8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure 1

8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure

8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure

 

What is a Cover System?

A cover system is a type of engineered barrier used to manage mine waste during operations or at mine closure.  A cover system may be constructed using naturally available materials, such as locally available soils and mine waste with appropriate physical and chemical properties. However, they may also be constructed from a combination of natural and synthetic materials, with synthetic materials providing physical properties that natural materials alone cannot.  A cover system’s main purpose is to provide a reclamation surface for mine waste storage facilities (e.g. waste rock dumps, tailings storage facilities) that provides a stable, reliable, and sustainable interface between the receiving environment and the mine waste (Figure 1)[1].

Figure 1:     Conceptual schematic of a cover system.

INAP, 2017[1].

Design objectives for cover systems can vary, however common objectives are typically developed to address physical stability and chemical stability. Physical stability of a cover system is related to the overall landform design and may address concerns related to surface water management, dust and erosion control. Cover designs can also address chemical stability whereby water movement through a cover system and into stockpiles of reactive mine rock is to be minimised, leading to reductions in contaminant loadings into the receiving environment.  We refer to the water movement through a cover system and into the underlying waste material as “Net Percolation”1.

Mining landscapes can be described at several scales: Region, Landscape, Landform, Landform Element and Microsite[2] (Figure 2).  A cover system design falls within the Landform Element scale and is an integral component of waste stockpile design, for example, waste rock dumps and tailings storage facilities.  Therefore, integration of cover system and landform objectives is critical, and both sets of objectives must contribute towards achieving the agreed returning land use.

Figure 2:     The mining landscape.

LDI, 2021[2].

How Can a Cover System Help You Minimise Your Environmental Impact?

Cover system functions and types are broad, and selection depends on a site’s climate and the environmental impact(s) requiring mitigation.

There are six general cover system functions, some of which include several variations for achieving the intended function (Figure 3)[1]:

  1. Reclamation/Revegetation;
  2. Erosion Protection;
  3. Store and Release Cover;
  4. Enhanced Store-and-Release Cover
    • Capillary Break Layer;
    • Seasonality Frozen Capillary Break Diversion (SFCBD);
  5. Barrier (Ksat ≤ 1 x 10-7cm/s);
    • Compacted Clay Layer (CCL);
    • Compacted Sand Bentonite (CSB);
    • Permanent Frozen Layer (or Permafrost Aggradation Layer);
    • Geosynthetic Materials; and
  6. Saturated Soil or Rock.

Figure 3:     Cover system function; a) Reclamation/revegetation; b) erosion protection; c) store-and release and enhanced store-and-release; d) enhanced store-and-release – capillary break; e) barrier; f) saturated soil or rock.

INAP, 2017[1].

Cover system designs may incorporate multiple functions.  For example, it is very common to have reclamation/revegetation, erosion protection, and store and release cover functions coupled with a primary objective, such as minimizing contaminated seepage.  In this example the cover design does not solely focus on minimising contaminated seepage but also captures the reclamation/revegetation and erosion functions providing a more robust solution. The overall solution establishes the target vegetation community and achieves acceptable erosion rates, which in turn have positive impacts to the cover systems ability to mitigate contaminated seepage.

Okane’s Approach

Okane’s approach to designing cover systems for mine waste has been developed over 25 years working on mine sites spanning the globe across all climate regimes. Unlike other geotechnical consultants we integrate mine closure with mine planning to deliver optimized cover system design solutions for our clients at all mining stages.  Over the years Okane has developed the following eight key factors that will ensure the success of your cover system:

  • Set clear returning land use expectations;
  • Define realistic cover system objectives;
  • Know your site and material characterisation;
  • Align your landform design and cover system design;
  • Watch out for your basal flow conditions;
  • Look at long term surface water management;
  • Validate the availability of desired cover material; and
  • Consider constructability.

Okane is the industry leading expert in cover system design and performance monitoring. Our multidisciplinary team can help you design, construct and monitor cover systems that will complement your final landform designs, and help you achieve your mine closure plans.

[1] International Network for Acid Protection (INAP), 2017.  Global Cover System Design--Technical Guidance Document.  November.

[2] Landform Design Institute (LDI), 2021.  Mining with the end in mind: Landform design for sustainable mining.  Position Paper 2021-01.  March.  Landform Design Institute.  Delta, BC, Canada.


Reclassification and Decommissioning of Tailings Storage Facilities 2

Reclassification and Decommissioning of Tailings Storage Facilities

Reclassification and Decommissioning of Tailings Storage Facilities

 

Tailings Storage Facilities are widely considered the most difficult structures for mines to decommission at closure. They are also typically the structure with the highest consequence to failure when considering economic effects, environmental impacts, and community consequences. From a geotechnical stability perspective, an ideal outcome for long term closure of a Tailings Storage Facility is to have it reclassified as a mine waste structure, such that it no longer retains flowable liquid.

