Designing heap leach facilities for closure from feasibility

Active drain down can become a significant component of heap leach closure cost. SRK Consulting specialists Jeff Parshley, Heather Thomson and Bronwen Forsyth discuss how early water-balance modelling, cover trials, materials planning and operational data can reduce uncertainty in closure design. 

For much of the mining industry’s history, heap leach closure has been treated primarily as an end-of-life activity. A facility would be designed, constructed and operated, with detailed closure planning deferred until mineral recovery was approaching its end. 

That approach is changing. Closure planning is now embedded across most Australian jurisdictions as a regulatory expectation from project inception, with Western Australia’s Mine Development and Closure Proposal (MDCP) and Queensland’s Progressive Rehabilitation and Closure Plan (PRCP) frameworks the most prominent recent examples.  

Globally, financiers and insurers are asking sharper questions about long-term liability and environmental risk. Among the industry’s most experienced closure practitioners, one message is coming through consistently: the closure cost of a heap leach facility is not set at end of life – it is set at feasibility. 

According to Jeff Parshley, Principal at SRK Consulting; Heather Thomson, Principal Mine Waste Engineer; and Bronwen Forsyth, Principal Geochemist, decisions made during feasibility and operations can significantly affect the uncertainty associated with this period. 

Looking beyond earthworks 

Closure discussions often focus on reshaping a heap, constructing a stable landform and installing a cover. These remain important tasks, but Parshley says they represent only part of the closure challenge. 

‘Reshaping, building landforms, putting covers on. That we’re very good at. The industry has been doing that for a long time,’ he says. 

‘Understanding how long you’re going to have to actively manage drain down, that is where the uncertainty and costs come in. Every year you spend actively managing drain down is expensive, and it can very quickly overwhelm the cost of the earthworks if it goes on long enough.’ 

The broader point is that closure cost may depend on more than the volume of material moved or the area covered. It can also depend on how long water and geochemical conditions require active management after operations end. 

Those conditions are influenced by the hydraulic behaviour of the heap, the effectiveness of the cover, the amount of water stored within the facility and the chemical characteristics of the residual material. Addressing these questions early gives project teams more opportunity to test assumptions against operating data. 

The cost of designing against uncertainty 

Thomson says detailed closure design sometimes begins without enough site-specific data to support decisions about covers, materials and long-term performance. 

‘We get to two years out from closure, and we start looking at the detailed closure design. There are no cover trials in place, so you’re designing against uncertainty. It’s too late to put cover trials in,’ she says. 

A cover trial is a field-based test used to assess how a proposed cover performs under site conditions. Depending on its purpose, it may help a project understand water infiltration, erosion, vegetation establishment or the availability and behaviour of construction materials. 

Without evidence from trials and operational monitoring, designers may need to adopt more conservative assumptions. In practice, this can affect cover dimensions, material specifications and design margins. 

‘So you’re building fat into your design. Building conservatism. Bigger covers, bigger margins. You’ve lost your materials that you could have been stockpiling through operations to build your cover. You’ve pushed yourself into a gap where you’re defensively designing rather than evidence based,’ Thomson says. 

The quotation reflects a broader design principle: where evidence is limited, uncertainty must be managed through assumptions and allowances. Starting investigations earlier creates more opportunity to refine those assumptions before they become embedded in detailed design. 

Closure planning as an evolving process 

In Australia, mine closure expectations are addressed through jurisdiction-specific planning and approval processes. The level of detail required can vary according to the jurisdiction, project stage and characteristics of the operation.

Forsyth says closure planning should evolve as information becomes available during the mine life. 

‘It’s about updating the closure plan as more information becomes available throughout the mine life cycle, including predictions of post-closure drain down and long-term water quality,’ she says. 

This distinction is important. A closure plan may establish objectives and intended outcomes, but the technical basis for achieving those outcomes should continue to develop through investigation, monitoring and operational experience. 

For heap leach facilities, operational information can improve understanding of drainage behaviour, seasonal water movement and the relationship between irrigation practices and solution recovery. It can also help project teams assess whether closure assumptions remain reasonable as the facility changes over time. 

Closure planning is therefore not simply a matter of updating a document at prescribed intervals. It is an opportunity to use emerging evidence to improve the design basis and reduce uncertainty. 

Building an operational evidence base 

The specialists identified several actions that project teams can consider during operations. The appropriate timing, scope, approvals and resources will depend on the site, but the common objective is to develop evidence before detailed closure decisions need to be made. 

Calibrate the water balance 

A water balance is an accounting and modelling tool used to understand the water entering, leaving and being stored within a facility.

‘One of the ways we can improve the accuracy of models is calibrating them with empirical data from the actual facility,’ Parshley says.

Operational shutdowns and transitions between leach cycles may provide opportunities to observe how drainage flows decline. Capturing those observations can help inform the assumptions used in closure water-balance models. 

