McCarthy, P L, Lee, M F and Bridges MC, 1988. WASM Annual Conference.

Application of Rock Mechanics to

Underground Mine Design
By P L McCarthy 1, M F Lee 2 and M C Bridges 3

Introduction Due to the large number of parameters involved, and their

inherent variability, all methods for the economic design,
The transformation of underground rock mechanics from a support, and extraction of mine openings, are probabilistic to a
"black art" to engineering science is proceeding very slowly. greater or lesser degree. The risks are less in a civil engineering
Few practitioners would claim that rigorous analysis has environment, where safety factors and design life may greatly
superseded experience and judgement in any "real life" exceed those used in mine production. In civil projects, the
operational situation. The Rock Mechanics engineer now has a client ultimately pays whatever is necessary to ensure stability
range of tools and procedures, which provide essential support and safety. In mining projects, the ability to ensure stability is
for his considered recommendations, but those tools and limited by the realizable value of the ore and the limited life of
procedures may be dangerous in the hands of a novice who openings, thus putting great pressure on operators to apply
accepts the results uncritically. judgement to decisions about the probability of failure. In a
mine, the risk of massive failure in a stope crown may be
The Rock Mechanics engineer (who, in the broad sense of the acceptable in comparison with the cost of intensive cable dowel
title, may be an engineering geologist, a production engineer or support, provided that the risk to operating personnel is
mine manager) must have a set of objectives when designing an minimal.
underground mining operation.
Role of Rock Mechanics in Mining
In a Rock Mechanics sense, "success" can mean:
Rock mechanics can be regarded as the management of the
• an optimum economic result through reduced costs, response (mechanics) of rock to mining.
increased mine life or extraction ratio, or increased
productivity; Rock mechanics therefore has an important role in mine
• increased practical knowledge of the mine planning, operations and safety. Miners cannot control the
environment; location and style of an orebody, the rocks surrounding it, or
the natural stresses. But, through control over stope and pillar
• increased confidence in planning and management; shapes, sizes, and sequences and the application of effectual
• a safer mining operation. support, they are able to manage the response of the rock to
mining. In some instances, effective management of the
It is important to note that physical failure of the rock material response will be critical to the viability of the venture.
(such as yielding of a pillar) may be part of a successful mining
plan. Specifically, rock mechanics has a key role in:

"Failure", by contrast, may include: • location and design of service facilities;

• size, shape, and sequencing of stopes and pillars;
• a poor economic result due to dilution, high • support of stopes and service facilities, which
direct costs or production delays; includes backfilling of stopes;
• lack of observation or recording which would have • blasting, rock cutting and comminution generally;
improved knowledge for future design;
• controlling groundwater.
• loss of confidence in people or methods;
• injury or fatality; Rock mechanics is a mature science. Soil mechanics, as a
• loss of areas of the mine to future production. recognisable discipline, began during the 1920s and 30s. Rock
mechanics is a post-war phenomenon, beginning with
applications to civil engineering during the 1950s and
developing rapidly during the 1960s and 70s, including
widespread application to mining engineering. During the
1970s soil mechanics principles were applied to developments
of mine backfill. Australia has, and continues to have a leading
1. Managing Director, AMC Consultants Pty Ltd, 19/114 William Street, role in rock mechanics and its application to mining.
Melbourne Vic 3000. E-mail: pmccarthy@amcconsultants.com.au
2. Principal Geotechnical Engineer, AMC Consultants Pty Ltd,
Consequently, there are now a range of established procedures
19/114 William Street, Melbourne Vic 3000.
E-mail: mlee@amcconsultants.com.au and proven techniques that can, with expertise and experience,
3. Principal Geomechanics Geologist, AMC Consultants Pty Ltd,
be called upon to develop efficient mining practices and to
19/114 William Street, Melbourne Vic 3000 correct operating problems as they arise.
Email: mbridges@amcconsultants.com.au
Application of Rock Mechanics to Underground Mine Design

However, great challenges remain ahead to apply rock More simply: we are given the rock and its stresses, we provide
mechanics to improve the efficiency and productivity of the new forces and support to the rock, so that we can influence
mining, through adoption of new technology and evolution of its response.
mining methods. Here, rock mechanics has a prime role, as
most developments in some way involve controlling, The relationship amongst these factors is illustrated by the
modifying, or exploiting the behaviour of rock. diagram:

In this process, rock mechanics' contribution to mining is Input Output

greatest when it is applied systematically. This can be (parameters) (response)
represented by the conceptual relationship: properties of rocks
stresses
Basically, the response of rock (to mining, or any other stress field
Model displacement
disturbance) is reduced to a function of four variables, or geometry of excavations
parameters: yield

• rock, particularly its geological structures; properties of support

• stresses in the rock, natural or imposed;

• forces that are applied to the rock during mining, This depiction emphasises an important point: efficient
which are effectively the geometry of the openings application of rock mechanics requires that each of the inputs
that are created; be of similar accuracy, and that the sophistication of the model
• support that is applied to the rock. be matched to the given accuracy of the inputs and the required
accuracy of the output.
The function that relates these four parameters to the response
is the model of behaviour of the rock, which can be empirical, The iterative application of rock mechanics to mining, in
physical or mathematical. concert with mine planning and operations, is illustrated in the
diagram below.

