Rock Mechanics.pdf - Mining and Blasting

Figure 13.26 Stope-and-pillar layout
to maintain favourable states
of stress in rock remnants (after
Goddard, 1981).
STOPE-AND-PILLAR DESIGN IN IRREGULAR OREBODIES
to remain sound under mining induced stresses needs to be located in rock free of
features which may allow slip to occur in the interior of the pillar. Local slip on
persistent discontinuities can cause pervasive problems, such as falls of rock from
the pillar surface, damaging seismicity and general degradation of the pillar rock
mass.
In close-spaced, parallel orebodies, an additional factor in siting pillars is the correct
alignment of pillar axes. Pillars whose axes are offset may result in stress distributions
favouring slip on planes of weakness oriented parallel to the orebodies. The general
principle is illustrated in Figure 13.26.
A suitable stope-and-pillar layout and extraction sequence for an orebody is established
through an iterative process. Having established suitable prospective locations
for pillars, a preliminary design for the set of stopes and pillars can be proposed. The
layout must fit with existing development. It must also be based on stope sizes and
shapes which satisfy both stability criteria as discussed in Chapter 9 and mine production
requirements. An analysis of stress distribution in this prospective structure
can then be conducted using a computational scheme such as the boundary element
method and assuming linear elastic rock mass behaviour of the rock mass, or perhaps
accounting for a sparse population of major discontinuities. In these preliminary
design analyses, the assumption of more complex constitutive behaviour of the rock
mass is rarely justified, because the initial problem is to recognise obvious defects
and limitations of the design, or perhaps to compare possible alternative layouts of
stopes and pillars.
Having determined the stress distribution in the set of pillars and stopes, it is possible
to map the zones of tensile stress or the regions in which compressive stresses satisfy
the criteria for particular modes of rock mass failure. The preliminary design can be
modified until unacceptable states of stress are eliminated from the layout, or some
compromise is reached between geomechanical and other engineering requirements
for operation of the mine. In some cases, it may be necessary to accept unfavourable
states of stress in parts of the layout and to specify operational measures to manage
them, in order to provide a set of stopes and pillars which can sustain established
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