Seismic Risk in Canadian Mines

Transcription

1 Seismic Risk in Canadian Mines Marty Hudyma Laurentian University Laura Brown Laurentian University Donna Cortolezzis Laurentian University ABSTRACT Hardrock mines in Canada are extracting orebodies at greater depths. With increased depth, come higher stresses, more difficult rock mass conditions and an increased potential for failing ground. In many deep mines, dynamic rock mass failures, also called mining-induced seismic events, have become a major operational risk. In Ontario and Quebec mines, very large seismic events (Nuttli magnitude +2) occur on approximately a weekly basis. Seventy percent of Ontario underground, hardrock mines have a seismic monitoring system. There are currently twenty-five seismic monitoring systems in Ontario and Quebec mines, seven of which have been installed in the last four years. Seismicity and rockbursting is widespread in the Canadian mining industry. Effective seismic risk management (in Canadian mines) is a key topic for ensuring workforce safety, while maintaining mine production. This paper will discuss the generalities of seismic risk and its current status in the context of deep Canadian mines. 1 INTRODUCTION Hardrock mines in Canada are extracting orebodies at increasing depths. Advances in mining technology in areas such as mining methods, mining equipment, ventilation and ground control are allowing companies to extract orebodies that may have been considered too deep to mine in the past. With increased depth, come higher stresses, more difficult rock mass conditions and an increased potential for failing ground. In many deep mines, dynamic rock mass failure, also called mining-induced seismic events, have become a major operational risk. In the spring of 2014, the Ontario Ministry of Labour held a series of meetings and public discussions with industry, union representatives and other stakeholders in the mining industry to identify mining related hazards (Ontario Ministry of Labour, 2014a). Several hundred 1

2 submissions were received, identifying 263 mining hazards. In June 2014, these hazards were risk ranked by an expert group comprised of industry and worker representatives. The top ranked risk was Occurrence of a Rockburst (Ontario Ministry of Labour, 2014b). A rockburst is a large seismic event that results in rock mass damage in a mine opening. 2 SEISMIC RISK Some generic risk terminology: Risk is broadly defined as the chance of something happening that will have an impact upon objectives. It is measured in term of consequences and likelihood (AS/NZS 4360:1999). Hazard is a source of potential harm or a situation with a potential to cause loss (AS/NZS 4360:1999). Likelihood is used as a qualitative description of probability or frequency (AS/NZS 4360:1999). Consequence is the outcome of an event expressed qualitatively or quantitatively, being a loss, injury, disadvantage or gain. There may be a range of possible outcomes associated with an event (AS/NZS 4360:1999). Owen et al. (2002) modified these generic terms for mining-induced seismicity, proposing that seismic risk could be expressed as a product of hazard, likelihood, and consequence. In a seismicity context: where: seismic risk = (seismic hazard) x (probability of rock mass damage) x (exposure) Seismic hazard is the probability of initiation of a seismic source to produce a magnitude of a certain size, in a given time period. Probability of rock mass damage is the likelihood of damage at a given location, considering local site characteristics and the proximity of seismic hazard. Exposure considers the mining elements at risk, usually referring to the mine workforce, but also may include mine equipment, production and ore reserves. 2

3 2.1 SEISMIC HAZARD IN CANADIAN MINES A textbook definition of seismic hazard is An estimation of the mean probability (over space and time) of the occurrence of a seismic event with a certain magnitude within a given time interval (Gibowicz and Kijko, 1994). This definition identifies some of the key challenges in understanding seismic hazard in mines: Mean probability there are a number of factors that influence the variable likelihood of occurrence of a seismic event, principally uncertainty and variability associated with the rock mass and major geological features. Space seismic hazard varies spatially within a mine. Some areas are more prone to large events, while some areas are unlikely to ever have a large event. Time seismic hazard varies in time. There are times in which a seismic event is much more likely to occur, such as after a mine production blast. Certain magnitude the size of an event to result in harm or potential loss at a mining operation varies considerably between different operations. Mining-induced seismicity has been reported in Canadian mines since the 1920s, with the first formal Canadian research on the problem starting in the 1930s (Hedley, 1992). Table I is a summary of large seismic events recorded in Ontario and Quebec mines by the earthquake monitoring system of Natural Resources Canada over the last ten years (Earthquakes Canada, 2016). The data shows an average of 34 large events (Nuttli magnitude +2) recorded per year, or almost three events per month. Table I Large mining-induced seismic events (Nuttli Magnitude, M n ) in Ontario and Quebec mines (Earthquakes Canada, 2016). M n +2.0 M n +2.5 M n +3.0 M n Cumulative Yearly Average (Jan-Sept)

