Management of geotechnical risks in mining projects

B K Hebblewhite

School of Mining Engineering

The University of New South Wales

Sydney NSW Australia

ABSTRACT

This paper discusses the concepts of core risk associated with mining projects and operations and illustrates the types of core geotechnical risks and consequences that may relate to a range of different mining methods, with recommendations on risk management procedures to ensure that the core risks are under constant and ongoing scrutiny and priority management focus. These core geotechnical risks can and should be identified at the initial mining method selection stage, during a feasibility study. They should form part of the auditing of the project and the decision-making regarding the project viability. They should then be built into the ongoing management structure of the project, right through into ongoing operations, with regular audit and review.

INTRODUCTION

It has long been recognised that underground mining operations involve a particular level and type of risk that is not encountered in most other industries – certainly not to such an extent, or on such an ongoing basis throughout the project life. This difference can be appreciated if one considers an analogy of the mining operation as a set of processes taking place within an overall system. By comparison with say a manufacturing system of processes, as discussed by Ward (2000), where all the processes take place using fairly consistent feedstock within a fixed, controlled factory environment, the mining system operates in an opposite manner. In mining, the ore reserve or minable deposit (the feedstock) is fixed in the ground and extremely variable, but the mining system (or overall operation of a set of processes) – akin to the “factory” in the manufacturing system – is mobile, moving through the input feedstock, or orebody, encountering a constantly changing environment.

The combination of the ever-changing “feedstock” of ground conditions, together with the constantly moving, or dynamic mining system, generates a variable profile of risks that must be managed throughout the mining project. This risk profile must be recognised from “Day 1” in the mine feasibility study as the orebody is characterized and mining method options are evaluated.

Amongst these types of risk profile, geotechnical risk must rank close to the top of the tree, possibly along with ventilation, as the most critical area or sector of an underground mining operation that can lead to significant problems – from both a safety and economic perspective. In years gone by, there was a view held that such risks were simply an inherent part of underground mining that could neither be avoided, controlled or managed in any way. Fortunately, this view is now part of mining history, and most enlightened mining engineers and mine operators now recognise that modern risk management techniques can be applied in the mining geotechnical field, to great effect, to manage such risks throughout the mining operation.

It is now time to take a further step forward and recognise that the risks within a mining operation can be categorized as occupying three levels, or superimposed layers. Firstly, all mining methods introduce their own special areas of core risk, that may vary from method to method, but are inherent and embedded in the generic characteristics of each method. These are then overlain by specific site or local mining condition related risks, and then a third overlay includes the day to day operational risks (see Table 1).

Table 1. Overlay of Mining Project Risk Levels

Level 1 Risks	Day to day operational risks.
Level 2 Risks	Specific site or mining condition-related risks.
Level 3 Risks	Core risks associated with mining method or system.

The industry is managing the top layer (level 1 risks) of operational risks well through mechanisms such as Safe Operating Procedures (SOPs), training etc. Likewise, the intermediate layer is being addressed through management initiatives such as Ground Control Management Plans. However, often we do not adequately recognise and address the underlying core risks early enough, or we recognise them at some point in the design/evaluation stage of a project, but then lose sight of them, or lose focus on them over time.

A further difficulty is the problem of the changing mine environment – again particularly with regard to geotechnical risks. Often a mine will modify or even change the mining method altogether during the course of the operation as a result of changing conditions, but may fail to revisit the core risks associated with the different mining systems now being adopted, or the management systems that were put in place to address the original core risks but may no longer be adequate or appropriate for the new method.

Definition of Core Risk

For the purposes of this paper, and to encourage wider recognition of the need to identify and manage core risks, the following definition is proposed:

The term “core risk” is used to describe any risk associated with a major hazard or potential hazard, that is an inherent feature of a generic mining method. Almost by definition, core risks cannot be totally eliminated, and must therefore be controlled and managed during the life of the mining method or system of work.

Under this definition, core risks may be of a technical nature, may be project or financial risks, and may or may not impact on mine safety. The core risks do not need to be present in every mining operation of a particular method, but they are very directly related to the method, rather than just some local, specific condition or circumstance, and they must be evaluated in every instance where that method is contemplated.

The subject of this paper – core geotechnical risks – is clearly of a technical nature in the main, with very significant safety (and potentially serious economic) implications.

