Page 414 - Rock Mechanics For Underground Mining

PILLAR SUPPORTED MINING METHODS

Figure 13.25 Calibrated stability
curves for standard pillar width and
varying pillar condition rating, Doe
Run mines, Missouri, USA (after
Roberts et al., 1998).

to predict the progress of pillar deterioration and ultimate collapse as adjacent pillars
are mined (Roberts et al., 1998, Lane et al., 2001).

13.7 Stope-and-pillar design in irregular orebodies

13.7.1 Design principles and methods
While single coal seams and lenticular or stratiform metalliferous orebodies are
amenable to the pillar analysis and design procedures discussed above, many metal-
liferous orebodies are not. They may be irregular in shape and several may occur in
close proximity. Further, partial or total recovery of pillar ore is routine metal mining
practice. In coal mining, it is frequently necessary to co-ordinate the extraction of sev-
eral contiguous seams. In all these cases, a more robust and versatile design procedure
is required to plan the stope-and-pillar layout and the extraction method itself.
In contemporary design practice, routine use is made of the computational tools
for stress analysis described in Chapter 6. Applied in conjunction with a suitable
conceptual model of the mine and appropriate rock mass properties and failure criteria,
they allow evaluation of alternative mining strategies in terms of orebody and host
rock mass response to different layouts of stopes and pillars. Usually, evaluation of
various feasible extraction sequences for stopes and pillars is an important part of
these studies.
An effective design practice which is essential for irregular orebodies is based on
an observational principle. It involves direct determination, interpolation, inversion
of data or estimation of geomechanical conditions in the orebody and near-field rock
mass. All the site characterization information specified for design of an isolated
excavation, described in previous chapters, is required for the design of an evolving
mine structure. This includes the pre-mining triaxial state of stress, the in situ strength
and deformation properties of the lithological units in the mine domain, and the
location, attitude and mechanical properties of major penetrative structural features
transgressing the zone of influence of mining. This information is revised and updated
as rock mass properties are evaluated during progress of stoping and as site data is
collected during mine development.
Design of a stope-and-pillar layout starts by locating pillars in parts of the orebody
interpreted to be free of adverse structural features. For example, a pillar intended
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