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 (K_sat ≤ 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.