In the course of new mine construction or of expanding mine workings, the mine development crews are the first to encounter the realities of stress-related strain behaviour of the rock.
These hazards are encountered generally before the more comprehensive seismic monitoring and rockburst management systems, common in production areas, are in place.
As such, strainbursting must be considered an important hazard facing mine personnel.
The nature of such hazards, although broadly foreseeable, is that their occurrence is unpredictable. Using a risk-based approach, Cementation Canada Inc., a mining contractor based in North Bay, Ontario, has developed practical guidelines for strainburst hazard awareness for development miners and shaft sinkers.
The objectives are to raise awareness, to highlight observable indicators, and to minimise exposure and mitigate the negative impacts of strainbursting, with the ultimate goal of zero harm.
This article has been written to cover the background to this challenge, explain how it fits into the Internal Responsibility System (IRS), and to provide details of how the program is being implemented.
The intent is to share this program to help improve safety in the Canadian mining industry and to gather feedback in order to continuously improve upon this initiative.
Around the globe, numerous incidents, injuries and fatalities have been associated with rockbursting. As noted by Potvin et al. (2000), despite all the excellent research and progress in the past decades, the problem is still not well understood. Brady (1990) suggests that given the pervasiveness of rockbursting, it remains the major unresolved ground control problem in underground mining.
In the past few years, Cementation’s workers have been in close proximity to several rockburst incidents at various operations. Fortunately most have been close calls but everyone will agree that even one injury is one injury too many. For a number of years, Cementation has carried out development and shaft sinking at significant depths in several different mining areas known historically for their seismicity.
As a mining contractor, we strive to safely develop and construct mines. The company has a responsibility for the safety of its workforce. One initiative that it is currently advancing is strainburst hazard awareness training following a rational, risk-based approach.
Development miners are the most exposed to the hazard and they are the primary target of the training. However, all employees at underground projects, including maintenance, construction, supervision, safety professionals, management and technical staff, are potentially exposed, and the training is to be provided to all of the underground team.
It is a value common to everyone in the province of Ontario that every worker returns home safe and healthy every day (Workplace Safety North vision). At Cementation, the belief in working safely and eliminating injuries must pervade everything we do. Such safety values are shared throughout the mining industry. It is these values that have guided this initiative.
The objectives of this strainburst awareness initiative are as follows:
- Promote awareness of the strainburst hazard amongst the workforce.
- Coach all levels of underground workers to recognise factors that are commonly associated with strainburst occurrence.
- Provide guidance on what they can do to reduce both the probability of becoming exposed and the consequences of strainbursting.
- Improve communication amongst all levels of the company and the mine owners about the strainburst hazard.
The objectives culminate in an effort to reduce the frequency or number of indicents involving workers in proximity to strainburst-prone rock by raising awareness to their possible causes, and to reduce associated consequences to achieve zero harm. This initiative also follows the regulatory guidelines of training workers in rockburst awareness in the Province of Ontario (Deck 1997).
Due to its unpredictable nature, rockbursting is an especially challenging hazard to manage as explained in North America by Blake and Hedley (2003).
From a South African perspective, Wagner (1982) states that rockbursts are the most serious and least understood problem facing deep mining operations all around the world. This presents an outstanding challenge.
The rockburst phenomena
A rockburst is described by Hoek (2007) as “an explosive failure of rock which occurs when very high stress concentrations are induced around underground opening.”
This is a technical definition based on the ‘cause,’ which contrasts with the more practical ‘effect’ definition provided by Kaiser et al. (1996); “Damage to an excavation that occurs in a sudden or violent manner and is associated with a seismic event.”
Rockbursts commonly are classified into three types: fault slip, pillar, or strainburst (Kaiser & Tannant 1999).
Fault slip bursts will occur when mining induced stress changes induce movement along a fault or slip.
Pillar bursts occur where pillars, in whole or in part, suddenly fail due to rock mass strength being exceeded by the stress in the pillar.
And finally, strainbursts occur along excavation boundaries where rock mass strength is exceeded by the stresses. A brittle, violent failure of rock adjacent to the boundary causes the damage.
Three distinct damage mechanisms may be encountered as described by Kaiser et al. (1996) – seismic shake down (rockfall), rock fracturing with rock mass dilatation (bulking), and violent ejection of rock.
Mine contractor’s perspective
As development miners, Cementation’s workers are mainly exposed to the boundary strainburst type and are most concerned with events involving violent ejection of rock. Development mining generally does not induce fault slip bursts or create bursting pillars.