Refined
modeling
techniques

rock analysis
properties and
and designs
stresses

new
information

operations
and
monitoring

new
field
experience

Quantitative assessments included testing rock specimens for was required. At the existing mine everyone knew from
density, dynamic modulus, uniaxial compressive strength and "experience" what size stopes could be mined and what ground
mode of failure. support was needed. The rock mass classification scheme was
therefore used to relate the new conditions to those known and
The rock mass strength was estimated with shear box tests, understood.
triaxial tests, deformability determinations (E and v), point load
testing and RQD logging, all contributing to the emerging Classifications were based on stress, strength, joint frequency,
picture of the rock conditions. bedding plane break frequency, the presence of water, and the
use and orientation of the proposed underground opening.
To translate the results of Rock Mechanics investigations into
useable information for mine planning, some common yardstick

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Application of Rock Mechanics to Underground Mine Design

The likely modes of ground behaviour were considered and the to be incompatible with the operational and practical needs of
most important fracture sets identified. Wedge failures occurred the mine.
in strike development, and unravelling of ground on fractures
was predicted in stope wall and back exposures. Rock mechanics remains a combination of art and science. It
should be seen as an aid to mine design, and not as a rigorous
A comparison of ground conditions between mines or areas is constraint on the options available. Rock mechanics is helpful
only meaningful if like values are being compared, whether in the early planning of a mine, particularly avoiding problems
they are absolute or only relative values. As an example, due to lack of local experience. It is essential in the later stages
bedding plane breaks were found to be an important measure of if a very high extraction ratio (approaching ;100% of in situ
rock quality, but many of the logged breaks were induced by reserves above cutoff) is planned.
drilling. To make a comparison between two areas it was
necessary to also record the core-to-bedding angle, core It is good practice to have a coordinated, ongoing programme
diameter, rock type, rock strength, type of drilling machine, involving, as a minimum:
core handling method, and even to note the care taken by the
driller. • collecting data on engineering geology, rock
properties and stress field;
Measurements of the virgin stress field showed the expected • documenting and explaining local ground behaviour;
magnitude-depth relationship, with the major principal stress
horizontal and perpendicular to bedding. However, the stress • refining information for particular areas or
field was skewed to proposed transverse pillars, suggesting that applications.
a rotational force might tend to loosen the pillars.
If these steps are not taken well before a problem occurs, then it
The test of such a rock mechanics programme is the is usually too late to seek a solution from rock mechanics
contribution it makes to mine planning and subsequent analysis because the fundamental data does not exist.
operations. Observation and recording applies to simple things such as the
occurrence of fracturing in raise-bored holes, which is an
None of the planning conclusions can be attributed solely to the excellent guide to stress orientation.
rock mechanics work. All area result of a team effort, with
experience playing the major role and with rock mechanics A successful programme depends on the enthusiasm and ability
methodology providing a set of reference points to assist in the of individuals, particularly the mine geologists who gather and
decision-making process. Perhaps the same decisions would present much of the data. Frequent positive feedback from mine
have been reached by experienced mining engineers without planners and operators is necessary to keep the programme
any specific rock mechanics advice. However, rock mechanics alive and healthy.
input serves to reduce the risk inherent in subjective decision-
making and to reduce the chance of a bad decision being made Trends in Underground Rock Mechanics
on the basis of a wrong assumption or poor analogy with
previous experience. Ten years ago, most major underground mines felt obliged to
support at least one in-house "Rock Mechanics Engineer". This
It is noteworthy that no mention was made of numerical person was either a mining engineer, geologist or surveyor who
modelling in this example. Although numerical modelling had decided to specialise in that area. The business of the rock
techniques were applied to mine design, they played a minor mechanics engineer was to perform complex rituals with
role. computers and field instrumentation, and then present the
results for consideration by the mine operators. The role of the
A contrasting example is provided by a second mine where Rock Mechanics Engineer, as one of the mining team, was
mining-induced stresses at depth frequently exceed the often misunderstood by management and by the engineer
rockmass strength. Rock mechanics investigations concentrated himself, who frequently felt ignored and also found that he had
on defining the stress environment, identifying major structures stepped outside of the management stream and had little
which may act as stress concentrators or stress relief paths, and opportunity for advancement. Most mines have now learned
constructing a numerical model which, after extensive that they can do without an in-house Rock Mechanics Engineer,
verification, is now used as a reliable predictive tool. relying instead on an improved knowledge of Rock Mechanics
by ordinary mining engineers (due to greater exposure during
The main application for this rock mechanics programme was undergraduate training) and on a greater use of specialist
in optimising stope dimensions and a mining sequence. Even consultants.
when good rock mechanics data is available in an established
mine, considerations such as production requirements (tonnes The reduction in staff Rock Mechanics positions in recent years
and grade) and development rates to open new stopes are likely may be due to:
to control decision making. The rock mechanics advice is
evaluated with the practical mining requirements and inevitable • better career opportunities for technical staff in
compromises are reached. production situations;
Where possible, the recommended sequences are followed.
• unsatisfactory experiences with transforming rock
Whenever rock mechanics investigation has identified an
mechanics theory into practical recommendations,
important outcome, such as a potential failure or rockburst, this
and
may dominate mine scheduling until stress relief has occurred.
• improved knowledge of rock behaviour and ground
Good rock mechanics programmes relate harmoniously with support around large openings as a result of operating
the mine planning process. Poor programmes concentrate on experience.
one area of study, such as numerical modelling, to the detriment
of practical observation and data-gathering. Conclusions The state of rock mechanics knowledge is such that an
derived from such poor, or "academic" programmes are likely experienced mining engineer is usually better placed to make