Run cover trials during operations 

Cover trials require enough time to provide useful information under representative conditions. Beginning them during operations can give a project team a longer performance record before final design decisions are made. 

The results can help designers assess whether proposed materials and cover configurations are likely to meet closure objectives. They can also identify where additional investigation or design refinement is needed. 

Plan for cover materials 

Materials suitable for constructing a closure cover may be encountered or generated throughout the operating life of a mine. Identifying, characterising and managing those materials early may provide more options than attempting to source them near the end of operations. 

Materials planning should therefore be connected to mine scheduling, waste-rock management and closure design, rather than being treated as a final-stage procurement exercise. 

Prepare for hydraulic characterisation 

Hydraulic characterisation examines the properties that control how water moves through the heap. Accessing representative material at depth may be difficult while a facility is operating, but the investigation can be planned in advance and integrated into the transition to closure. 

Advance planning can help define the information required, potential sampling locations and the operational conditions under which the work can be completed. 

Consider water and geochemistry together 

Drainage infrastructure should be considered both as a hydraulic system and as a possible pathway for air movement. 

‘Oxygen entry into these landforms, facilitating the oxidation of residual sulfides, is a long-term geochemical issue,’ Forsyth says. ‘That includes oxygen transport into the drainage layers and pipework that may act as conduits for air entry post-closure.’ 

When sulfide minerals are exposed to oxygen and water, oxidation can affect drainage chemistry. This means that infrastructure designed to move water may also influence the geochemical conditions within a closed facility. 

Considering these interactions during design can help teams identify monitoring needs and assess whether drainage features could affect long-term closure performance. 

Designing for a permanent landform 

Long-term modelling is essential to closure planning, but it also involves uncertainty. Conditions far into the future cannot be predicted with the same confidence as near-term operational performance. 

Thomson says it can therefore be more useful to think of the closed facility as a permanent feature of the landscape, rather than relying only on a nominated modelling period. 

‘I like to think about it as perpetuity rather than 1000 years or 500 years, because you can model something for 1000 years, but how can you realistically predict that?’ she says. 

‘If you take the approach of designing for a permanent feature of the landform, that helps you frame it better.’ 

Parshley expresses the principle more directly. 

‘Closure is forever. That’s the way you need to think about it. It’s in perpetuity,’ he says. ‘I’ve yet to see an engineer sign off on a design for in perpetuity.’ 

These observations do not remove the need for defined modelling periods or performance criteria. Instead, they highlight the importance of considering how a landform will respond to uncertainty and change. 

Resilience, redundancy, stress testing and adaptive management can all contribute to that approach. Progressive closure and operational monitoring can also provide evidence of performance before responsibility for the facility enters its final closure phase. 

From designing for closure to operating for closure 

The phrase ‘design for closure’ is well established, but the specialists argue that design intent must also be reflected in operating decisions. 

‘It requires a different kind of thinking than you will typically get from operations staff,’ Parshley says. ‘There need to be ways of incentivising operators to make changes so that you’re facilitating, and hopefully reducing, the cost of closure.’ 

Operational teams must manage immediate priorities including throughput, recovery and cost. Closure decisions, by contrast, may produce benefits many years later. Connecting closure outcomes with operational planning can help address that difference in time horizon. 

This could include identifying closure-related data requirements, protecting suitable cover materials, reviewing water-management assumptions and incorporating progressive closure opportunities into routine planning. The details will differ between facilities, but the underlying principle is consistent: operational choices can either preserve or limit future closure options. 

Starting with the end in mind 

Asked what advice they would give a developer commissioning a first heap leach facility, all three specialists return to the importance of early planning. 

‘Take the time and make the effort to understand what it’s really going to take to close it,’ Parshley says. ‘A lot of people gloss over that one.’ 

‘Design upfront. Put the cover trials in. Stockpile your materials. Build your liner to the right area. Do it all from day one,’ Thomson says. 

‘Understand your drain down and water-management options upfront and keep validating those predictions with real data as you operate,’ Forsyth adds. 

Their central message is that closure should be treated as a design and operational consideration from feasibility onwards. Early trials, appropriate data collection and progressive validation of water and geochemical models can provide a stronger basis for later closure decisions. 

For heap leach facilities, the end of mineral recovery is not necessarily the end of active management. Understanding that distinction at feasibility can help project teams make informed decisions throughout the life of the facility. 

About the contributors 

Jeff Parshley is a Associate Corporate Consultant (Mine Closure) and Director, Mackay School of Earth Sciences and Engineering, University of Nevada, based in Reno. He has extensive experience in mine closure, including heap leach closure planning and design. 

Heather Thomson is a Principal Civil Engineer in SRK Consulting’s tailings group, based in Perth. She specialises in the design, operation and closure of mine waste facilities, including tailings storage facilities and heap leach pads. 

Bronwen Forsyth is a Principal Geochemist with SRK Consulting. She specialises in mine waste geochemistry, water quality and the assessment of long-term environmental performance.