Building Mined Rock Stockpiles for the Future

The Current State of Play

Despite the global pandemic, national economies like Australia have shown resilience due in large part to the strength of the resources sector. [1] Despite the benefits this performance brings to investors, governments, and communities, it is the sector’s environmental, social, and governance (ESG) performance which is dominating public discourse and impacting investment decisions. License-to-operate remained the leading risk to businesses within the resource sector from 2019 to 2020. [2]

The reason – Inability to predictably control the impact on environmental and heritage values of the land on which we operate.

Where then, can operators focus efforts to control these impacts? From an environmental perspective, the mine features which frequently represent the largest potential source of contaminants are mined rock stockpiles (MRS) or waste rock dumps (WRD). These constructed landforms can contain significant existing contaminant loads, and have the potential to continue producing contaminants which can be mobilized; this process is generally known as metal leaching and acid rock drainage (ML/ARD) or acid and metalliferous drainage (AMD). These landforms dominate our post-mining landscapes yet their design and construction are often only undertaken with immediate operational expenditure (OPEX) in mind. It is this singular focus which compromises not only our industry’s environmental performance, but also our legacy and long-term liability.

The Opportunity

By considering additional performance metrics such as projected seepage water quality and landform evolution during MRS / WRD design and construction, we can better understand the potential impacts of post closure liability on net present value (NPV). Understanding closure liability from an NPV perspective allows closure professionals to communicate upwards within their business and reach consensus about how full lifecycle environmental goals can be realised through smart landform engineering.

Fundamentally, limiting ML/ARD or AMD from an MRS or WRD is about managing the supply of oxygen into reactive waste, then limiting contaminant transport through surface water management and/or reduction of water through the waste (net percolation; NP). The way in which we manage or place mined rock, in combination with site-specific controls (e.g., oxygen availability, rainfall, temperature, surface water) largely drives the potential for negative impacts. Therefore, with years of research undertaken and an industry understanding of ML/ARD or AMD processes, some historically used MRS or WRD dumping and construction practices are no longer acceptable.

A Path Forward

At Okane, we couple geochemical and hydrological modelling with mine-planning processes to evaluate a range of construction scenarios in respect to closure goals and NPV. We empower operators to make smart decisions about how legacy and liability are controlled.

Our process, rooted in the observational method,[3] takes a site-specific approach through risk and opportunity analysis. Using Okane’s approach to integrated life of mine planning:

We seek to understand landforms and build a Conceptual Site Model (CSM) that considers climate, hydrological setting, and material characteristics.
We seek to build consensus on the site-specific environmental and social risks through collaborative risk assessment processes that includes stakeholders from across the operational setting.
Our team of geotechnical engineers, geochemists, and environmental scientists address knowledge gaps to update the CSM.
We develop landform design and construction options based on the CSM, considering site-specific risks and opportunities that have been agreed on.
We evaluate options from a multi-criteria perspective, considering hydrological and geochemical performance through modelling.
Options are then integrated into mine-planning software to project associated costs, residual risk, and conformance to regulatory requirements.

We present the range of options and their associated performance to our clients and stakeholders to inform decision-making and facilitate consensus building.

The landform option carried forward then becomes a fully integrated part of the mine plan, and operators no longer need to make unverified assumptions about the potential environmental impact of their MRS or WRD. Okane’s approach empowers operators to:

Control uncertainty with regards to latent water quality impacts.
Provision for projected schedules and cost.
Plan with confidence to manage liability and achieve post-mining land uses.
Communicate the suitability and benefits of a given approach.

Okane has partnered with clients across the mining industry for 25 years and has established a clear Roadmap to Closure by managing closure performance from feasibility assessment through to relinquishment.

1 Constable, T (2020) Minerals Council Australia, accessed July 2021 https://www.minerals.org.au/news/mining-largest-contributor-australian-economy-2019-20

2 Mitchel (2020) Ernest and Young, accessed July 2021. https://www.ey.com/en_au/mining-metals/10-business-risks-facing-mining-and-metals

3 Terzaghi and Peck (1967) Soil Mechanics in Engineering Practice, 3rd Edition | Wiley