Paper Structure

Against this background, the balance of this paper will review a selection of major underground mining methods (both coal and metalliferous) to illustrate the core geotechnical risks associated with each of them and the potential consequences if they are not appropriately managed. Comments on the management of core risks are also provided.

Prior to presenting the review of major methods, a brief discussion of the findings from the 1999 Northparkes Mine Disaster (Bailey, 2003) is provided. The author of this paper was involved in the Coronial Inquest (as a technical adviser to Counsel Assisting the Coroner) held into that accident in which four lives were lost. It was during that role that recognition of the need for greater focus on core risks became very clear.

NORTHPARKES FINDINGS & RECOMMENDATIONS

In reviewing the specific incident relating to block cave mining, the Coroner (Bailey, 2003) made the following recommendation (No. 3):

“3. Any mine operator intending to employ the process of block cave mining to identify and analyse the elements of all the risks associated with its block cave operations and develop and maintain hazard management procedures for the management of:

(a) The void above the muckpile;

(b) The height of the muckpile;

(c) The airblast hazard and shall include all the appropriate controls for the airblast at all openings or potential openings into the caving zone.

Management of the major hazards in a block cave mine must include recognition of the fact that these three issues are interrelated and cannot be managed as discrete elements.”

On a more general note, the Coroner recommended:

“6. Mine design and operation – The mining industry should incorporate the following specific recommendations into all relevant codes of practice or industry guidelines for safe mine design and operation:

(a) Identification of the core risks that are inherent in the proposed mining operations/methods under consideration, at the time of the initial feasibility study stages of the project;

(b) The above core risk identification should generate both a means of comparison of alternative mining method options, at the feasibility stage, as well as a subsequent package of priority management strategies for elimination or control of these core risks to an acceptable level, throughout the future life of the project;

(c) At the feasibility and design stages of any mining project, the project should be subjected to a rigorous process of independent audit, by a team that is at least external to the dedicated project team. Such an audit process should address both the economic and technical aspects of the project, and must include an assessment of the core risks identified and the proposed means of addressing such risks;

(d) The above audit process should be repeated at regular “milestone” stages of a mining project (not necessarily by an external team), from conceptual planning through to and during operations. Such ongoing audits should include review and scrutiny of initial planning and design issues in the light of changing conditions or circumstances, to ensure that the critical safety-related design issues and management strategies continue to be both appropriate and adequate;

(e) The responsibility for initiating and conducting such audits, and for the key actions arising from them, must be clearly defined and assigned within the management organizational structure.”

The Coroner further recommended, in relation to education and training (Rec. 13), that all relevant mining and geotechnical courses be reviewed, and that:

“Specifically such review shall seek to ensure that, the disciplines of core risk identification and fundamental principles of rock engineering behaviour, relative to different mining methods and ground conditions, is incorporated as necessary.”

MAJOR MINING METHODS

This section of the paper considers four major mining methods and identifies a series of core geotechnical risks considered to be relevant to these methods (that is not to say there are not other core geotechnical risks). The choice of mining methods and the lists of core risks are intended to be illustrative rather than complete or definitive lists. The methods to be considered are:

· Block caving (metalliferous)

· Longwall mining (coal)

· Open stoping (metalliferous)

· Room and pillar mining (coal and metalliferous).

Geotechnical Principles

As a means of assessing the inherent geotechnical engineering principles (and potential rock behaviour) involved with each method, the approach described by Brady & Brown (1993) is considered an appropriate starting point, and a useful checklist for mining method selection on geotechnical criteria. This relates to the regional issue of rock mass response to mining, and is illustrated in Figure 1.

As indicated in Figure 1, the mining methods listed (and any other variations or additions) can be grouped according to whether they are naturally supported, artificially supported or unsupported. This is very much a function of the competency of both the orebody and the surrounding near-field rock mass. As indicated in the figure, there is a sliding scale of consequent magnitudes of displacements in the country rock, which is generally opposed to a similarly sliding scale of level of strain energy storage in the near-field rock mass. Room and pillar mining and open stoping are at one extreme (low country rock displacement, high stored strain energy), whereas block caving is at the other extreme of high displacement and low stored energy. Longwall mining is in between, but closer to the high displacement end of the scale, with some degree of artificial support in the form of longwall face supports.

