Difference between revisions of "Mining in the Arctic"
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In arctic regions, the ability to access water for production purposes can be a challenge due to permafrost and cold temperatures.
Many water bodies in arctic regions are shallow, and may freeze solid during the winter, rendering water access from surface sources impossible during winter months. Mines may also have difficulty accessing water via wells, as there is no liquid water in permafrost. In addition the bedrock layers may be dry due to the impermeable nature of permafrost.
These extremes of temperature can also lead to difficult operating conditions. Water supply systems need to be constructed in a manner those pipelines do not freeze, causing the failure of a critical process, such as dewatering or water supplies to heap leaches.
The Spitsbergen mine in Norway solved their water access issue problem by heating water to 25 degrees Celsius at a power plant, and shipping it via truck to the mine workings for use. Modern practices see water being transported from deep lakes to the mine via piping.
All pipes must be heated and insulated. In Svalbard all pipes are heated with self-regulated heating wires. At the end of the work week and before periods of interruption, water is removed from the pipes using dry compressed air to prevent the pipes from freezing. In situations where water is used as a drilling fluid, the water is mixed with an anti-freeze agent such as calcium or sodium chloride.
== Heating and Ventilation ==
== Heating and Ventilation ==
Revision as of 17:42, 5 February 2016
- 1 Introduction
- 2 Camp Life
- 3 Infrastructure Requirements
- 4 Transportation
- 5 Water Management
- 6 Heating and Ventilation
- 7 Geotechnical Issues
- 8 Tailings Storage Facilities
- 9 Mine Waste & Tailings Covers
- 10 Additional Risks
- 11 References
Mining in the Arctic is a difficult task, as many associated costs and design challenges exist. The cold climate, permafrost, and seasonal restrictions due to the remote location of most Arctic mines influences various elements of a mining operation including:
- Camp life
- Infrastructure requirements
- Access and shipping
- Water management
- Heating and ventilation
- Geotechnical issues
- Tailings storage facility
- Mine waste and tailings covers
Through the implementation of mitigation strategies and cutting-edge designs, mines have been able to overcome the above challenges and successfully operate in Arctic conditions.
Due to the remote location of arctic mines, operations must be fly in fly out with worker dwellings on site. In order to maintain operation efficiency, the mine must always be operating, and have a constant labor force on site ready to work. An arctic mine site must have on site administrative and accommodation facilities. Site facilities must be engineered to be habitable in the the harsh arctic environments. The Raglan mine currently has a 400 room, hotel style complex, that workers live in during their 2 week shifts at the mine. This process works well, as the mine is an extremely efficient, self-contained arctic mine.
The lack of infrastructure in Arctic regions makes mine development difficult. Constructing brand new facilities for non-permanent facilities is often too costly and difficult for temporary development .
- Most remote mines are remote in uninhabited regions with little to no infrastructure
- Large investment is require for mine development before operations can commence
- Mines in similar situations have built totally self-contained mines
- Mines have a concentrator, a power plant, fuel storage, fresh water supply, accommodations and administrative buildings
- All necessary features for creating mines in remote location
The biggest issue with infrastructure is foundation construction
- Different foundation types exist
- Pads and Wedges
- Pads and wedges are a type of foundation in which timbers are laid in alternating layers on a prepared gravel base or exposed rock outcrop
- They are used on soils that have low bearing capacities, and have been widely used in northern permafrost areas for small buildings which are not likely to suffer damage from a reasonable degree of movement. Preserved wood should be used in the construction of the pads and wedges.
Pilings are a type of foundation in which a pile or post is driven or drilled into the ground to a solid bearing level or bedrock, or set into permafrost to provide structural support They are made of wood, concrete or steel, and are used at sites with soil of low bearing capacities and in permafrost applications. Some advantages of pile foundations include: they have a small cross-sectional area, which minimizes heat transfer from the building to the soil; they can be drilled or driven into frozen permafrost, which ensures stable support even if the soil in the active layer has low bearing capacity; there is minimal differential movement; and they can provide anchorage against wind uplift. Lifting forces may occur due to annual freeze and thaw cycles but this can be overcome by a combination of proper anchorage in the permafrost and the incorporation of a "slip sleeve" arrangement at the active layer. Disadvantages with this system include difficulties in drilling during the thawing of the active layer, the expense of skilled labor, and the expense of transporting and maintaining heavy drilling or pile driving equipment in remote areas..
