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Chapter 2

GeotechnicalCharacterizationfor Mass Mining

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Massmin 2004 39

1 INTRODUCTION

In the future environment of surface and undergroundmines, we face both a trend towards the development ofsuper pits and super caves, and a tendency for theunderground development of ageing pits. These superprojects, or long-life ore factories, sprout from the calls forincreased production and more cost effective solutions formass mining projects, and perhaps from the belief that biggeris better. The move to fewer, larger, longer life projectsexposes operations to an elevated level of risk from singular

events. One of the most crippling and potentially fatal eventsin these mass mining operations is geotechnical failures.

Safe and efficient operation of large scale open pit andunderground excavations thus requires an even strongerfocus on geotechnical issues. As geotechnicalinvestigations rely on information supplied by the entirerange of mining related disciplines, effective integration ofthese services is becoming even more crucial.

Having the geotechnically related risks and benefitsdefined at an early stage may well differentiate the projectfrom others, all queuing up to obtain financing, whether frominternal or external sources, in todays ever-competitivemarketplace.

In the authors experience, the introduction of an

integrated geological structural hydrogeological -geotechnical approach, as early as possible in thedevelopment of a project, saves time and expensethroughout all phases of the project life.

2 FUNCTIONAL SILO MENTALITY

A geotechnical investigation requires the integration ofdata from a number of sources. The geotechnical engineerwill thus be required to interact with a number of individuals.Depending on the status of the project these individuals willrange from: Exploration/resource geologists...to define the geological

model and resource footprint,

Structural geologiststo assist with the interpretation ofthe rock mass fabric and applying geological and spatialcontext around the geotechnical data,

Mine geologiststo describe the performance of theencountered geological conditions,

Hydrogeologiststo provide input into the impact ofground water on the excavation stability,

Engineering geologiststo characterise the rock massfrom strength and performance requirements,

Mining engineersto develop the mining methodology(including blasting) from the geological and geotechnicalinterpretations,

Mine planning engineersto plan the mine geometry,layout and scheduling in the most optimised fashion.

The authors have noted that the lack of cross-discipline

integration is commonplace in the industry. The industryas a whole suffers from a 'functional silo mentality,whereby tasks are undertaken in an assembly linefashion, often in isolation or with an incompleteunderstanding of how the results impact on the entiremining project or operation. Cross-disciplinecommunication is often the critical issue.

This is evident for example, when the explorationgeologists often do not communicate well with mining andresource geologists, who do not communicate well withgeotechnical engineers, and so on. Often, neither task teamunderstands the others requirements, nor how they canhelp address the issues those teams face for the ultimatebenefit of the study.

The way consultants are used in the Australianindustry often exacerbates this problem, in thatcompanies select people for specific tasks on aperceived value for money basis. Therefore, oneconsulting firm may be used for geology, another forresource estimation, another for geotechnicalengineering, and another for mining engineering. Thispresents additional challenges in ensuring the effectiveflow of information and knowledge across the task teamsand, therefore, the entire operation.

The functional silo mentality is a substantial challengefacing the industry and, therefore, focus is required oncross-training consultants and clients so that they areaware of issues facing each of the mining disciplines and

can communicate across these disciplines. This helps toeffectively break down the functional silos to allow the moreeffective flow of information and understanding throughoutthe mining process, and thus should result in more informeddecisions.

Santiago Chile, 22-25 August 2004

AbstractThe need for an integrated approach in mining is a key element during all phases of the evaluation of a mineable deposit.

The integration of the geological, structural, hydrogeological and geotechnical disciplines, as feeds into the mine

planning and scheduling process, are essential elements in the conversion of a deposit into a mine. Having the

geotechnically related risks and benefits defined at an early stage may well differentiate the project from others queuing

up to obtain financing, whether from internal or external sources. From the initial greenfields site, from scoping to drill

out and the establishment of an advanced project, pre-feasibility, feasibility and ramp-up into operations it is beneficial

to establish the links between the geotechnical characteristics and mining risks.

This paper will examine the timing, functional silo mentality, the influencers and modifiers on mine design, the benefits

resulting from integration and some operational examples to illustrate the benefits of this approach.

Mining geotechnical investigations:The need for an integrated approach

Allan Haines, T. Campbell McCuaig and Esther Theron, SRK Consulting, Australia

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3 TIMING

Geotechnical investigations should be incorporated intoan exploration or mining project at a very early stage. Thespecific investigation can take many forms depending onthe type, scale and location of the project. It should haveequal status alongside the geological, resource,metallurgical, mine planning and financial modelling studies.It is equally important to know that:

the resource existsin a certain form and location, that it can be mined safely and economically by open

pit or underground methods, that the ore can be processed, that it has a market that can be reached, and that the project will produce a viable return on investment.