Figure 1. Rock mass response to mining (after Brady & Brown (1993))

Block caving

On the basis of the above criteria and definitions, Table 2 is a summary of some core geotechnical risks associated with block caving. Note that some of these risks represent events that almost certainly will happen as a result of using this method (e.g. surface subsidence) whereas others are potential risks generic to the method, that may occur if the method is not managed appropriately. It should also be noted that a number of these risks are inter-related.

Table 2. Block caving - Core geotechnical risks.

Hazard	Consequence
Uncontrolled, dynamic, large scale caving event	Airblast; damage to draw points/other infrastructure; loss of control of cave propagation; premature cave propagation to surface.
Caveback hang-up	Development of excessive void leading to airblast potential; production disruption threatening economic viability.
Undesirable cave propagation outside orebody	Dilution and consequent threat to mine economics; loss of control of cave propagation; damage to adjacent/overlying infrastructure.
Inrush	Mud/water flooding of mine; workforce safety; surface damage.
High level, concentrated surface subsidence on breakthrough	Surface damage; safety hazard on surface; disruption of aquifers etc.

Longwall mining

Table 3 presents some major core geotechnical risks associated with longwall mining.

Table 3. Longwall mining – Core geotechnical risks.

Hazard	Consequence
Surface subsidence	Disturbance/damage to surface features (natural and man-made), and to sub-surface, such as aquifers.
Face instability/periodic weighting	Loss of face/roof control; production disruption; equipment damage; operator safety threatened.
Caving hang-up	Windblasts (range of consequent safety implications); excessive pillar and face loading; unpredictable subsidence.
Structural geology disruption to panel blocks	Production disruption and potential sterilization of reserves leading to major economic impact; adverse face ground conditions
Abutment stresses on development	Adverse conditions/potential failure in gateroads and chain pillars.

Open stoping

Table 4 presents some major core geotechnical risks associated with open stoping.

Table 4. Open stoping – Core geotechnical risks

Hazard	Consequence
Hanging wall/back failure	Dilution; safety of operators; damage to equipment; regional instability; production disruption.
Crown/rib pillar failure	As above.
Rockburst	Safety of operators; local instability; equipment damage; excess development costs; delayed development/production.
Fill/barricade failure (where stopes backfilled to allow extraction of adjacent ore).	Fill liquefaction/slurry inrush leading to flooding; operator safety threatened; loss of access; sterilization/dilution of ore in operating stopes.

Room and pillar mining

Table 5 presents some major core geotechnical risks associated with room and pillar mining.

Table 5. Room and pillar mining – Core geotechnical risks

Hazard	Consequence
Room/intersection falls	Operator safety; equipment damage; loss of access; production disruption.
Local pillar collapse	Production disruption; loss of access.
Regional pillar failure / “pillar run”	Major loss of reserves; production disruption; potential safety issues; possible subsidence consequences.
Regional closure/creep	As above.
Rockburst	Safety of operators; local instability; equipment damage; excess development costs; delayed development/production.

MANAGEMENT CONSIDERATIONS

The above examples illustrate a selection of core geotechnical risks for four major mining methods. As can be seen, these can vary between the methods, and some relate to hazards that will inevitably occur, but have to be managed (e.g. subsidence); as opposed to others which are potential hazards with serious adverse consequences which are directly related to the characteristics of the method, but are avoidable or can be mitigated against, if managed correctly (e.g. airblasts or rockbursts).

Feasibility study review/method selection

A critical feature of virtually all these risks is that they are ever-present, throughout the life of the mining operation, and so must be subjected to ongoing management control. In most of the cases, the risks are ones which should be identifiable and to some extent at least, quantifiable, at the time of a project feasibility. As such, it is suggested that when the feasibility study is being conducted, the various core risks must form part of the evaluation process for selecting the mining method and establishing the viability of the project. Each core geotechnical risk will have certain economic and safety considerations in terms of both consequence and control, which must be taken into account in the project evaluation. Management strategies for the core risks must be developed from this stage of the project.

Risk Assessment

A project risk assessment should be conducted as part of the feasibility study and should incorporate addressing the core risks within it. However, given the nature of core risks – particularly the fact that they are not likely to be eliminated (and so must be “lived with” throughout the project), it is important that some key additional steps be taken as a result of the risk assessment.