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Footings or Pier Foundations There are several types of footings or pier foundations: individual footings carrying one column or post, combined footings carrying more than one column or post, and continuous or strip footings carrying a wall. Individual footings are preferred to continuous footings in permafrost areas since there is less structural damage if there is movement, and the footings can usually be individually adjusted to correct for the movement. Piers are square or cylindrical units with large bases to distribute the loads for an acceptable bearing pressure. These foundations are very stable provided they are formed within the permafrost, or above the permafrost on thaw-stable soil. If formed within the permafrost, provision must be made to ensure that the ground below the footing does not thaw. Protection must be provided against frost heave on piers and columns, and consideration must be given to the long-term bearing capacity and creep settlement of the frozen foundation soil..
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The remote location of mineing projects makes it difficult to access required resources and supplies
Road Construction Issues
- Permafrost, ground conditions of either soil or rock that remains at or below 0 degrees Celsius for long periods
- Due to severe ground deterioration, maintaining roads in permafrost conditions is up to 10 x more expensive than in non-permafrost conditions.
- Issues consist of:
- Water runoff, in melting seasons
- Road deformation due to ground disturbances
- Road bank collapse/ road subsidence
- Large cracks
- Can severely damage local environment 
- Routine (day to day) maintenance to fix any issues
- Frequent periodical patrol to detect and fix any issues
- Heat extraction using air convection in embankments on permafrost
- This cools the embankment in an effort to maintain cool/ frozen ground
- Reduces snow accumulation on embankment sides to prevent degradation
- Performance monitoring, using Ground- Penetrating Radar
- Insulating permafrost to mitigate thawing
- Using polyurethane insulation
- Soil stabilization, to reduce frost action in subgrade soils
- Specialized pavement, to reduce heat transfer to underlying subgrade soils
- Lightweight cellular concrete for thermal protection under roadways and for thermal protection.
Arctic mines do not face the same dewatering issues faced by mines in southern regions. Permafrost forms an impermeable layer through which water can not flow, resitricting the inflow of water into mine workings. The major challenges of keeping mines dry are associated with surface dewatering, and keeping permafrost frozen.
In arctic regions, the ability to access water for production purposes can be a challenge due to permafrost and cold temperatures. http://pubs.usgs.gov/pp/0696/report.pdf Many water bodies in arctic regions are shallow, and may freeze solid during the winter, rendering water access from surface sources impossible during winter months. Mines may also have difficulty accessing water via wells, as there is no liquid water in permafrost. In addition the bedrock layers may be dry due to the impermeable nature of permafrost. https://www.icmm.com/document/3660 These extremes of temperature can also lead to difficult operating conditions. Water supply systems need to be constructed in a manner those pipelines do not freeze, causing the failure of a critical process, such as dewatering or water supplies to heap leaches.
The Spitsbergen mine in Norway solved their water access issue problem by heating water to 25 degrees Celsius at a power plant, and shipping it via truck to the mine workings for use. Modern practices see water being transported from deep lakes to the mine via piping. All pipes must be heated and insulated. In Svalbard all pipes are heated with self-regulated heating wires. At the end of the work week and before periods of interruption, water is removed from the pipes using dry compressed air to prevent the pipes from freezing. In situations where water is used as a drilling fluid, the water is mixed with an anti-freeze agent such as calcium or sodium chloride.
Heating and Ventilation
Ventilation is a fundamental design aspect of all underground mine operations. It is undertaken to introduce fresh, cool air to the working faces of the operation and to remove stale, affected air; to remove contaminants such as dust, methane and diesel emissions; and to help control heat and humidity . Mine ventilation typically attributes to 40% of an underground mine’s electrical power consumption and is comprised of many essential elements including compressors, pumps, piping, fans and tubing, heating and cooling, ventilation raises, and barricades and regulators . When considering the challenges of mining in the Arctic, a major ventilation concern is the heating of air within the operation.
