8 Factors to Consider when Designing Mine Cover Systems During Closure

Summary

• A cover system is a stable engineered barrier/interface between mined materials and the surrounding environment for reclamation and closure of mine waste storage facilities.
• Key factors to consider when designing a cover system include: climate; post-mining land use expectations; landform and cover system objectives; material characterization and availability, long-term surface water management and monitoring program; and constructability.
• Okane takes a risk-based approach that combines mine and closure planning to develop a cover system that not only helps sites meet their operational and closure objectives, but also achieves the agreed post-mining land use.

What is a Cover System?

A cover system is an engineered barrier to prevent water and/or oxygen from interacting with mined materials, which can change its composition and/or result in significant impacts, such as acid and metalliferous drainage (AMD).

A cover system is used to manage mine waste storage facilities during operations or at mine closure.

You can build a cover system using naturally available materials, such as locally available soils and mine rock (with appropriate physical and chemical properties).

However, you can also build a cover system from a combination of natural and synthetic materials. Synthetic materials can provide 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. mine rock stockpiles, tailings storage facilities). This surface provides a stable, reliable, and sustainable interface between the receiving environment and the mined materials (Figure 1)[1].

Figure 1: Conceptual schematic of a cover system. INAP, 2017[1].

What are the objectives you need to focus on when designing cover systems?

Design objectives for closure can vary, based on:

• post-mining land use (PMLU) plans for each mine site domain (e.g., mine rock stockpile, tailings storage facility, leach pads, or open pits);
• water quality criteria;
• overarching operational objectives; and
• regulatory requirements (e.g., cover system performance assessment timeframes).

In collaboration with the International Network for Acid Prevention (INAP), Okane developed the Global Cover System Design. This technical guidance document summarizes best practices for mine operators in understanding, evaluating, designing, implementing, and monitoring cover systems for their site.

Okane has also developed a Cold Regions Cover System Technical Guidance Document, in collaboration with Mine Environment Neutral Drainage (MEND) and Aboriginal Affairs and Northern Development Canada (AANDC). This guidance document outlines key attributes of cold regions and addresses potential challenges of cover system design in these climates.

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 system designs can also address geochemical stability whereby water movement through a cover system and into stockpiles of reactive mine rock is minimized, leading to reductions in contaminant loadings into the receiving environment.

Note: We refer to the water movement through a cover system and into the underlying waste material as “Net Percolation”.

Mining landscapes and cover systems

Mine landscapes also need to be considered when determining what type of cover system the mine needs.

Mine landscapes can be classified at different scales (Figure 2)[2]:

• Region: All operating mine sites in a valley or region.
• Landscape: A single mine property.
• Landform: A distinct topographic feature constructed through mining activities, such as a mine rock stockpile, open-pit, or tailings storage facility.
• Landform element: A single feature on a landform to allow the landform to meet its overall design objectives (e.g., drainage swale).
• Microtopography (or microsite): Small-scale modifications typically implemented to support biodiversity and functionality(e.g., woody debris, surface roughening, mounding, boulder placement).

Integration of cover system and landform objectives is critical, and both sets of objectives must contribute towards achieving the agreed PMLU. The agreed PMLU(s) are typically identified through consultation with rightsholders, local communities, and regulatory bodies (INAP, 2017).

Figure 2: The mining landscape. LDI, 2021[2].

How can a cover system minimize environmental impact?

All cover system types depend on the agreed upon PMLU, site-specific climate conditions, material availability and characteristics, and performance criteria to meet the site’s operational and closure objectives.

You can minimize your environmental impact in six ways.