Historical Tailings Practices

Tailings Storage Facility design, construction, maintenance, and closure have gone through a number of changes and geotechnical advancements in the past century.  The triggers for this have, unfortunately, largely been reactionary following past catastrophic Tailings Storage Facility dam failures (e.g., Brumadinho, Brazil 2019, Stava Dam, Italy 1985, El Cobre, Chile 1965).

According to a 2019 research article, on average three of the world’s 3,500 tailings dams fail every year.[1] Mining is a temporary land use, so Tailings Storage Facilities were never meant to be permanent structures on the landscape. Ensuring they can be properly decommissioned helps deleverage the geotechnical, environmental, and economic risk operators hold into closure.

New Tailings Standards

The Global Industry Standard for Tailings Management (GISTM) [2] is one of the globally recognized standards created to provide a framework for safe tailings management over all phases of a Tailings Storage Facility’s lifecycle. These phases include:

    1. Project conception, planning and design;
    2. Initial construction;
    3. Operation and ongoing construction (may include progressive reclamation);
    4. Interim closure (including care and maintenance);
    5. Closure (regrading, demolition and reclamation); and
    6. Post-closure (including relinquishment, reprocessing, relocation, removal).

Proactive actions adopted throughout these phases can help Tailings Storage Facility operators increase the likelihood of obtaining decommissioned status for their facilities. This, in turn, will reduce (or eliminate) the need for long term care and maintenance and minimize liability and risk into post closure.

Some proactive measures that can be progressively adopted over a Tailings Storage Facility lifecycle include:

    • Selection of the proper construction method and design criteria of impoundment structures;
    • Consideration to alternative tailings product in operations (e.g., conventional, thickened, paste, dry stack, etc.);
    • Tailings dewatering to reduce liquefaction potential;
    • Progressive reclamation to control water infiltration; and
    • Integration of the Tailings Storage Facility closure plan into the Life of Mine plan.

Tailings Dam Reclassification Guidelines

The Canadian Dam Association is working on revised guidelines to provide guidance on the reclassified of tailings dams as a mine waste structure[3].

A barrier that previously impounded tailings is considered a mine waste structure if it meets all the following criteria:

    1. Ponded water will not propagate a failure or uncontrolled release of contents;
    2. Contents do not and cannot flow (i.e., are not fluid like) and do not rely on a barrier structure to prevent an uncontrolled release;
    3. Contents do not and cannot pipe through the structure or foundation; and
    4. Conditions will not develop in the future that could violate the previous three criteria.

The ability to limit the phreatic surface within a Tailings Storage Facility is one important facet when working towards these draft Canadian Dam Association criteria. In this decommissioning scenario, all fluid from the Tailings Storage Facility would be removed and tailings would be drained. The remaining solid tailings and impoundment structures could then be reclassified as a solid waste structure.

Okane’s Approach

Okane works closely with our clients at all phases of a Tailing Storage Facility lifecycle to evaluate technical solutions for tailing operations and closure alternatives.  There are many factors that influence which decommissioning approach is suitable for a Tailing Storage Facility. These factors may include the volume and geochemistry of impounded fluid, tailings composition, characteristics of groundwater and surface water at site. They also include future mine schedules and plans, returning land use objectives, reclamation efforts completed to date, community and stakeholder’s interests, as well as current regulatory requirements and those that were in place at the time of construction.  We have worked with our clients to design transition plans from liquid tailings to dry stack tailings and comingled disposal with mined rock.

Okane’s approach is to apply site-specific closure objectives and risk profiles to evaluate a site’s optimized Tailings Storage Facility operations and decommissioning scenario. Our team of geotechnical engineers apply expertise from tailings closure projects around the world and tailings operations experience to design solutions that achieve both physical and geochemical stability. This methodology has been adopted by many operators to practically manage the geotechnical, environmental, and economic risks of Tailings Storage Facilities as well as to provide a clear pathway to decommissioning and closure.

 

 

[1] Lyu, Z., Chai, J., Xu, Z., Qin, Y., & Cao, J. (2019). A Comprehensive Review on Reasons for Tailings Dam Failures Based on Case History. Advances in Civil Engineering, 2019. https://doi.org/10.1155/2019/4159306

[2] International Council on Mining and Metals (ICMM), United Nations Environment Programme (UNEP) and Principles for Responsible Investment (PRI). (2020). Global Industry Standard on Tailings Management. August 2020. https://globaltailingsreview.org/global-industry-standard/

[3] Alberta Energy Regulatory Manual 019: Decommissioning, Closure, and Abandonment of Dams at Energy Projects. Published January 2020. https://static.aer.ca/prd/documents/manuals/Manual019.pdf


Climate Change Resiliency and Adaptation 3

Climate Change Resiliency and Adaptation

Climate Change Resiliency and Adaptation

Climate change resiliency must include the capacity for adaptation. Adaptation put simply is to respond, to reassess and to restore. To differentiate climate adaptation as part of policy framing from adaptation that occurs in ecosystems, climate adaptation involves deliberate responses to climate change. A resilient framework involves a systems-based approach to adaptive processes to respond to environmental changes .