Cold-climate mines, such as those located in the Arctic, face additional challenges in the area of mine ventilation due to the risk of frost build-up and ice formation. Frost and ice are introduced to the operation when an air mass, containing warm humid air, undergoes a process of auto compression as it travels up a ramp, shaft, or otherwise increases in elevation. The dry bulb temperature reduces to a point where the air mass reaches its water saturation limit and the excess water is precipitated as fog or airborne ice crystals. If the surface temperature is below 0 degrees Celsius, as is the case with all Arctic mines, frost or ice will begin to develop.
Ice and Frost Hazards
Many hazards are attributed to the presence of ice or frost due to poor heating and ventilation in underground mining operations. One hazard associated with mining in the Arctic is the potential for water in the shaft to freeze, ultimately disrupting hoisting operations and damaging shaft support members including cables and pipes. Freezing pipes in other areas of the mine, such as the water main, also poses as a problem, as well as the formation of large ice blocks within the exhaust, which act as a crushing hazard to workers and equipment upon falling. Care must also be taken when mining with specific methods, such as caving, which result in direct exposure to surface air. Upon entering the stopes, the cold air from surface can cause significant damage to the operation and equipment if not treated before entering the operation.
The heating of intake air is a common method of mitigating the adverse effects that Arctic climates pose. Multiple methods of heating air have been practiced within the mining industry, with the following methods being the most common:
- The use of steam coils operated by boilers that burn either wood, coal, fuel oil, or natural gas. Though this practice was once common, it has now been outdated as more environmentally sound practices have been established;
- The use of electric resistance heaters underground, though not as common in practice, as they are quite expensive to operate;
- The use of waste heat from compressor stations;
- The use of controlled recirculation to heat intake air. Although this method is effective, it must be temporary halted during blasting or the exposure to high concentrations of contaminants;
- The use of circulating glycol or heat pump systems to transfer heat from the exhaust to the intake when the two shafts are located close together. 
Though multiple methods of heating air currently are, or have been used within the mining industry, the most common method of heating air is through the use of natural gas and/or propane gas heaters . Due to natural gas being less expensive and more environmentally friendly, it is the preferred fuel option when operating ventilation heaters . Where natural gas is not available, as is the case in most Arctic mines, propane gas is the next viable option as it can be transported to the mine and stored on site in an industrial propane tank.
Rules of Thumb
Some general ventilation and heating tips and tricks to consider when operating a mine within in the Arctic are as follows:
- Downcast hosting shafts should have air heated to at least 5 degrees Celsius 
- Fresh air raises require a temperature of 1.5 degrees Celsius;
- The efficiency of heat transfer in a mine heater are as follows:
- 90% for a direct fired heater using propane, natural gas or electricity
- 80% for indirect heat transfer using fuel oil;
- When mine air is heated directly, it is important to maintain a minimum air stream velocity of approx. 2,400 fpm across the burners for efficient heat transfer. If the burners are equipped with combustion fans, lower air speeds (approx. 1,000 fpm) can be used;
- When mine air is heated electrically, it is important to maintain a minimum air stream velocity of approx. 400 fpm across the heaters, otherwise, the elements will overheat and eventually burn out .
Mining in arctic climates presents unique geotechnical challenges at underground mines. A major engineering consideration is the effect of permafrost (low temperatures?) on the rock matrix. In the arctic cold weather can affect ground conditions in two ways including permafrost, and seasonal frost. Permafrost is ground that remains permanently frozen. While seasonal frost is ground that thaws periodically. Permafrost typically occurs at areas with a mean annual temperature of 0oC, usually above the 55oN latitude. Seasonal freeze-thaw cycles present significant complications for the construction of roadways and surface infrastructure .
There are two failure categories associated with permafrost: 1. Shallow failures: Involving thawed soils 2. Deep-seated failures: involving frozen and thawed soils. Shallow failures are “skin flows”, involving the movement of the active layer or the zone of soil above the permafrost that freezes and thaws seasonally. They may occur in backfill areas. Deep-seated slides involve the movement of large masses in the form of relatively intact block materials.  Ground stability can be maintained through ground refrigeration, to ensure ground material remains frozen, or by preventing the flow of heat from mine workings into permafrost.
The extremely low temperatures present at Arctic Mines affects the characteristics of resin used in conjunction with rock bolts, as resin will not cure properly outside of its intended temperature range. Mines in cold environments sometimes utilize specially formulated resin for use in low temperatures. Such a case is found at the Svea Nrod longwall coal mine in Norway:
- Comparison made between existing 1.8 m long, fixed-head fully grouted bolts and 22 mm diameter, 2.4 m long pre tensioned KT bolts in conjunction with Lokset resin cartridge.