In the course of new mine construction or of expanding mine workings, the mine development team is the first to encounter the realities of stress-related strain behaviour of the rock. These hazards are encountered generally before the more comprehensive seismic monitoring and rockburst management systems, common in production areas, are in place.
As such, strainbursting must be considered an important hazard facing mine contracting personnel.
As recognised in the guideline issued by the Ontario Ministry of Labour (Deck 1997), only a rough evaluation of the potential for bursting at a mine can be made before mining begins.
Rockbursting is dependent on the interaction between rock properties, geological structure, mine layout and sequencing etc.
The full assessment of bursting potential is not possible until a considerable amount of data has been collected in the course of operations, which occur later than the execution of initial development work.
Contractors are able to bring varied experience from other projects to each new job. However, they do not have the level of site-specific knowledge, nor the technical expertise the owner may have for rockburst hazard planning at any given mine site. However, it is the contractors’ workers and contracting crews working at the face. They rely on clients to share their site specific knowledge.
The nature of this site specific expertise may include geological, geotechnical, ground control, seismic monitoring, mine planning and so forth.
So the question is posed – what can we do as contractors? We can implement strategies with the underground team to raise their awareness of the strainburst hazard using general guidelines.
This is especially important given the nature of the workforce in mine contracting, with miners moving from project to project – all wi
th differing ground conditions. Cementation can continually update the guidelines by tapping into its clients’ knowledge and with experience gained on multiple sites. Our goal is to share this development with all of our clients as well as the industry in general. It is all about the common safety values that we all share.
Internal responsibility system
The basis of the Occupational Health and Safety Act in the province of Ontario is the IRS which was described by Dr. James Ham when his Royal Commission looked into mining health and safety in the mining town of Elliot Lake in 1976. Strahlendorf (2013) describes the IRS as meaning:
“Everyone is personally and directly responsible for health and safety as an intrinsic part of their job. Work and occupational health and safety are not separate activities. Everyone is doing occupational health and safety 100% of the time.”
Cementation promotes the IRS as an important part of any safety management system, and more importantly, as a crucial part of a healthy safety culture. It believes that management has a duty to provide the knowledge and tools for workers to complete tasks safely, each and every day.
In turn, it expects its employees to communicate concerns and hazardous conditions to their co-workers and supervisors. This approach is the basis of Cementation’s Strain Bursting Hazard Awareness sessions. It wants to inform employees of this difficult to predict hazard with significant consequences. Cementation will encourage its employees to actively participate in the identification of key indicators and communication of them to the appropriate personnel, and will introduce a process to facilitate this.
The company has chosen to follow a risk-based approach as commonly applied in mine safety. Risk is the product of the probability of some thing occurring and the consequences of it happening. Using a risk approach, it asks key questions whose answers formulate the guidelines.
For the challenge of reducing harm from strainburst incidents, we ask the following questions about the probability and consequence of strainbursting:
- What factors increase the likelihood of strainburst occurrence?
- What indicators can be recognised and how do we ensure that our underground workers, especially those at the face, can recognise these factors?
- What can we influence that might reduce the probability of a strainburst occurence?
If a strainburst is likely to occur, what can we do to reduce the consequence of that event? What can be done so that the worker might avoid the hazard? What controls can be applied to reduce the hazardous consequences of strainbursting?
It is these questions and their answers that form the basis of the hazard awareness training program.
What factors increase the likelihood (probability) of strainburst occurrence? To answer this fundamental question, look at the underlying cause of strainbursting – it occurs where the rock stress exceeds the rock mass strength in brittle rock.
So where should we look? Look for factors that induce increased stress or decreased rock-mass strength.
What are the factors causing high stress? A general rule is that you get higher stress with depth. However, locked in stresses from previous geological history can lead also to high stress at shallower depths. Stress fields can change near faults and other structures.
There are stress changes, risers, etc. induced by excavating holes in the ground. These mine induced stress patterns are higher with over extraction, poor pillar layout, poor excavation shape, etc.
Also, sudden changes in excavation profile shape/size can all lead to high stress conditions locally around the excavation.
As was mentioned previously, hard brittle rocks are most prone to bursting. Strainbursts are less likely in weak, yielding rock such as evaporites.
Contrasting rock types where weaker and stronger rocks are in contact – the stronger (stiffer) rocks will take on more stress from the weaker rock and become susceptible to bursting. Intrusive geological structures such as the trap dykes in Sudbury that tend to be stronger and more brittle.