Managing Climate Change Risk in the Mining Industry

Mine operators and owners aspire to be industry leaders in sustainable mining, which includes a commitment to conducting activities ethically and transparently. Most climate change policy focuses on reducing future greenhouse gas (GHG) emissions, and net-zero targets. Achieving these commitments requires transparency to the site-specific activities, infrastructure, equipment, data, and information used in the analysis.

Too frequently, challenges arises when there isn’t sufficient ‘line-of-site’ from corporate climate change strategies to site-based performance. Achieving the outcomes driven by corporate strategic initiatives requires clear communication of purpose and action at all levels of a mine site’s operations, from the board room to a haul truck driver on shift.

To effectively manage climate risk, it is also critical that climate change policies consider the adaptive capacity of operating and legacy sites. This requires a deliberate approach to understanding, assessing, and responding to the potential impacts of climate change at a site-specific level.

Adaptation for Climate Change

When incorporating climate change scenarios into designs, Okane starts at the site level. Okane uses the Köppen-Geiger climate classification system to help create a deeper conceptualization of climate conditions on a site-specific basis. It is important to remember that the world, and climate do not function on the average. As we like to say at Okane “The average does not exist!”.

If we think about a weather forecast, we do not decide to wear a coat or not based on the average. We make these decisions based on what the highs and lows are. The Köppen-Geiger classification includes seasonality, whether that be precipitation or temperature, which automatically getting us to stop thinking about ‘the average’. This approach gets us into the right thought space, which can then be adjusted based on orographic, slope, and climate change effects. Applying the Köppen-Geiger climate classification system helps develop a clear conceptualization of what is possible and helps set up a robust numerical modelling program.

Which Climate Change Scenario?

The advice most often given with respect to managing risk is to ‘be conservative’. For modelling climate change, that is often interpreted as using RCP8.5 scenario . Interestingly, the RCP8.5 scenario is not always the conservative approach!

Okane takes the approach of understanding the probability for each RCP scenario at a given location and site. Modelling scenarios by applying numerical models to evaluate geomorphic, geotechnical, and geochemical stability, ensures Okane develops an understanding of the consequence effects of each scenario. Only when probability and consequence are brought together can you properly inform on risk.

For example, simulating the RCP8.5 scenario may seem conservative with respect to water quality risk, but this RCP scenario will generally add more water to the watershed than other scenarios. This changes the assimilative capacity of the watershed, not necessarily by volume but by changes in intensity and duration. In this scenario, designs based on RCP8.5 may not protect the surrounding environment.

‘Base Case’ Climate Database for Modelling

An all-too-common approach is to develop a climate database based on average climate conditions. Remember though, “The average does not exist!”.

At Okane, we encourage using as long a climate database as possible (ideally 100 years) and modelling each year within that database. With this approach, performance can be statistically evaluated based on the model output, rather than the average climate year. In this way, the influence of watershed storage capacity and antecedent conditions influence each subsequent season or year modelled.

Beyond the above, our approach is to include an RCP scenario representing the base case climate database (i.e., historical data adjusted for the RCP scenario), and to simulate alternate climate change scenarios as part of sensitivity analyses to comprehensively evaluate consequence effects and inform on risk.

What is Downscaling?

General circulation models, also known as global climate models, or GCMs, are mathematical models that predict future climate at a large scale. Large scale being defined as 80 to 300 km. While this does provide some insight into climate change on regional scales, it is too coarse for site-specific climate databases. Features that determine local climate phenomena, such as orographic effects, will not be accounted for by the large scale of GCMs.

GCM output must be downscaled, either through dynamic downscaling or statistical downscaling. Okane use the latter method, which is a two-step process consisting of:

i) The development of statistical relationships between local climate variables (e.g., surface air temperature and precipitation) and large scale predictors (e.g., pressure fields); and

ii) Application of statistical relationships to the output of GCM experiments to simulate local climate characteristics in the future.
This method allows for creation of site specific climate change databases that are based on local historical climate.

What Next?

Developing climate change resiliency requires more than corporate policy on GHG emissions. It requires a site level understanding of probability and consequence to inform on risk, and to achieve true adaptive management. Give us a call, shoot us an e-mail, or catch us on LinkedIn.

We would be happy connect with you and talk climate change resiliency and adaptation!

 

References

Nelson, Donald R.; Adger, W. Neil; Brown, Katrina (2007). "Adaptation to Environmental Change: Contributions of a Resilience Framework". Annual Review of Environment and Resources. 32: 395–419. https://www.annualreviews.org/doi/abs/10.1146/annurev.energy.32.051807.090348

Schwalm, Christopher R.; Glendon, Spencer; Duffy, Philip B. (2020-08-18). "RCP8.5 tracks cumulative CO2 emissions". Proceedings of the National Academy of Sciences. 117 (33): 19656–19657. doi:10.1073/pnas.2007117117. ISSN 0027-8424. PMID 32747549. https://www.pnas.org/content/117/33/19656