4 Observations from Table I: The yearly number of large events (Mn +2.0) varies from 21 to 56, with an average of 34. The variation from year to year depends on mining conditions at various mine sites and the number of deep mines in operation. Over the last 10 years, there is no clear trend to indicate an increasing number of events. There were increases in some years (2008 and 2012), followed by event rate decreases (2009 and 2013). The data for the first nine months of 2016 suggest that 2016 will have the highest number of large events in the last ten years. The number of events in 2010 is comparatively low due to a labour disruption at a number of mines for several months. Work by Hudyma and Beneteau (2010), suggests that these statistics are conservative, with some large events not being detected and some events being mislabelled as mine blasts. Table II shows, from 2011 to 2016 (September 2016), the Sudbury basin mines experienced 152 mining related seismic events with a magnitude of M n +2.0 or greater (Earthquakes Canada, 2016), or approximately 2.2 events per month. The Sudbury Regional Seismic Network (Hudyma and Beneteau, 2010) recorded 315 events during the same time period an average of 4.6 events per month. Table II shows that the Earthquakes Canada seismic record is essentially complete for mining-induced seismic events with a Nuttli magnitude of greater than The Earthquake Canada seismic record is incomplete for events with Nuttli magnitude less and Table II Events recorded in the Sudbury Basin ( ) Number of Mining Related Events Nuttli Magnitude Earthquakes Canada (2016) Sudbury Regional Seismic Network The Earthquake Canada network is comprised of 160 sensors, of which 22 are in Ontario. The Sudbury Regional Seismic Network covers 60 kilometres (East-West) and 40 kilometres (North- South) in the Sudbury basin with 26 sensors, providing greater sensitivity to mining related seismic events in the area. 2.2 ROCKBURST DAMAGE IN CANADIAN MINES The large events, recorded by Earthquakes Canada, occur predominantly in deep mines. Table III lists fifteen Canadian mines that have workings (or planned workings) of greater than 1000 metres. All of these mines have recorded large events in the last seven years. In eleven of the 4

5 fifteen mines, rockburst damage in the mine has been reported in technical publications. For the other four mines, there are no published references of rockburst damage. Depth (m) Table III. Deep Canadian mines and the recent occurrence of rockbursting. Mine Province Max Event Recorded Rockburst Damage Rockburst Reference >1000 Copper Cliff Ontario M n +3.8 (1) Yes Morissette et al. (2014) >1500 Coleman Ontario M n +2.9 (1) Yes Townsend and Sampson- Forsythe (2014) >1500 Morrison Ontario M n +2.3 (1) Yes Anderson (2014) >1500 Nickel Rim Ontario M n +2.4 (1) Yes Jalbout and Simser (2014) >1500 Garson Ontario M n +2.8 (1) Yes Shnorhokian et al (2012) >1500 Hemlo Ontario M n +3.0 (1) Yes Coulson (2009) >1500 Fraser Ontario M n +2.9 (1) Unknown No published reference Copper >1500 Totten Ontario M n +1.6 (1) Unknown No published reference >1500 Macassa Ontario M n +2.6 (1) Unknown No published reference >1500 Westwood Quebec M n +3.0 (1) Yes Iamgold (2013) >2000 Lockerby Ontario M n +2.6 (1) Unknown No published reference >2000 Red Lake Ontario M n +3.1 (1) Yes Goldcorp (2012) >2500 Kidd Ontario M n +3.8(1) Yes Counter (2014) >2500 LaRonde Quebec M n +3.8 (1) Yes Turcotte (2014) >2500 Creighton Ontario M n +3.8 (1) Yes Morissette et al. (2014) Notes: (1) Reference for largest seismic event (Earthquakes Canada, 2016) From the data in Table II and Table III, it can be concluded that mining-induced seismicity and rockbursting are considerable problems in deep Canadian mines. Many of these operations have plans to mine at greater depths in the future. In the Sudbury area alone, there are at least three additional orebodies under study or consideration for mining at depths greater than 2000 metres (Vale Victor project, KGHM Victoria project, Glencore Canada Onaping Deep). For many Canadian mining companies, the future of mining is at depths in excess of 1000 metres. In the hardrock mining industry in Ontario, mining-induced seismicity is a widespread problem. As of October 2016, the Ontario Mining Association lists 26 active underground, hardrock mines (Ontario Mining Association, 2016). Eighteen of these mines operate a seismic monitoring system as part of their ground control program. 5