1. Reclamation/Revegetation: Cover systems that promote vegetation growth can be used to minimize erosion risks and support natural ecosystems like creating wildlife habitat, restoring native plant species, promote agricultural opportunities, and provide recreational spaces and aesthetically pleasing landscapes (INAP, 2017). It can also intentionally limit invasive vegetation species that competes and hinders the growth of targeted vegetation (Okane, 2024).
2. Erosion Protection: Some sites may only require cover systems that control dust emissions and minimize erosion risks on their site. In this case, erosion protection cover systems protect landforms and cover systems from eroding and promote landform stability.
3. Store and Release Cover: This type of cover system limit net percolation by storing water during precipitation events and then gradually releases the stored water through evaporation or transpiration. It minimizes water available for oxidation of the sulfide minerals within mine rock. Store-and-release cover systems help prevent rapid contaminant loading and the subsequent impacts of AMD generation.
4. Enhanced Store-and-Release Cover
   • Capillary Break Layer: A coarser-textured and unsaturated material can be incorporated below the finer-textured ‘store-and-release’ material to enhance the cover system’s water holding capacity. This helps enhance evapotranspiration and reduces further net percolation. Capillary break cover systems can also be used to mitigate the upward migration of contaminants from mined materials into the cover system and subsequent vegetation
   • Seasonality Frozen Capillary Break Diversion (SFCBD): This enhancement involves a layered system with an active, well-drained surface layer on top of the layer with higher water content (due to its finer texture). This system allows lateral movement to take place and, thus, reduces further net percolation into the landform.
5. Barrier (K_sat ≤ 1 x 10^-7cm/s);
   • Compacted Clay Layer (CCL): CCLs can be used to decrease hydraulic activity or manage oxygen ingress to exposed materials at acceptable levels (INAP, 2017).
   • Bentonite Amended Cover System: If the cover system objective is to minimize water infiltration but no finer-textured materials are available, then a barrier with bentonite can be used (MEND, 2012). Amending soils with bentonite is known to increase storage capacity and reduces hydraulic conductivity.
   • Permanent Frozen Layer (or Permafrost Aggradation Layer): In permafrost climates, it is possible to take advantage of frozen material as a barrier layer, especially if the frozen material stays at a high water content (MEND, 2012).
   • Geosynthetic Materials: Incorporating geosynthetic materials, such as geosynthetic clay liners, bituminous geomembranes, or polypropylene, can significantly minimize net percolation and oxygen ingress (MEND, 2012).
6. Saturated Soil or Rock: Saturated cover system isolates reactive mined materials by maintaining them under a water table using an inert, coarse-textured material as the cover  (INAP, 2017).This limits oxidation and acid generation while managing water fluctuations to prevent exposing the reactive materials.

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 minimizing 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 storage facilities has been developed for 30 years working on mine sites spanning the globe across all climate regimes. We integrate mine closure with mine planning to deliver optimized cover system design solutions for our clients at any phase of the mine lifecycle.

Our interdisciplinary team has developed the following eight key factors that can enhance the performance of your cover system:

1. Set clear PMLU expectations:

1. • Ongoing collaboration with mine operators, rightsholders, community members, and regulators can help with identifying post-mining land use plans. Mine material management should align with land use capabilities and closure objectives (INAP, 2017).

2. Define realistic cover system objectives:

1. • Closure conceptualization builds the knowledge base and promotes realistic expectations of cover system performance. Long-term landform and cover system performance should be evaluated under site-specific conditions and processes to avoid unrealistic expectations of longevity (INAP, 2017).

3. Know your site and material characterization:

1. • Material texture is the first component to material characterization.
   • Particle size distribution informs hydraulic conductivity and water retention capacity of the cover system material (INAP, 2017).

4. Align your landform design and cover system design:

1. • Landform design is critical for designing cover systems, and both landform and cover system designs are dependent on the site’s closure objectives.
   • Landform design should prioritize geotechnical stability, which is a priority for all closure regulations. This can be achieved by aligning landform with slope, shape, and surface water management criteria (INAP, 2017).

5. Watch out for your basal flow conditions:

1. • Understanding how catchment run-on and lateral groundwater seepage enter the mine domain is important for evaluating cover system performance (INAP, 2017).

6. Look at long term surface water management:

1. • Surface water management structures are essential to prevent erosion features, such as rills, gullies, stream channels, and mass wasting and debris flows, from forming (INAP, 2017).
   • Poor surface water management and landform instability are common causes of cover system failure worldwide (INAP, 2017).

7. Determine the availability of desired cover material:

1. • Availability of potential cover material is important. This involves assessing and understanding their characteristics, including volume, suitability, variability, access, and ownership (INAP, 2017).

8. Consider constructability:

1. • Construction matters. For example, top-down cover system construction may leave the bottom slopes exposed to runoff, while bottom-up construction may result in cover system materials being dislodged or washed away during cover placement of the lower layers (INAP, 2017).
   • Poor cover system construction quality assurance and control can lead to preferential pathways for advective fluxes of oxygen into mine domains, which can result in generating acid and metalliferous drainage (INAP, 2017).

Read more about how Okane has applied our expertise in cover systems to support varying project objectives, regulatory requirements, risk profiles, and site-specific conditions around the world:

• Increasing TSF Capacity Through Cover System Design (Australia)
• A Cover Design Optimized for Haulage, Cost, and Revegetation (Australia)
• Constructible Cover Systems at the Faro Mine Remediation Project (Canada)
• Dust Management through Closure Landform Construction (Canada)
• Covering for the Future (Finland)
• Adjusting Covers for Climate Change (U.S.A.)

Curious about which cover system type is the right place to start? Explore the possibilities with the Okane Cover System Calculator!

References

[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.