
such as depositionally associated rock structure, local alteration of country rock, and
the nature of orebody–country rock contacts, may impose particular modes of rock
mass behaviour.

12.3.2 Disposition and orientation
These issues are concerned with the purely geometric properties of an orebody, such as
its depth below ground surface, its dip and its conformation. Conformation describes
orebody shape and continuity, determined by the deposit’s post-emplacement history,
such as episodes of faulting and folding. For example, methods suitable for mining
in a heavily faulted environment may require a capacity for flexibility and selectivity
in stoping, to accommodate sharp changes in the spatial distribution of ore.
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