Also, weak zones may shed stress to nearby brittle rocks or allow very small scale slips that result in strainbursts.
While drilling, miners can look for changes in drill behaviour such as lower drilling speeds, jerking/jamming of rods, pitching/caving of holes, or even face splitting or cracking. These may be an indication of ground changes or high stresses.
Thus having recognised the main causal factors, what can be seen underground, what observations can be made that may indicate strainbursting conditions?
There are many indicators; the presence of any one of these indicators does not necessarily mean that bursting is imminent. However, the more indicators that are present, the more likely that bursting will occur.
Mines with seismic and micro-seismic monitoring systems in place can note increased levels of local mine seismicity or swarms of regional seismicity that may indicate that bursting is more likely.
As development mining contractors we rely on timely communications from the mine operator of these indicators. Having looked at the factors and indicators of burst-prone ground, can we now reduce the probability of strainburst occurrence?
We can reduce the probability of strainbursting by:
- Minimising the effect of rock stress: using proper geometry, sequence and rate of mining. Examples include: avoiding narrow sill pillars, using proper backfill, crossing structures/faults at favourable orientations and/or locations, use seismic data in mine planning, layouts, etc.
- Attempt to drive high stress away from the excavation boundary by using destress blasting or drilling.
Generally the mine owner controls these variables based on their site-specific knowledge. However, observations by development miners and input from contractors can provide feedback which will help mine owners improve the mine design and thus the safety of the workplace.
If a strainburst is still likely and despite steps to reduce the likelihood, what can be done to reduce the consequences of such an event?
One approach is to avoid the consequences by retreating, barricading or using remote equipment. These methods require judgment and re-entry protocols. Often mine operators have good experience with re-entry protocols that is shared with contractors.
Another approach is to try to induce the strainburst activity in a safe controlled manner. An example is hosing down the rock face in question with cold water from a safe distance. If the face is close to bursting, the thermal shock of the cold water can be used to induce the burst.
A third option is to reduce the extent of the damage by installing energy absorbing ground support. There are many new innovative products on the market for such dynamic ground support. These systems have held up well in bursting conditions.
However, as a development miner, one must be aware that we are in burst-prone ground when installing these support elements. We must continue to educate development miners on these new ground control processes and materials.
An employer has a duty to inform its employees of all workplace hazards. In the mining industry, those hazards typically include operating mobile equipment, working at heights, and falls of ground to name just a few.
Cementation Canada has developed a presentation to guide its crews in a conversation regarding the dangers of strainbursting.
To do this, Cementation focused on enabling employees to recognise the hazards and in addition to reporting those hazards to management, to consider what else they themselves can do to keep safe when such hazards are
A large part of the awareness discussion centres on communication. It’s stressed to the employees that they be aware of the factors which may lead to strainbursting and should they be identified, to communicate those signs to a supervisor and management so they can get the proper people involved in order to mitigate the risks.
The primary communication method available to workers is a Five-Point Safety System card.
Each employee is given a Five-Point Card at the beginning of their shift which has a written line-up from their supervisor for the upcoming shift. There is a series of questions and checks which the worker and supervisor sign off on over the course of the shift.
These checks, along with an area for Further Action Required, enables the worker to express any safety concerns they may have, including sections specific to ground control.
Each card is submitted to the supervisor at the end of the shift to report on the shift’s work, and to ensure there are no outstanding concerns.
Additional means of communicating potential strainbursting hazards includes the use of the ground control log book located near the wicket.
Provided the worker is aware of relevant factors related to strainburst risk, they can be written down. Notes on ground conditions for specific areas are written after each shift by the supervisor and checked by other supervisors, management, and departments like geology and ground control.
For these systems to be effective, it is essential for all personnel to contribute to the process, while it is important that the miners at the face are able to communicate what they see and hear to their front line supervisor.
This information then goes up the line to management to ensure that the proper professionals can get involved to minimise the potential risk to workers.
Safety System Card
Of course, supervisors are required to visit the face on a regular basis. They are also responsible to recognize these hazards and are therefore included in this training program.
Management is responsible for ensuring that the workers recognize the hazards (the main premise of this program, training workers to be able to do so) and creating a work environment where they are comfortable in reporting their observations.
During the awareness sessions held at the project site, it was found that one of the ways to generate discussions with the crews was to engage them with questions about their own experiences of strainbursting events.
