Drawpoint Management

Drawpoint Management

Introduction

Despite significant advances in remotely operated machinery, working around underground mine drawpoints remains hazardous.

Some of the activities that may necessitate workers to be in the vicinity of drawpoints include:

• Drilling
• Charging and blasting
• Loader operations (mucking or bogging)
• Filling process
• Technical services work
• Ventilation maintenance

A recent review of mine practices found that there is significant variation in how effectively each mining operation assesses and deals with the risks associated with drawpoints and stope brows.

Risk management strategies should aim for mine and operational designs that remove the need for personnel to work near drawpoints, thereby reducing the exposure to workers.

Drawpoint management

Several underground mines across Queensland were inspected by Resources Safety & Health Queensland (RSHQ) to determine the effectiveness of control measures relating to drawpoints. The findings were:

• Significant variation in the definition of an open brow
• Variation between mines, and in some cases within the same mine, on when the transition from conventional loader operation changes to remote loader operation should be made
• Variation in bunding standars, with all mines using bund height from the floor without consideration of the drive height
• At some mines, hazards associated with open drawpoints are being normalised
• Lack of formal risk assessment for working near open drawpoints with reliance on experience
• Lack of formal training for secondary blasting
• Lack of understanding of what constitutes the need for internal incident reporting, in particular, incidents associated with open drawpoints.

Defining an 'Open Brow'

The project found that there were differing opinions among workers and supervisors as to when a brow is open (cracked) or choked. It is vital to ensure there is a common understanding on when a brow is open or choked; the definition of an open brow should be when it is just 'cracked'.

Consequently, there were differences between mine sites on when to convert from conventional loader operations to remote loader operations. In some mines the trigger was a cracked brow to other mines where the trigger was when the bucket of pins of loader are level with the brow. The application of the latter trigger will result in a very wide-open drawpoint when the drive is large.

The definition of an open brow should be when it is just cracked, this should also be the trigger to convert from conventional loader operation to remote loader operation.

Explosive usage

The project found that common areas for improvement is the traceablity of explosives, particularly for their usage in secondary blasting. The project identified that some sites were adequately recording the explosive quantities being transported or the explosive quantities being used at the draw point. The authority holders have obligations to ensure the security and traceability of explosives throughout their entire life cycle.

The project found that the placement of explosives at draw points varied from mine to mine. A review of risk assessments found that key controls were absent, or the risk assessments did not have adequate consideration for the protection of explosives from the associated hazards such as friction or static electricity.

It was identified that, in some mines, onsite training did not follow a structured and formalised training program and was heavily reliant on experienced operators to train others on secondary blasting activities. The application of an industry recognised training standard such as an RIIBLA training competency specifically for specialised secondary explosive training is an improvement opportunity.

Training competencies should be implemented specifically for specialised secondary blasting activities.

Drawpoint controls

Bunding

Bunding is the most common form of protection from uncontrolled riling of stope material. Bunding can be an effective control for managing the risk, however if strength of the bund is unknown it can be attributed to a low-level engineering control and may be ineffective. Notwithstanding the strength characteristics of the bund if the bund is of an adequate size it may still provide an effective physical barrier from the hazard.

The bund dimensions and location are critical in effectively controlling the risk of uncontrolled rilling of stope material. There was a good understanding of the required bund height across various mine sites. In the case where the drive has higher backs, material has further to rill and consequently, for a bund to be effective, drive height must be considered. Development overbreak is common risk and further emphasis is needed fro the bund height to be measured relative to the backs.

Additional considerations include:

• Height and placement from the brow position
• The shape of the bund
• Consolidation of the bund by 'patting' down a bund crest will make the bund more effective and reduce the likelihood if material rilling down from the bund itself.

It is important to consider the rill angle when considering bund placement. An increase in fines within the stope will reduce the rill angle and subsequently the placement of the bund will need to be adjusted.

See figure 1 below for an example of how to determine where to place a bund considering rill angle, drive height and overbreak.

Bund dimensions and locations are critical in effectively controlling the risk of uncontrolled rilling of stope material.

Remote system of work

The application of remotely operated machinery has resulted in reducing worker exposure to drawpoint hazards.

Advances in wireless initiation system technologies has decreased the requirement to go back to charge rings in close proximity to open drawpoints. This system is not yet widely used and is relatively new technology but should be considered.

Support and Brow integrity

All mines recognised that it is unacceptable for workers to be directly under the brow or inside the stope. However, there is variation between sites on acceptable distance that workers are allowed to approach an open brow.

It can be difficult for workers to determine and continuously monitor their position relative to the drawpoint. When work is required for workers to approach the drawpoint, operators must establish an agreed safe distance to the drawpoint and install demarcation for when workers go near them.

Ground support integrity is a critical issue during secondary blasting activities within a stope drawpoint. Ground support can be damage from secondary firings and render the support ineffective. This may lead to an increase in rock fall hazards.

Secondary blasting within a drawpoint must be the last resort to be considered. If oversize is fired, drilling and blasting should be the preferred option as plastering oversize with explosives are seldom effective. The provision for secondary blasting must be included during the planning phase and may include the development of cuddies or bomb bays for firing oversize. These areas must be adequately supported with the aim to reduce blast-induced damage to ground support.

Operators must establish an agreed safe distance to the drawpoint and install demarcation for when workers near them.

Effective management of an open drawpoint

Effective management of drawpoints should incorporate every stage of the mining lifecycle and should start at the prefeasibility or planning phase.

Risks associated with drawpoints can be reduced by planning approciately and the following points should be considered:

• Avoiding uphole stoping where possible; in some cases uphole stoping cannot be avoided; drill and blast standards should be developed for application to uphole stoping.
• Approximate drill and blast techniques and quality control to achieve improved fragmentation with the object to reduce secondary blasting.
• Developing clear and understandable definitions on when a brow is open or closed
• Dumping rings forward may increase in the distance between a design brow and next ring to charge.
• Appropriate geotechnical analyses of critical structures and joint angles in relationship to the brow and assessment of hanging wall stability to inform on stope stability and the risk of material rilling from within the stope.
• Developing bunding standards ensuring all factors that influence bund placement are considered.
• Providing increased levels of supervision and recorded inspection of tasks associated with open drawpoints.

Conclusion

Working close to open brows or close proximity to drawpoints poses a significant risk to workers and Mine Managers should ensure effective management strategies and the effective controls detailed in this document are implemented.

The review of practices at mines found that there are inconsistent standards as to when a brow is considered to be open (cracked) or closed (choked). The review also found that there are differing opinions on acceptable distances to safely work from an open brow.

Risk associated with drawpoints can be more effectively managed through mine planning and prefeasibility study, development of standards for bunding and uphole stoping, and risk monitoring including geotechnical assessment and supervision of workers.

Additional improvement will also be gained through the early adaption of advances in technology in remote working practices.

Through better understanding of the risks associated with open drawpoints and ensuring proper management, the risk can be greatly reduced.

Authorised by Hermann Fasching - Chief Inspector of Metalliferous Mines and Quarries

Contact: Trevor Brown, Deputy Chief Inspector of Metalliferous Mines and Quarries , +61 7 4447 9253 Trevor.Brown@rshq.qld.gov.au

Issued by Resources Safety & Health Queensland