Webinar: Determining and Justifying the Optimal Pushback Number. Economic Effect

Pushbacks represent controlled phases of pit development, consisting of unique adjacent volumes that can be mined sequentially with existing equipment while respecting practical geometric limits. In strategic mine planning, pushbacks allow management to control mining intensity across different areas and coordinate development over time. Each pushback must extend the pit depth at its lowest point, giving the entire operation a defined direction. This approach differs fundamentally from conventional mining where multiple zones are worked simultaneously without a clear mining front.

Conventional mining offers flexibility by providing access to most mining faces simultaneously, enabling quality blending to meet processing plant requirements. However, this approach results in higher stripping ratios over the deposit's life because large areas must be opened to access ore. Mining with pushbacks delivers lower operating costs, better equipment productivity, and more predictable planning that translates to higher NPV. The tradeoff is reduced flexibility when switching between pushbacks and potential production shortfalls if pushback sizes are too conservative.

Each pushback must consist of unique connected blocks, which is essential for planning pit roads and maintaining safe slope angles on pit walls. Every new pushback should connect to the previous one and share transport links to ensure continuous material flow. Before planning begins, engineers must know exactly what machinery will be used for mining, loading, and transport to ensure equipment can operate effectively within each pushback. Additionally, each pushback must contain enough quality minerals to keep the processing plant running while the next pushback is being opened.

Effective pushback creation requires balancing technical, economic, environmental, and operational factors. Geotechnical stability assessment determines whether slope angles need adjustment and how geological changes affect bench configurations in the new mining area. Economic optimization must verify that bringing new portions into the pit maximizes net present value rather than simply extending mine life. Grade requirements for the processing plant are often overlooked—the reserves entering the strategic plan must match plant specifications. Equipment and operational constraints must also be evaluated, particularly whether existing equipment layouts suit the new mining geometry.

Traditional pushback creation often focuses on short-term profitability while neglecting long-term implications such as future mining phases or operational bottlenecks. Geological uncertainty is frequently underestimated—mining engineers assume grade continuity based on existing pit observations without verifying conditions in the pushback area. Commodity and market condition fluctuations are rarely incorporated into pushback models, leading to decisions that must be revisited after two or three years. Cross-functional collaboration with geologists, processing engineers, and environmental teams often comes too late in the planning process when decisions are already irreversible.

Automated pushback generation using pit optimization software provides efficiency, speed, consistency, and scalability as a foundation for planning. However, automation cannot accommodate all operational flexibilities and may have challenges with input dependencies. Manual pushback creation is time-consuming and subjective but allows customization, precision, and control over geotechnical issues. The recommended approach combines both methods: start with automated pushbacks to establish boundaries, then use manual adjustments to fine-tune constraints based on engineering experience and site-specific knowledge.

Satellite pits—smaller units of pit development within or around the main existing pit—offer an alternative when pushbacks face geometric limitations, though they restrict achievable depths and slopes. Transitioning from surface mining to underground methods becomes viable when open pit expansion is constrained, potentially using decline access to extract remaining minerals. Robotics and automation address the equipment constraints of limited mining widths by enabling smaller, automated drilling and loading equipment to operate in confined pushback geometries where conventional large equipment cannot function efficiently.

K-MINE's pit optimizer calculates optimal pit outlines incorporating cut-off grade, extraction rates, slope stability, and mining costs. From these results, engineers can generate multiple pushbacks automatically by setting parameters including price adjustment coefficient, number of pushbacks, working area width, and minimum pushback area at top and bottom elevations. Each pushback can have individual extraction volume targets and priority metrics, plus capital expenditure data for detailed financial modeling. The software allows unlimited scenario tabs for comparing different pushback configurations before selecting the best option for detailed planning.

Manual pushback creation requires engineers to define limits using polylines, surfaces, or solids that constrain block selection inside or outside specified boundaries. The hybrid approach starts with automatic pushback generation and then applies manual restrictions to refine shapes—for example, using a specific wireframe from optimization results to define the first pushback while letting automation generate subsequent phases. K-MINE's dynamic design plugin enables rapid pit design from wireframes, creating complete pushback projects with benches, ramps, and topography intersection in minutes rather than hours of manual bench-by-bench drafting.

Analysis of scenarios ranging from three to seven pushbacks demonstrates clear economic patterns. Fewer pushbacks create large overburden spikes early in development, delaying ore access and reducing NPV through extended payback periods. With three pushbacks, ore targets are met only from the fourth planning interval after massive initial stripping. Increasing to five or six pushbacks progressively reduces overburden peaks, improves schedule feasibility, and raises NPV. Seven pushbacks show the highest NPV with stable overburden volumes, though design complexity may make implementation impractical.

Each pushback should start at the pit surface and extend to maximum depth with the bottom targeting the mineral zone—the exception being transitions between mining methods where development is already established. Pushback width should equal two to five working area widths of the primary equipment, with three to four being optimal for most operations. Every pushback must extend from the previous one unless deposit geometry is uneven, where stripping one section may be more efficient than starting a new area. The fundamental scheduling principle is to begin developing each pushback as late as possible while reaching minerals as quickly as possible—this timing balance maximizes project NPV.

Every deposit requires a unique strategy, but general guidelines apply across operations: leverage the benefits of different pushback widths while remaining aware of potential downsides. Calendar mine planning using pushbacks consistently yields better economic outcomes than conventional approaches. Consider resource depletion strategies including stockpiling lower-grade material that will become profitable as commodity prices increase or processing technology improves. Success ultimately depends on having qualified personnel to execute these strategies and maintaining cross-functional collaboration throughout the planning process.