Experienced miners were quick to share some of their past experiences from working at other mines across Canada. Many stories were shared from deep mines in various areas across the country.
Sharing of these stories is very important to raising awareness of the risks of strainbursting, especially for those miners who are relatively new to the underground environment.
A first-hand explanation from an employee who has seen and experienced the powerful consequences of a strainburst is extremely valuable.
It was eye-opening to hear the stories come forth, with statements like, “The scary part when working in some of those headings was when things became quiet, because you were so used to the face and walls spitting.”
The hope is that in providing the crews with additional information and raised awareness about strainbursts, that more incidences will be reported and measures can be put into place which will reduce the chance of loss.
Feedback from awareness sessions
In general, the sessions have been well received. The amount of discussion generated is above average as compared with typical safety meetings, which is a positive sign.
One Superintendent who attended a session stated that he felt there was some really good information provided to the crews, and thinks the crews were provided with knowledge that they will be able to take with them to different mines as their mining careers move forward.
A client safety coordinator echoed this notion in stating, “I think it is important that Cementation is educating its employees on the risks of strainbursting because the work force moves around so much. They are bound to see examples of it over the course of their careers.” A site safety supervisor simply commented, “this is good, we need to keep reminding ourselves of this.”
Cementation’s plans are to continue to develop this initiative to educate its employees regarding the risks of strainbursting.
In continuing with a risk-based approach, it will work with its clients’ ground control engineers in order to identify areas where strainbursting will pose a significant risk.
By having the Corporate Safety and Training department involved, the awareness level of strainbursting hazards has been increased with many individuals who can then carry this knowledge forward into the workplace and to share with other projects.
It is known that strainbursts are a significant safety hazard and they are challenging to predict. However, there are several indicators that assist with the identification of this hazard.
Cementation is developing a strainburst hazard awareness initiative to improve workplace safety for underground workers – this will include hazard awareness training for workers, and the introduction of a strainburst hazard awareness card to help report and communicate observations from underground.
This approach is to empower the worker, through awareness and knowledge to report upon, communicate and act on areas of concern. This abides by the intent of the internal responsibility system.
The guideline will be generic and it plans to use it across relevant projects. Site specific input for each mine is always required in addition to the guidelines, while incorporating and using the expertise of clients.
Like all guidelines, this initiative is under continuous development, so feedback is essential and welcomed. The focus is to encourage the early reporting of key identifiers of strainbursting potential by educating employees as to what are those identifiers, so that actions can be put in place to help achieve the goal of zero harm.
Blake, W & Hedley, DGF 2003, Rockbursts: case studies from North American hard-rock mines, Society for Mining, Metallurgy and Exploration, Littleton.
Brady, BGH 1990, Rockbursts and Seismicity in Mines, A.A. Balkema, Rotterdam.
Deck, BK 1997, Management of Rockbursts – Statement of Policy and Health and Safety Guidelines, Ontario Ministry of Labour Health and Safety Guideline.
Hoek, E 2007, Practical Rock Engineering, RocScience
Inc., Toronto, viewed 14 July 2014https://www.rocscience.com/hoek/corner/Practical_Rock_Engineering.pdf
Kaiser, PK & Tannant, DD 1999, ‘Lessons learned for deep tunnelling from rockburst experiences in mining.’ Proceedings Vorerkundung und Prognose der basistunnels am Gotthard und am Lötschberg, A.A. Balkema, Rotterdam, pp. 325-337.
Kaiser, PK, Tannant, DD & McCreath, DR 1996, Canadian rockburst support handbook, Geomechanics Research Centre, Laurentian University, Sudbury.
Potvin, Y, Hudyma, M & Jewell, RJ 2000, ‘Rockburst and seismic activity in underground Australian mines-an introduction to a new research project.’
Proceedings of the International Conference on Geotechnical & Geological Engineering, TechnomicPublishing, Lancaster, on CD- ROM.
Strahlendorf, P 2013, ‘The Internal Responsibility System’, Presentation given to the Workplace Safety North Mining Health and Safety Conference, Sudbury.
Wagner, H 1982, Rockbursts and Seismicity in Mi
nes, South African Institute of Mining and Metallurgy, Johannesburg.
*Information for this Special Report provided by Alun Price Jones, Technical Director, Ryan R. Lyle, Senior Geotechnical Engineer, and Steve Wrixon, Superintendent of Safety and Training, Cementation Canada Inc., North Bay, ON. The article is based on a technical paper presented at a recent conference of The Australian Geomechanics Centre (ACG).