- Because the mine is situated below a glacier and in an area of permafrost, Minova had to draw on expertise to produce a specifically formulated resin for use in low temperatures. This was especially important for the cable bolts .
Engineering problems due to Permafrost in Placer mining
To access economic material, placer mining operations must melt permafrost. At most large placer mines in Alaska, permafrost is melted using water. Using water at ambient temperature is most effective and economic. Steam or hot water penetrates frozen ground more readily, however, they produce irregular bodies (“horses”) of unthawed ground in the contact region between bedrock and gravel, the location where gold is typically found.
Engineering problems due to permafrost in shaft sinking
In the Perchora coal mines of northern Russia vertical shafts are preferred to inclined shafts when excavating in frozen ground . Every effort should be made to preserve the original ground surface conditions when permafrost is close to the ground surface. If the surface is disturbed, accelerated thawing may occur, and thawing will follow the shaft causing damage to the structure. Permafrost integrity can be preserved by developing a collar shaft around the entrance and circulating cold air to preserve the frozen state of the surrounding ground. Furthermore, water lines should be insulated in mining operations with permafrost. The water lines produce heat and thaw the adjacent ground.
Tailings Storage Facilities
The most unique challenge associated with mining in the Arctic is the presence of permafrost which extends several hundred meters below the ground. Permafrost is thick and stable, so rather than attempting to remove or thaw it, engineers have developed solutions which utilize the stable characteristics of permafrost to design tailings storage facilities (TSFs) . As a result, Arctic dam designs rely heavily on the integration of permafrost to act both as an impermeable boundary between the tailings and the surrounding environment, and to provide a solid foundation for dams, dikes and infrastructure . The design of TSFs not only benefit from the presence of permafrost but also from the cold temperatures which significantly reduce the chemical reactions occurring within the contained tailings, such as sulphide mineral oxidation rate, also known as acid rock drainage .
Tailings Management in the Arctic possess several design challenges including :
- Extremely low temperatures in the winter and a very short, mild summer
- Permafrost degradation below mine waste facilities
- Settlement of structures
- Ice entrainment
- Ice within mine waste occupies a volume (melting can damage geosynthetics).
- Convective cooling of mine waste resulting in aggradation of underlying permafrost
- Geochemical weathering, low temperatures slow oxidation process
- Animal Migration
Since tailings storage facilities (TSFs) in the Arctic rely on permafrost to create an impermeable boundary, there has been increased concern on the long term effects of climate change on the stability and security of TSFs  . With the winter season shortening, areas which once remained in a constant deep freeze are beginning to thaw. The thawing of permafrost results in increased pore pressure, increased possibility of piping below the foundation and increased flow paths for subsurface flow/an increase in overall soil permeability. All of these factors have been seen to increase the potential for seepage and cause settlement which increases the risk of the dam over turning .
When permafrost and/or the cores of these frozen dams thaw, there is an increased risk of seepage and settlement. Sites including Mt. Nansen in the Yukon and the Red Dog Mine in Alaska have had major issues with contamination due to this uncontrolled thaw. In order to mitigate the risk that thawing poses, it is very important to monitor the internal temperature of dams through the installation of thermistors and to perform routine ground surveys .Dams in areas where permafrost melt is a risk have begun taking additional measures to insure the designs remain stable under non-permafrost conditions, this includes keying dams directly into bedrock .
Dams in the Arctic are typically built on bedrock or permanently frozen till. These dams rely heavily on permafrost to act as an impermeable boundary. Degradation of permafrost, a growing concern in the Arctic, will result in differential settlements of the dam as well as creating issues for seepage and stability. In order to ensure the permafrost remains frozen, cores and/or thermosyphons can be installed in to the dam to cool the foundation. Additional methods of increasing the stability of dams is through the keying of dams directly into the bedrock foundation.