6 2.3 RELATION BETWEEN EVENT MAGNITUDE AND ROCKBURST DAMAGE Past studies have shown that there is a crude relation between event magnitude and rockburst damage. A study based on data from Brunswick Mining (Hudyma, 2004) investigated event magnitude and the result rockburst damage. Detailed records were kept comparing size of seismic events and resulting damage at Brunswick Mining. Data was collected over a continuous three-year period from 1993 to 1995, for 207 events Richter magnitude 0. The damage reported from each of the 207 events was rated using a 5-level damage classification system (Table IV). The rock mass in all case histories was supported with 2.4 metre resin rebar, welded wire mesh, and in many cases twin strand 6 metre long cablebolts. The 5-level damage classification scale is similar to the scale proposed by Kaiser et al. (1992). Table IV Five-level rockburst damage classification scale (Hudyma, 2004). Class Description Tonnes Displaced Need / Ease of Rehabilitation A No Damage 0 No Rehab required B Minor Rock Damage < 1 tonne No Rehab required C Minor Support Damage 1 10 tonnes Easily rehabilitated D Major Support Damage tonnes Ground can be rehabilitated, but may be slow and difficult E Severe Rock and Support Damage More than 100 tonnes Rehabilitation will be very difficult or may not be possible Table V and Figure 1 summarise the results of the rockburst data collected at Brunswick Mining. The probability of occurrence in Figure 1 is calculated as the number of occurrences of rockburst damage for each magnitude divided by the total number of events of that magnitude. Table V Richter magnitude and damage from 207 events at Brunswick Mining (Hudyma, 2004). Richter Magnitude Damage Severity A B C D E Total Total

7 Figure 1 Cumulative probability of rockburst damage for events from Richter magnitude 0 to Richter magnitude +3. The blue line refers to the cumulative probability of occurrence of rockburst damage severity B, C, D, and E (in Table IV). The green line refers to the cumulative probability of occurrence of rockburst damage severity C, D, and E. The orange line refers to the probability of occurrence of rockburst damage severity D and E. The red line refers to the probability of occurrence of rockburst damage severity E. Some general conclusions from this data: Events in the range of Richter magnitude 0 rarely caused observable damage (no observed damage ~90% of the time). The probability of damage and severity of damage increases as event magnitude increases. Very large seismic events (Richter +2 ), do not always result in major (D) or severe (E) rockburst damage. In this dataset, less than half of the Richter magnitude +2 events cause major (D) or severe (E) damage. 7