The main benefit of an integrated approach is that thegeotechnically related risks can be understood andquantified as early as possible, especially where a minedesign is sensitive to these parameters. The maingeotechnical risks or issues that need to be addressed froman early stage can be related to the following: rock mass characteristicswhich can be managed with

empirical methodologies, but require geological context toproperly establish their spatial variability

structural fabrican evaluation of the controls on the 3Ddistribution of rock damage (discontinuities), and theinteraction between primary, secondary and tertiarystructures and the excavation,

ground waterwater pressure can significantly reducethe stability of the excavation,

in-situ stressesand their redistribution during miningneeds to quantified,

seismicitycan have a detrimental impact on the stabilityof the rock mass,

mine excavationand its interaction with the geotechnicalenvironment can be modelled to determine the

development of adverse tensile and shear stresses thatmay lead to failure of the excavation profile.

All of the project scientists and engineers play a roleduring the development of a deposit from initial explorationor discovery through project scoping, pre-feasibility,feasibility, detailed engineering design, start-up, productionand closure. The project risk profile can be changed byinformed decisions taken by engineers, financiers,stakeholders and government that are involved in theemergence of a new mining project.

From the initial greenfields site, from scoping to drill outand the establishment of an advanced project, pre-feasibility, feasibility and ramp-up into operations, it is

beneficial to establish the links between the geotechnicalcharacteristics and mining risks.

There is a real benefit in investing time and capital duringearly exploration to provide initial estimates of geotechnicalrisk. In our experience there is the potential to reduceexpenditure during feasibility if the integrated geotechnical-geological model is understood right up front.

The geotechnical engineer must identify what will be thekey influenceson mine design and be aware of that whichmay subsequently modifythe design.

These aspects are normally assessed in relation to therisk profile for the project and whether a competitiveadvantage can be obtained by optimising the designprocess. It is the application of the influencing and modifying

components that can result in significant cost saving duringthe project life and which may ultimately extend theoperational lifeof a mine. This is illustrated in Table 1 for asteep slope strategy as applied to open pit design.

4 BENEFITS FROM INTEGRATION

The main benefits resulting from an integrated approachcan be categorised into savings in time and associated costwith a better scope for communication among theprofessionals involved.

Incorporating geotechnical investigations as early aspossible in a project study can be beneficial in guiding futurework, especially into feasibility. With this approach, it ispossible to reduce the dependence on long and costlyfeasibility studies in which the geotechnical risk profile hadnot been adequately defined earlier on.

The estimate of capital and operating cost expenditurecan be better quantified early on, by following a morerigorous and integrated approach. By getting the overallslope angle for an open pit closer to its expected value at anearly stage, there can be substantial cost savings inestimates of waste stripping. For example, the differencebetween a 45and a 50overall pit slope, for a pit perimeterof approximately 3000m over a slope height of 150m, canresult in an additional US$ 15M of waste stripping. In thisexample there is also a corresponding shift in the pit crest ofapproximately 25m.

These values are illustrated in Figure 1 and Figure 2.

5 PLANNING ON A RISK BASIS

The follow extract from a 1997 paper by Oskar Steffen onthe planning of open pit mines on a risk basis states thecase with regard to the value that should be attached togeotechnical information during the development of amining project.

"As in the case of mineral resource estimation, thedetermination of slope angles is dependant on theunderstanding of geological and geotechnical informationand the confidence of design is equally based on the degreeof certainty which applies to the data available. Unlike thecase of mineral resource estimation, where the exploration

is specifically targeted to provide ore reserve information,the requirement for slope design only becomes necessaryonce a prospective mineral resource has been discovered.Exploration core has usually limited value for slope designpurposes as the target areas for slope design are notnecessarily the same as for the orebody. Drillingrequirements for geotechnical purposes also differconsiderably from that for mineral exploration. Hence alimited campaign for geotechnical purposes is usuallyundertaken in addition to whatever value can be obtainedfrom the original exploration campaign.

It is therefore not uncommon to have a slope designwhich has a much lower degree of confidence than thatpertaining to the mineral resource definition. By definition,

the mineable reserves within the resource are determinedby applying a mine design which could economically exploitthe resource."

It is again emphasized that the evaluation and definition ofthe geotechnical risks as early as possible in thedevelopment of a project must be understood by all. Thegeotechnical risks described earlier in this paper must beinvestigated and quantified using the most appropriatetechniques. In the case of caving it is almost impossible tofully define the nature of the cave without exposing the orematerial in an exploration decline.

The geotechnical evaluation should advance at the samerate as that for the resource model.

6 OPERATIONAL EXAMPLES

Case 1

A SLOS operation in Australia has experienced difficultieswith oversize in the stope drawpoints. The causes of the

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Figure 1: Cost saving in waste stripping (per m of pit perimeter) with slope steepening

Figure 2: Shift in pit crest with slope steepening