The use of frozen core dams, as seen below, are very common in Arctic operations, such as the Ekati Diamond Mine. These dams require both the core and foundation to remain saturated in a frozen state even after mine closure . The frozen pore water provides both strength and impermeability to the core of the dam. Any zones of unfrozen ground, also known as taliks, must be identified and either avoided completely or frozen with thermosyphons. In order to ensure the core remains frozen in the long term, the dam and core should be constructed in the winter and extra fill material should be used to provide insulation. Thermal modelling can be done to predict the effects of climate change on the long term life of the dam, and if at any point during operation or post-closure, additional thermosphyons should be added. These dams require on going monitoring .
The below figure depicts a typical frozen core dam which has thermosyphons aligned vertically, similarly to the alignment of a picket fence, and can also be parallel to the ground surface. Each thermosyphon creates a localized frozen layer and when installed along the length or width of the damn, can create a frozen barrier in order to prevent the underlying bedrock from thawing. Examples of frozen core dams include, The Ekati Diamond Mine in the NWT and “Leslie Long Lake Frozen Core Dam” .
Mine Waste & Tailings Covers
One of the most important aspects of closing a mine waste storage facility is the design and construction of a cover system. Several different types of covers are used in the management of mine waste rock and tailings, including:
- Isolation covers;
- Barrier covers;
- Store-and-release covers;
- Water covers, and
- Insulation covers .
The purpose of a cover system may vary from mine to mine; however, the objectives of the system usually include chemical stabilization of acid-forming mine waste, dust and erosion control, contaminant release control, and a growth medium for vegetation . These covers help eliminate direct contact with mine waste by creating a physical barrier between the environment and contaminants.
Many covers are designed for temperate climates. As a result, the majority of covers do not take into account the issues that may arise in cold conditions, such as the Arctic. Insulation covers however, are a relatively new technology that are specifically designed to take advantage of colder climates. Two of the main purposes of insulation covers are to ensure that frozen mine waste remains frozen, and to increase the extent of permafrost into the mine waste . This is achieved by covering the tailings with the following methods:
- Using a thick layer of inert waste rock to contain the active layer or
- Using a thin layer of inert rock, underlain by a fine-grained high water content layer .
Fine-grained materials are preferred, as they tend to remain saturated with water, providing more insulation per unit depth. When fine-grained material is not available, esker material is commonly used. There are several mines in North America that have constructed or plan to construct insulation covers, including but not limited to Meadowbank, Cullaton Lake, Diavik, Ekati, Lupin, Nanisvik, and Raglan mines . The below figure shows different insulation cover designs used in three different Canadian mines .
Design Considerations for Cold Regions
There are several factors that contribute to the design of cover systems, regardless of the location; however, there are several design considerations that are exclusive to cold regions:
- Permafrost degradation below mine waste facilities: Placing a mine waste facility on permafrost could alter the ground temperature and pressure, leading to permafrost degradation.
- Ice entrainment: Ice can occupy volumes within the storage area that could otherwise be occupied by waste rock or tailings.
- Other frozen ground features within mine waste: It is possible that massive ice lenses may grow, mounds of earth-covered ice may form, and cracks may cause the formation of ice wedges.
- Convective cooling of mine waste: Tailings dams constructed from coarse rock will be subject to convective cooling. Convective cooling could help facilitate permafrost aggradation, however it could also result in ground processes that could have negative effects on cover performance.
- Geochemical weathering: Cold temperatures could help control AMD, however other measures besides cold temperatures should be taken to control AMD .
Frozen Ground Effects on Cover Performance
- Freeze-thaw effects: Freeze-thaw cycles are known to increase the permeability of soil and rock.
- Frost susceptibility: If any of the following or a combination of the following were to occur, the cover system may freeze if the material used is susceptible to frost, if there is enough water available and/or if there is sufficient cooling.
- Migration of fines through covers: It is possible for fine tailings to rise through the cover .
Hydrological Effects on Cover Performance
Hydrology in cold regions is complex and must be taken into consideration when examining cover performance. A large amount of infiltration and runoff can occur during the snowmelt season. The amount of runoff is relatively high in cold regions, as permafrost eliminates natural groundwater inflow. As a result, systems should be designed to divert runoff away from the cover system . In addition, groundwater flows travelling through interflow drains have the potential to form sheet-like masses of layered ice . In order to prevent this from occurring, interflow drains should be monitored to ensure no blockage is occurring.