8 Seismic source effects, path effects and local site conditions would have significantly influenced the degree of rockburst damage reported. However, the data give insight into the probability of rockburst damage. Morissette (2015) conducted a study of rockburst damage associated with 324 large seismic events at three Ontario mines. It was found that for events with Nuttli magnitudes of +1.0, less than 1% of the events resulted in more than 10 tonnes of rockburst damage. For events with a Nuttli magnitude of +2.0, only 5% of the events resulted in more than 10 tonnes of damage. For events with a Nuttli magnitude of +3.0, approximately 30% of the events resulted in damage of more than 10 tonnes. The results of these two studies have similar conclusions. There is a general trend of increasing damage with increasing event magnitude. However, the severity of damage for any event size is variable, with magnitude being only one of the many factors influencing rockburst damage. 3 MANAGING SEISMIC RISK IN CANADIAN MINES If seismic risk is taken as the product of seismic hazard, probability of rockburst damage and exposure, then: seismic risk = (seismic hazard) x (probability of rock mass damage) x (exposure) If seismic hazard, or the probability of rock mass damage or exposure are reduced, seismic risk is reduced. If seismic hazard, or the probability of rock mass damage or exposure are eliminated, seismic risk is eliminated. Unfortunately, in the context of seismic risk, it is not possible to eliminate seismic hazard, rock mass damage potential or exposure. A more practical approach is to manage seismic risk through reduction of seismic hazard, rock mass damage or exposure. 3.1 REDUCING SEISMIC HAZARD The fundamental factors related to seismic hazard can be generalized to: premining stress conditions, rock mass and geological features, and influence of mining activities. Of these three factors, control can only be exercised over the third factor the influence of mining activities. The premining stress conditions and geological environment are predetermined and there is very limited ability to change them. Mining factors that can be adjusted to reduce seismic hazard include: blast size, stope size, pillar size and shape, stope 8

9 sequence, mining method, type and timing of backfill, destress blasting and mining rate. Each of these factors is related to increasing seismic hazard. Mikula (2005) and Volume 2 of the Rockburst Research Handbook (Kaiser et al., 1996) discuss in more details some of the mining factors that have been found effective in reducing seismic hazard. Usually, there is a significant additional financial cost and negative effect on mine production to implement seismic hazard reduction measures. 3.2 REDUCING ROCKBURST DAMAGE POTENTIAL Heal (2010) and Potvin and Wesseloo (2013) discuss a number factors related to dynamic loading and rockburst damage. Table VI lists these factors, separating them into factors over which there is limited or no control, and factors over which there may be some control. Table VI Factors related to rock mass dynamic loading and rockburst damage (after Heal (2010) and Potvin and Wesseloo (2013)) FACTORS RELATED TO ROCKBURST DAMAGE LIMITED OR NO CONTROL SOME CONTROL Distance from the location of the Excavation span seismic event Rock mass attenuation Excavation orientation compared to rock mass discontinuities Rock mass conditions at the damage Type of surface support and type and location length of rock mass reinforcement The level of stress at the damage Condition of ground support and location reinforcement Energy radiation pattern Rockburst damage mechanics Excavation related ground motion amplification Seismic source mechanism Ultimately, there is some potential to change some of the factors related to rockburst damage. However, significant a priori information about future rockburst locations is needed to change excavation span and orientation. Often this information is only available long after the mine development has been created. This leaves ground support and reinforcement as the remaining factor to reduce rock mass damage potential. Usually, there is a significant additional financial cost and negative effect on mine production to implement rock mass damage reduction measures. 9

10 3.3 REDUCING EXPOSURE There are a number of means of reducing workforce exposure to seismic risk, including: Post blast re-entry periods to temporarily keep workers out of areas likely to experience a large seismic event, Post blast exclusion zones to keep workers away from changing ground conditions, Reduce the number of people exposed, by removing non-essential workers (mechanics, electricians, etc.) from high risk areas, Change of mining practices, such as: minimizing hand-held mining, keeping workers away from unsupported ground, using bored raises instead of drop raises for stope slot raises, production drilling and blasting from outside stopes, mining under backfill, Use of remote/automated equipment, and Implementation of just-in-time development mining. Usually, there is a significant additional cost and/or mining delays to implement exposure reduction measures. 3.4 TRAINING AND EDUCATION While some of the Ontario mines have faced the challenge of mitigating seismic risk for years, for many mines the problem is relatively new. Seven of the twenty-five seismic systems currently operating in Eastern Canada have been installed in the last four years. The details of learning to operate a seismic monitoring system are relatively straightforward. There are effective training courses and manuals for this. However, for many mines, there are many other facets to effective seismic risk management, including: improving the quality of the seismic data recorded in Canadian mines, learning to interpret seismic data, understanding areas of high seismic hazard in a mine, identifying areas or conditions prone to strainbursting, starting to appreciate the cause-effect relations between mine production blasting and seismicity, learning which geological features in the mine are more prone to large events, training front line supervisors and the workforce to understand how seismicity and rockbursts affects them, determining when, where and how to implement development destress blasting, developing local seismic risk procedures and protocols, such as re-entry protocols and exclusion zones, choosing an appropriate dynamic resistant ground support system, and determining which parts of the mine would benefit most from this enhanced system, 10

