Page 33 - Rock Mechanics For Underground Mining

IMPLEMENTATION OF A ROCK MECHANICS PROGRAMME

1.5.2 Mine model formulation
Formulation of a mine model represents the simplification and rationalisation of the
data generated by the site characterisation. The aim is to account for the principal
geomechanical features which will be expressed in the deformational behaviour of
the prototype. For example, lithological units are ascribed average ‘representative’
strength and deformation properties, major structural features are assigned a regular
geometry and average shear strength properties, and a representative specification is
accepted for the pre-mining state of stress. The need for this phase arises from the
limited details that can be accommodated in most of the analytical or computational
methods used in design.
It is clear that significant discrepancies may be introduced at this stage, by failure
to recognise the engineering significance of particular features of the mine geome-
chanical setting.

1.5.3 Design analysis
Having defined the prevailing conditions in the rock mass in an analytically tractable
way, the mechanical performance of selected mining configurations and excavation
geometries can be predicted using appropriate mathematical or numerical techniques.
The analytical tools may be relatively primitive (e.g. the tributary area theory for
pillar design) or advanced, employing, for example, computational schemes which
may model quite complex constitutive behaviour for both the rock mass and various
fabric elements. In any event, the design analyses represent the core of rock mechanics
practice. Recent rapid development in the power of available computational schemes
has been responsible for significant advances, and improved confidence, in the quality
of rock structural design.

1.5.4 Rock performance monitoring
The objective of this phase of rock mechanics practice is to characterise the oper-
ational response of the rock mass to mining activity. The intention is to establish
a comprehension of the rˆoles of the various elements of the rock mass in the load-
deformational behaviour of the rock medium. The data required to generate this
understanding are obtained by displacement and stress measurements made at key
locations in the mine structure. These measurements include closures across pillars,
slip on faults, and levelling and horizontal displacement measurements in and around
the active mining zone. States of stress may be measured in pillars, abutments and
in the interior of any rock units showing signs of excessive stress. Visual inspections
must be undertaken regularly to locate any structurally controlled failures and areas
of anomalous response, and these should be mapped routinely. Finally, data should
be collected on the production performance of each stope, and the final configuration
of each stope should be surveyed and mapped. The aim in this case is to seek any
correlation between rock mass local performance and stope productivity.

1.5.5 Retrospective analysis
The process of quantitative analysis of data generated by monitoring activity is in-
tended to reassess and improve knowledge of the in situ mechanical properties of the
rock mass, as well as to review the adequacy of the postulated mine model. Review
of the conceptualisation of the host rock mass involves analysis of the role of major
structural features on the performance of the structures, and identification of the key
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