Page 183 - Rock Mechanics For Underground Mining

6 Methods of stress analysis

6.1 Analytical methods for mine design

Basic issues to be considered in the development of a mine layout include the location
and design of the access and service openings, and the definition of stoping procedures
fororeextraction.Theseissuesarenotmutuallyindependent.However,geomechanics
questions concerning stoping activity may be more pervasive than those related to the
siting and design of permanent openings, since the former persist throughout the life
of the mine, and possibly after the completion of mining.
The scope of the problems which arise in designing and planning the extraction
of an orebody can be appreciated by considering the implementation of a method
such as room-and-pillar mining. It is necessary to establish parameters such as stope
dimensions, pillar dimensions, pillar layout, stope mining sequence, pillar extraction
sequence, type and timing of placement of backfill, and the overall direction of mining
advance. These geomechanics aspects of design and planning must also be integrated
with other organisational functions in the planning process. It is not certain that this
integration is always achieved, or that economic and geomechanics aspects of mine
planninganddesignarealwayscompatible.However,itisclearthatsoundminingrock
mechanics practice requires effective techniques for predicting rock mass response to
mining activity. A particular need is for methods which allow parameter studies to be
undertaken quickly and efficiently, so that a number of operationally feasible mining
options can be evaluated for their geomechanical soundness. Alternatively, parameter
studies may be used to identify and explore geomechanically appropriate mining
strategies and layouts, which can then be used to develop detailed ore production
schemes.
The earliest attempts to develop a predictive capacity for application in mine design
involved studies of physical models of mine structures. Their general objective was to
identify conditions which might cause extensive failure in the prototype. The difficulty
in this procedure is maintaining similitude in the material properties and the loads
applied to model and prototype. These problems can be overcome by loading a model
in a centrifuge. However, such facilities are expensive to construct and operate, and
their use is more suited to basic research than to routine design applications. An
additionalandmajordisadvantageofanyphysicalmodellingconcernstheexpenseand
time to design, construct and test models which represent the prototype in sufficient
detailtoresolvespecificminedesignquestions.Thegeneralconclusionisthatphysical
models are inherently limited in their potential application as a predictive tool in mine
design. Base friction modelling provides an exception to this statement. If it is possible
to deal with a two-dimensional model of a mine structure, and to examine discrete
sections of the complete mine layout, the procedure described by Bray and Goodman
(1981) provides a useful and inexpensive method for design evaluation. The method
is particularly appropriate where structural features exercise a dominant role in rock
mass response.
A conventional physical model of a structure yields little or no information on
stresses and displacements in the interior of the medium. The earliest method for

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