11 incorporating new mining equipment to reduce workforce exposure, and modifying mining practices (such as development practices, stope design, pillar design, stope sequencing) to reduce seismic risk (Mikula, 2005). Many of these considerations are site specific and may need modification to be transferred between mines. Importantly, however, by understanding and implementing these measures appropriately, mine engineers can become proactive about managing seismic risk, rather than purely reactive to seismicity and rockbursting. Through proactive seismic risk management, the effect rockbursting and seismicity on workforce safety and mine production can be significantly reduced. There is a long learning curve to effective seismic risk management, needing a dedicated engineering effort and a commitment from mine management. Many of the Canadian sites which effectively manage seismic risk have had a continuity of senior ground control personnel for several years or more. A key issue is how do we encourage, develop, train and retain our next generation of ground control personnel. Without highly trained ground control personnel, seismic risk management will be difficult in the future. 3.5 A GENERIC SEISMIC RISK MANAGEMENT PLAN In 2008, the Australian Centre for Geomechanics produced a Generic Seismic Risk Management Plan for Underground Hardrock Mines (Heal et al., 2008). The intent of this document was to provide a blueprint to bring together the most common seismic risk considerations for a mine site. Effectively, the Seismic Risk Management Plan becomes a companion document to the ground control management plan or the mine design package found in many Canadian mines. The Seismic Risk Management Plan focusses on documenting the local procedures and experiences with seismicity and rockbursting, so that this information becomes part of the seismic knowledge base for the current and future engineers at the operation. The ACG s generic Seismic Risk Management Plan is a public domain document. It can be found at the following link: nderground_hardrock_mines 4 SUMMARY Seismic risk is prevalent in Ontario and Quebec mines. Seventy percent of Ontario underground hardrock mines now have a seismic monitoring system to aid in managing seismic risk. For several of these operations, seismic risk is a new and emerging problem. 11

12 The relation between event magnitude and rock mass damage is complex. Often mine operators are surprised and relieved when a very large seismic event does no appreciable damage. However, mine operators are also sometimes perplexed that relatively modest sized events result in a disproportionately high level of rock mass damage. There is limited ability to reduce event size (seismic hazard), but there are numerous effective ways to be proactive about reducing rockburst damage and limiting exposure. Understanding the three main components of seismic risk helps to identify opportunities to reduce risk. To maintain workforce safety, maximize production and minimize costs and delays, there is a need to learn, develop and implement measures to become proactive about seismic risk. 5 ACKNOWLEDGEMENTS We gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council (NSERC), Agnico-Eagle Mines, the Australian Centre for Geomechanics, Glencore Canada, KGHM FNX Mining Company, Vale Canada Limited. 6 REFERENCES Anderson, T. (2014) A comparison of shallow and deep mining. In proceedings Seventh International Conference on Deep and High Stress Mining. Editors: M. Hudyma and Y. Potvin. Australian Centre for Geomechanics. pp AS/NZS 4360 (1999) Risk management. Standards Association of Australia. Coulson, A. (2009) Investigation of the pre to post peak strength state and behaviour of confined rock masses. Unpublished PhD thesis, University of Toronto. Counter, D.B. (2014) Kidd Mine dealing with the issues of deep and high stress mining past, present and future. In proceedings Seventh International Conference on Deep and High Stress Mining, (Editors: M. Hudyma and Y. Potvin), Australian Centre for Geomechanics. pp Earthquakes Canada (2016) GSC, Earthquake Search (On-line Bulletin), Nat. Res. Can., {10 October 2016}. Gibowicz, S.J. and Kijko, A. (1994) An introduction to mining seismology. 1st edition. San Diego, Academic Press, 396p. Goldcorp (2012). Goldcorp Annual Report. Retrieved from: {02 February 2015.} 12

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