The Challenges of Cold Region Mine Closure

Mining generates necessary material for everyday life across our globe. As a global industry, there are mines located in the most remote areas of our planet from scorching sites in Australia, to the frigid tundra of Alaska, Nunavut, Yukon, and the Northwest Territories. The process for developing sustainable mine closure plans and constructing effective cover systems in cold weather regions that reduce the potential impacts of mine waste has proven to be fundamentally different than for temperate regions. Compared to milder regions, cold weather mine closure requires meticulous planning and experience to do so responsibly and sustainably.

Attributes of a Cold Weather Region

Cold regions are geographic locations around the world that experience extreme climate conditions, prolonged cold being the most significant, and where the temperature, snow depth, ice cover on lakes, and depth of ground freezing is similar (Andersland and Ladanyi, 2004). In terms of cover systems and performance, a cold weather region is considered anywhere that experiences sufficient ground frost to affect design stability and effectiveness.

An additional attribute to consider when defining a cold weather region is the extent of ground freezing. There are regions where ground frost only occurs seasonally, and others where permafrost exists. When planning for cold weather mine closure, a thorough analysis of the ground frost patterns must be considered to effectively engineer a sustainable cover system.

Challenges of Cold Region Mine Closure

Cold region mine closure presents many challenges for mine operators and changes requirements and processes. The most substantial challenge when handling cold weather mine closure is the natural freezing and thawing cycle. Freeze/thaw cycles can make it difficult to maintain an effective cover system over extended periods and requires an exceptional understanding of specific climate conditions in the area.

Another challenge that cold weather mine closure presents is limited infrastructure and availability of materials. It’s common for cold weather mines to be remote, and as a result can make it difficult to acquire the materials needed to build effective cover systems and landforms. The transport of both material and labour can also pose a challenge, as some sites may only have seasonal accessibility via ice roads or access roads with seasonable road bans or weight restrictions. In addition, extreme temperatures can create hazardous working conditions and cause strain on equipment.

Another challenge associated with cold weather mine closure is that on a cold site, water flow partitioning is different to that at a temperate site. Glacial melt and snowmelt create much higher rates of runoff requiring management by site water management infrastructure. Frozen soil causes runoff to increase, potentially resulting in the erosion of cover systems in place, reducing the cover systems’ storage capacity over time.

Additionally, the presence of permafrost can be hard on excavation equipment, which can lead to higher expenses and extra time on the project. Further, digging into permafrost can damage it, causing it to melt and increasing liquid water content of soil. This melting and damage to permafrost can ultimately cause permanent changes to the landscape of the site.

Cold Regions Cover Systems

As previously discussed, cold region mine closure poses many challenges compared to mine closure in temperate regions. Each cover system design must be adapted to function with site-specific conditions and account for varying levels of ground frost. Outlined below are some considerations when designing a cold weather region cover system.

Water Cover Systems

Maintaining water cover systems in cold weather regions pose challenges when it comes to maintaining their effectiveness. Freezing conditions make it unlikely this type of cover system will perform as required and effectively mitigate hazardous materials consistently. In cold weather regions, it is generally recommended to avoid this type of cover system.

Erosion-Protection Cover Systems

For sites where erosion and dust control are an issue, an erosion-protection cover system is generally considered. In this case, the goal is to create a stable landform that will reduce contaminant-containing particulates from being transported off the landform.

In cold weather regions, the method of developing a vegetative layer to inhibit erosion may not be practical. Cold weather creates an environment difficult to sustain plant-life due to a shorter growing season. To create an effective layer of vegetation for an erosion-protection cover system, some options to consider are:

• Include a thicker layer of topsoil that creates a thicker rooting zone and enhances storage capacity.
• Use a soil amendment to add soil nutrients and provide an initial layer of rapid establishing vegetation to stabilize the surface and provide appropriate conditions for establishment of native vegetation.
• Use mulch material to reduce erosion effects until vegetation can develop.

The method of placing a layer of gravel or riprap over waste material is still a viable option for an erosion-protection cover system; however, the material should be appropriately coarse.

Store-and-Release Cover Systems

The risk of using a standard store-and-release cover system in cold weather regions is that the materials available for construction generally have lower storage capacities. This is a significant challenge when using this type of cover system because limited storage capacity can increase net percolation, and frozen soil causes an increase in runoff, which creates higher water input for water management infrastructure.

Enhanced Store-and-Release Cover Systems

An enhanced store-and-release cover system is designed much like a standard store-and-release cover system, but additional layers are included to reduce net percolation. This type of cover system is generally utilized when the site experiences shorter seasonal events, in which case a standard store-and-release wouldn’t be unlikely to meet storage capacity needs.

Additional layers that may be included in an enhanced store-and-release cover system are:

• Capillary break layer;
• Seasonally frozen capillary break diversion;
• Low permeability layer.

Addition of these layers will create higher storage retention in the soil above. Once enough vegetation is established and the lower layers eventually thaw, the layers will act as a store-and-release cover system during the summer.

Barrier-Type Cover Systems

Standard barrier-type cover systems primarily use a compacted clay (CCL), compacted bentonite (CSB), or permafrost layer. The main challenge when using a CCL or CSB type layer in a cover system in cold weather regions availability of these materials. Additionally, a CCL is susceptible to freezing/thawing and requires a substantial thickness of overlying material to reduce the propensity for layer breakdown and resulting reduction in performance. A CSB layer is also not recommended for cold weather regions, however the benefit compared to a CCL layer is the material is able to maintain low rates of hydraulic conductivity even after it has thawed due to its greater swelling capacity.

A seasonally frozen capillary barrier is one of the more useful types of cover systems in cold weather regions. This type of cover system uses frozen soil to its advantage by diverting water runoff downslope, away from the underlying mine waste. This type of cover system is most often used in regions where ground freezing occurs year-round.

Okane’s Approach

As leading experts of cold weather mine closure, we have over 25 years of experience implementing closure plans in cold weather regions. In 2012, Okane was a direct contributor in creating the Mine Environment Neutral Drainage (MEND) Cold Regions Cover System Design Technical Guidance document that is referenced industry wide.

When designing cover systems for cold regions, our team of experts carefully analyze the weather of the area, paying close attention to the freezing/thawing cycles, climate change scenarios, and the potential duration of seasonal events. Before construction begins, we conduct a thorough Failure Modes and Effects Analysis (FMEA) to understand any potential risks to the designs and develop risk mitigation plans accordingly. Once construction begins, we provide onsite support throughout the entire process for quality assurance. We continuously monitor cover systems to ensure they are meeting the site-specific closure objectives and maintaining effectiveness through closure and post-mining land use.

References

Andersland O.B. and Ladanyi, B. 2004. Frozen Ground Engineering. John Wiley & Sons.

MEND. 2012. Cold Regions Cover System Design Technical Guidance Document. http://mend-nedem.org/wp-content/uploads/2013/01/1.61.5c.pdf