1. Document the thinking (and potential geotechnical mechanisms involved) behind both the hazard itself and the potential consequences, as well as the reasoning behind the risk rating or scoring system used. This documentation must be sufficient to mean something to someone in years to come – probably someone who is not even employed by the company now, but will fill a crucial technical or management role in the future, when this risk is still in need of management. This person needs to know exactly what was meant by a particular risk scenario, and why it was rated in a certain way.

2. Once the controls for the risk have been identified and well documented, ensure that the responsibility for those controls is assigned to a person and a position within the management structure of the operation – again to ensure both accountability and responsibility for any action plans, and that as people change, that responsibility gets passed on to the new incumbent of the position.

Regular Audits

It should be standard practice to regularly audit the performance of a mining operation with respect to the core risks that are present in the mining system. Such an audit should also revisit any major risk assessments that had been carried out previously on the project, including the original project feasibility study. In particular, as mining conditions or economic circumstances change, there may be a different mining environment within which the same mining method is applied. Alternately, there may be a change in the mining method which will introduce new core risks that must be evaluated, and new management controls introduced.

Northparkes Experience

In the case of the Northparkes disaster, there was evidence presented that indicated that mine management had correctly identified the risk of airblast occurring through the 1 Level drive, years before the event occurred in 1999. (One should possibly also go back to the original project feasibility study when block caving was selected as the method to be adopted, to check whether core risks were adequately identified at that time, not to mention addressed in subsequent design and management practice).

As a result of the risk assessment that identified the airblast hazard on 1 Level, certain steps were subsequently taken to address this risk (including construction of a bulkhead). Thereafter, it appears that the focus of attention switched to other matters (including risk of airblast in other parts of the mine).

Problems were encountered with cave hang-up. Cave inducement activities were undertaken to cause the cave to propagate (with only mixed success, until the time of the accident). Ground conditions were changing, as was the proximity of the caveback to the surface. The air gap between the caveback and the muckpile was increasing.

However, the core risk of large scale, uncontrolled caving resulting in an airblast hazard on 1 Level was not revisited, and so the controls that had been put in place a year or more earlier were also not revisited, in terms of assumptions made (for bulkhead placement and design), or adequacy of the controls in the light of other changing conditions and changing ground conditions and behavioural mechanisms.

CONCLUSIONS

This paper is not written as a critique of the Northparkes disaster in 1999. Rather it is intended to use the lessons learned from analyzing the circumstances leading up to that disaster to provide guidance for future mining operations. These lessons focus on a number of issues, including the following:

· Every mining method involves an element of risk.

· There are certain generic, core risks that are inherent to a particular mining method or system of work in a mining operation.

· In underground mining, geotechnical risks are inevitably going to be amongst the top priority core risks that must be addressed.

· The mining industry should ensure that core geotechnical risks are identified – right from the feasibility stage of a project - to assist, not only in the most appropriate choice of mining method (for technical as well as economic reasons), but also in developing appropriate management systems and control measures.

· The management system must place the core risks of the system on a recurrent, or continuing agenda so that they are regularly reviewed to ensure controls continue to be both adequate and appropriate.

· Given that core risks are likely to require ongoing management, possibly for the life of the project, it is essential that any risk assessments focusing on them must be extremely well documented so as to be meaningful and able to stand scrutiny years after they are conducted.

· The outcomes of such risk assessments must very clearly assign responsibility for any controls, not just to an individual person, but to a position within the organization so that as personnel change, the responsibility for management of core risks remains assigned within the management structure.

· Finally, it is good management practice to conduct regular project audits during which the major risks and key project decisions are challenged.

REFERENCES

Bailey J (2003) Findings and recommendations: Inquest into the deaths of R Bodkin; M House; S Osman; and C Lloyd-Jones; on the 24P^th^P November, 1999 at the E26 Lift 1 Mine, Northparkes Mines, Parkes, New South Wales. NSW Coroners Court, 18 March, 2003.

Brady B H G & Rock mechanics for underground mining (2P^nd^P edition),

Brown E T (1993) Chapman & Hall, London, 1993.

Ward B (2000) Personal communication.