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Application of Rock Mechanics to Underground Mine Design

informed decisions on mine design than a theoretician, and this openings. Models were simplistic because complexities such as
situation is unlikely to change. major structures, ubiquitious joints and time-dependent
behaviour could not be modelled, so that attempts to apply the
The realistic Rock Mechanics specialist should have the results to mine design were often questionable.
following personal objectives:
It has taken twenty years to develop the Finite Element method
• visit a many mines as possible, as frequently as as a reliable tool and, more important, to understand its
possible, to maintain his experience base; limitations. Even now, many models are misunderstood or
• go underground whenever possible and be observant; misapplied by the end users. A long gestation period is
therefore likely for applications of fractal geometry. The
• understand the capabilities and limitations of research is now possible with the availability of high-speed
modelling methods and design techniques; computers with colour graphic displays essential to the
• be able to use the methods, or interact with specialists interpretation and comprehension of such complex phenomena.
on mine applications, and interpret the results; The CSIRO Division of Geomechanics has begun work in this
area and is worthy of support by industry. It might be
• be able to present information to those making
appropriate to direct five to ten percent of research funding
operational decisions in a way which does not pre-
toward such fundamental work, which can have no immediate
empt the decisions by ignoring the realities of costs,
application in operating mines.
scheduling and production;
• be prepared to draw conclusions from inadequate and
contradictory data, using experience and judgement.
Conclusion
This review has attempted to present a realistic view of the
One clear trend is to simplify the presentation of information at
application of rock mechanics to underground mine design.
each stage of the process. Field instrumentation should provide
Because many technical papers treat the rock mechanics
a clear indication of the parameter being measured, or connect
component of design in isolation, it is easy to forget that in the
into a data recording system capable of presenting results in
end the results must be applied in dynamic operating situations.
summary form. An alarm may be triggered at a preset
Rock mechanics is an aid, which helps engineers understand
threshold.
how the rock behaves. This understanding was once a product
of experience, observation and intuition in a mining engineer or
Computer analyses or modelling should result in readily
mine "captain" who had developed a sympathy for the rock as a
comprehended colour graphics or imagery. Written reports
material. Today, we can be less conservative in design because
should provide discussions and conclusions which can be
of the support offered by analysis. In the end, this translates to
understood by non-specialists. Tabulations of measurements or
reduced operating costs and an enhanced ability to be
computer output should be relegated to appendices.
competitive in international markets.

The Future
There is considerable scope for refinement and application of
rock mechanics methods which are accepted and understood. In
stress measurement for example, instruments such as the
CSIRO Hollow Inclusion cell provide results which require
experienced interpretation. Such experience may not be
available at a mine site, in which case stress measurement
should be used as part of a "package" of technical assistance.

There is potential for developing closer links between

geological data bases, mine planning and numerical modelling
software. For example, the link now being developed between
the Datamine geological database and Microstation computer-
aided design systems could be extended to producing modelling
details for the BEFE three dimensional stress analysis system,
which has been developed by CSIRO Division of
Geomechanics.

The second area of interest for the future involves new areas of
scientific and mathematical research. In the past ten years, a
wholly new field of mathematical research has developed
which is sometimes described as "the dynamics of chaos", of
which fractal geometry is an important part. Fractal geometry
recognizes the inherent "fuzziness" of the universe, and the
self-similarity of form and structure that may pervade a
medium (such as a rock mass) of various scales.

Some applications such as analysis of turbulent flow have

already emerged, and others may include a radical new view of
geological and geomechanical phenomena. The situation is
similar to that which applied in the early 1960's when the
availability of mainframe computers first made possible
applications of the Finite Element method to the design of rock

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