A major concern attributed to mining in the Arctic is the dangerous wildlife that inhabits the project’s land. Though wildlife is notoriously overlooked, it is a major factor to consider when designing infrastructure and health and safety protocols.
Typical Arctic wildlife includes deer, foxes, caribou, bears, reindeer, wolves and a variety of birds. Although most of the Arctic species listed pose little threat to the safety of employees, numerous cases of bear encounters have drawn attention to the need of greater infrastructure and policies and procedures at mine sites, which address such issue . Bears are of greatest concern to mining operations, as they are easily drawn to the site by food or edible garbage and once provoked, can have potentially dangerous defensive or predatory behavior. With most bears weighing in at approximately 200 kilograms, encounters with bears are a difficult scenario to walk away from unharmed .
Methods to mitigate the exposure to dangerous wildlife, particularly bears, include the installation of fences surrounding the property perimeter, the use of armed guards to escort employees on the property, the use of radio communications to warn employees of any potential hazards, and the installment of proper waste disposal and food storage facilities . As well, proper warning signs and procedures if an employee were to encounter a bear should be made visible on site with the accompaniment of wildlife awareness and safety training.
- Government of Canada, Natural Resources Canada (2016). Raglan Mine - Quebec.Retrieved from Natural Resources Canada: http://www.nrcan.gc.ca/mining-materials/publications/aboriginal/bulletin/8814 .
- R Noble (2013). Canadian Mining Journal. Two of Canada's more Isolated Mines Continue to Impress
- Murchinson, P. (n.d.), A Bumpy Road: Highways and Thawing Permafrost, Retrieved February 2016.
- ,Transportation Association of Canada. (2010, June). Developing and Managing Transportation Infrastructure in Permafrost Regions. Retrieved from Transportation Association of Canada: http://tac-atc.ca/sites/tac-atc.ca/files/site/doc/resources/primer-permafrost2010.pdf.
- Tumbde, A., & Caldwell, J. (2011, December). Ventilation. Retrieved February 2016, from InfoMine: http://technology.infomine.com/reviews/ventilation/welcome.asp?view=full
- De Souza, E. (2016, February 2). Mining in the Arctic. (Q. U. Student, Interviewer) Canada.
- De Souza, E., & Wilson, A. (2015). Arctic Mine Air Heating. Air Finders Inc.
- American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (2015). 2015 ASHRAE Handbook - Heating, Ventilating, and Air-Conditioning Applications (SI Edition). United States of America: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
- De Souza, E. (2016, February 2). Mining in the Arctic. (Q. U. Student, Interviewer) Canada.
- Wandinger, M. (2006). Water-related issues in permafrost mining, with special emphasis on the coal mining in Spitsbergen (Norway). Muchen: International Mine Water Association.
- Lindholm, M. (2014). Advanced Topics in Mine Water. CMS Project.
- Journeaux Assoc. (2012). Engineering challenges for Tailings Management Facilities and Associated Infrastucture with regard to Climate Change in Nunavut. Goverment of Nunavut.
- Wills, B. A. (2016). Wills' Mineral Processing Technology - An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery (8th edition). Oxford: Elsevier Ltd.
- Hockley, D and Rykaart, M. (2009). Mine Waste Covers in Cold Regions. MEND Report 1.61.5a. Retrieved from: http://mend-nedem.org/wp-content/uploads/2013/01/1.61.5a.pdf .
- O’Kane, M and Wels, C. (2003). Mine Waste Cover System Design – Linking Predicted Performaance to Groundwater and Surface Water Impacts. In proceedings of the Sixth International Conference on Acid Rock Drainage, Cairns, Queensland, Australia, pp. 341-350. Retrieved from: http://www.okc-sk.com/wp-content/uploads/2012/02/MineWaste.pdf.
- Journeaux Associates. (2012). Engineering Challenges for Tailings Management Facilities and Associated Infrastructure with regard to Climate Change in Nunavut. Report L-11-1472. Retrieved from: http://climatechangenunavut.ca/sites/default/files/tailings_final.pdf.
- McIntosh, J. (2014, May 08). Suncor attack: Why black bears kill and safety in the field. Retrieved February 02, 2016, from CBC News - Calgary: http://www.cbc.ca/news/canada/calgary/suncor-attack-why-black-bears-kill-and-safety-in-the-field-1.2636687