Achieving an optimal pit design requires maximizing economic benefits while adhering to strict geotechnical and economic parameters (Appianing & Mireku-Gyimah, 2015; Phillips, 1972). Below are a few factors to consider when developing an optimal pit design:

• Pit optimization

Pit optimization modelling involves assessing the grade and profitability of the block, considering all mining, processing, and transportation costs (Appianing & Mireku-Gyimah, 2015; Phillips, 1972). Pit optimization software like Whittle and Hexagon’s MinePlan use results from block modelling, alongside pit design parameters such as bench slope, height, and width, ramp and haul road gradient and width, berm width, and slope angle to inform and generate pit layouts and values (Appianing & Mireku-Gyimah, 2015).

• Geotechnical considerations

Slope stability is a critical factor influencing pit design. As the slope angle of a pit increases, so does mine productivity and profitability. However, steeper angles also increase the risk of instability. Research conducted by Abdellah et al. (2022) highlights that a 20-degree slope exhibits lower profitability and a reduced risk of instability compared to a steeper slope angle at 70 degrees, which yields higher profitability and instability risks. Processes such as dewatering can be employed to maintain an acceptable safety factor for pit wall stability at a chosen slope, as dewatering helps reduce pore pressure behind the pit walls (Smith, 2021).

• Environmental considerations

Incorporating rehabilitation costs avoids suboptimal cut-off grades and results in a more accurate NPV (Kalitenge, 2021). A study by Kalitenge (2021) shows that not accounting for rehabilitation costs when determining cut-off grade can inflate the NPV by 1.70% compared to a scenario where these costs are fully considered.

• Operational considerations

As described above, the optimal cut-off grade considers all mining, processing, and transportation costs. Mine operating costs, and therefore the optimal cut-off grade, are highly sensitive to material handling costs. Material handling costs generally make up about 40% of the total mining costs (Ben-Awuah et al., 2017). It is necessary to consider the potential effects of incorporating post-mining land use into integrated mine planning. It may be necessary to alter the mine development sequence, or haulage strategy, to optimize the potential for post-mining land use.

Optimized Pit Design for Post-mining Land Use

Incorporating post-mining land uses into the pit design and pit optimization process, particularly those supporting the global transition to renewable energy, can enhance project NPV. For instance, when assessing the pit value with the integration of pumped storage hydropower in mind, it may be valuable to consider the value, or future revenue, of the pumped storage hydropower facility when evaluating ore cut-off grades and the final pit shell geometry.

Pit limits can be designed to be deeper and wider to maximize economic resource and accommodate a post-mining land use like pumped storage hydropower. While extending pit limits to obtain subeconomic grades may result in increased near-term extractive costs, it has the potential to optimize the pit for pumped storage hydropower, improving the full project lifecycle costs and the project NPV.

Alternatively, sterilization of economic ore, typically near the cut-off grade, may be considered to achieve the optimal pit geometry or reservoir for pumped storage hydropower or bring the development of pumped storage hydropower earlier into the project lifecycle. Additionally, exploring subeconomic grades can provide materials for the site, such as non-acid generating materials for cover systems or for reshaping and constructing closure landforms.

While algorithms like the Lerch-Grossman algorithm (LGA) help determine the most economical pit based on costs for mining, processing, and ore revenue, they often overlook the potential revenue of post-mining land uses. Incorporating the potential revenue from renewable energy facilities into pit value assessment may assign a positive value to subeconomic grades to optimize repurposing the pit for post-mining land use.

Typically, post-mining land use and mine rehabilitation are considered at the completion of mining activity, requiring a large capital investment after earning the potential revenue from mining has ended. This results in a large negative project NPV at the end of the mine life.

Incorporating the post-mining land use and development of renewable energy facilities earlier in the mine life may provide an opportunity to construct the facility while the mine is profitable and realize revenue from the renewable energy facility earlier in the mine lifecycle. By incorporating renewable energy facilities into the project lifecycle, closure costs can be offset by cash flow generated by the facility.

Pumped Storage Hydropower as Post-mining Land Use

Today, several projects demonstrate the potential of open pit post-mining land use transformation into pumped storage hydropower facilities:

• Kidston Gold Mine, North Queensland, Australia

Decommissioned upper and lower mining pits at the Kidston Gold Mine offer a vast water-holding capacity and are being repurposed as upper and lower reservoirs to support 250MW of power generation for the Kidston Pumped Storage Hydro Project (Queensland Government, 2023).

• Mount Rawdon Mine, Queensland, Australia

Mount Rawdon Mine’s transformation into a pumped hydro generation facility comprises reusing a decommissioned mine pit as a lower reservoir and a purpose-built valley fill dam as an upper reservoir (Queensland Government, 2023).

• Tent Mountain Mine, Alberta, Canada

Plans to convert Tent Mountain Mine into a pumped hydro energy storage facility involve leveraging existing mining assets, including a 300-meter drop between two large water reservoirs as part of the energy plant infrastructure (Evolve Power, 2023).

• Marmoraton Mine, Ontario, Canada

The former iron Marmoraton Mine is undergoing transformation into a pumped storage hydropower project, repurposing the open pit as the lower reservoir and reshaping the by-product mine rock stockpiles to form the upper reservoir (Government of Canada, 2023).

These projects illustrate the opportunities for repurposing open pits into pumped storage hydropower facilities.

Okane’s Approach

The considerations above highlight integrated mine planning to optimize a pit for a pumped storage hydropower facility, but other potential renewable energy facilities, such as solar and wind farms, are also potential considerations for post-mining land use. Beyond reoptimizing pit designs, early planning of strategic mine sequencing to construct mine rock stockpiles can help prepare for transitions to solar and wind farms and other land uses.

At Okane, we offer integrated mine planning that explores innovative post-mining land use potential while prioritizing resource management. Our team of interdisciplinary experts collaborates closely with clients, community members, Indigenous rightsholders, and relevant stakeholders to identify opportunities to incorporate post-mining land use into the initial pit identification and development of the life of mine plan.

During closure vision workshops, we leverage our expertise in regulatory requirements to determine the studies needed to inform a closure strategy with the highest post-mining land-use value. By developing an integrated mine plan that considers detailed mine design, pit scheduling, and haulage modelling, we ensure a smooth transition from mining operations through closure to post-mining land use, delivering full lifecycle economic and environmental benefits.

We are committed to helping create a better tomorrow. Okane’s approach not only optimizes resource value but also supports sustainable and responsible post-mining land uses for future generations. To learn more about how we can help, please contact us at info@okaneconsultants.com.