Underground mine planning and design in complex orebodies is a multifaceted process that requires an intricate balance between maximising economic value, ensuring geotechnical stability, and maintaining operational continuity. As orebodies become deeper, more structurally complex, and economically marginal, mining engineers must employ advanced tools, data integration, and cross-disciplinary collaboration to optimise designs.
By Keith Sungiso
In this insightful interview, Engineer Nigel Mukonoweshuro BSc. (Hons) Mining Engineering, GMDP, CPM, MSAIMM, MAusIMM shares his expert perspective on the challenges and innovations in underground mine design and planning.
How do you determine optimal stope dimensions and orientation for maximum ore recovery and geotechnical stability in a complex orebody?
Determining the optimal stope dimensions and orientation requires a balance between maximising ore recovery and geotechnical stability. 3D geological modelling helps to understand the orebody’s geometry and structure, and these are followed up by geotechnical investigations to determine key matrix like your Rock Mass Rating (RMR) from Uniaxial Compressive Strength (UCS) and triaxial strength of the host rock. Stope orientation is usually parallel to the dip of the orebody to minimise dilution, and this should be favourable to reduce stress concentrations. Preliminary stope designs are based on stability checks, and historical data based on similar ground conditions. Numerical modelling and stability analysis then help to simulate extraction and refine the preliminary designs. Dilution and ore recovery factors are also analysed. Economic and scheduling constraints also come into play evaluated by cut-off grades, type of equipment (their access and productivity), and sequencing of blocks to minimise stress build-up. Other key considerations include hydrogeological (water inflow) and ventilation factors.
How do you incorporate evolving resource models and grade control data into long-term mine plans without disrupting production continuity?
Immediately, three key terms should come to mind, mine planning stages (strategic (long term), tactical (medium term) and operational (short term)), data reconciliation and risk management. The first step is to establish rolling strategic (long–term) and tactical (medium) term plans. These plans should be flexible and subject to periodic review. Resource model updates should be done regularly and integrated into the existing plans. Reconciliations and variance analysis will be used to refine the estimation parameters and planning assumptions applied through the determination of your F-factors. Reconciliation should be an implicit part of the mining process, and reconciliation targets should be a key performance indicator for well-run mines.
Scenario planning and sensitivity analysis are then utilised to assess the stability of the strategic (long-term) and tactical (medium-term) plans. Your grade control data should feed into your operational (short-term) plans on a weekly and monthly basis and then progressively into the tactical and strategic plans in that order. What is also key is to have your mine models designed in modular blocks or phases to allow updates to only affected areas/zones, thus making the process manageable. A cross-functional review process should also be established to allow for the integration of different functions (geology, mine planning, operations and finance) and trigger action planning when major shifts and alignment to production priorities. Your model updates should align with the operation’s budget cycles or annual planning and production reforecasts. A good record of the updates and changes also supports accountability and future planning audits.
With increasing pressure on cost efficiency, what innovations in underground design or scheduling have yielded the most value in your operation?
Some of the most valuable innovations in our underground mine design came with digital integration and strategic design choices. Development design optimisation software helps us to reduce development costs. Stope optimisation software allows us to rapidly reforecast and auto-schedule production utilising real-world operational rules. Drill and blast optimisation software has allowed for less overbreak and optimised fragmentation, which reduces secondary blasting. The use of digital tools and technology incorporating LiDAR scanning allowed for seamless development and stope excavation measurement for reconciliation processes. The use of semi-autonomous equipment has also reduced our labour costs, improved productivity and allowed for the removal of man from high-risk tasks/zones.
How do you manage trade-offs between ore recovery, dilution, and safety when designing stopes and production sequences?
Firstly, a multi-criteria approach is key to an advanced mine planning approach. An integrated value-based design framework is utilised, measured by the Net Smelter Return (NSR) or value per tonne of ore mines. Modelling and analysis of trade-offs for:
- Stope shape optimisation is done using modelling software and economic outcomes are compared for different designs.
- Dilution vs Safety, where wider stopes increase recovery but lead to unstable spans and hence wall failure, so safe stope spans should be employed, and
- Ore recovery vs safety, where maximum recovery will require tighter stope boundaries near wall contacts and increase risks of overbreak and unsupported voids. This is regulated by the enhancement of ground support design and sequencing adjustment to control exposure of man (lead and lag of stopes).
Overall, the mine design should be practical and flexible. Production sequencing will require a balance of ground conditions and value attained from extracting the stopes (block evaluation). The trade-offs highlighted above should be quantified and compared to evaluate stope designs. Monitoring of the actual recoveries and dilution provides a feedback loop that allows for design improvement in future mining.
Given Zimbabwe’s energy supply challenges, how do mine plans incorporate power reliability, ventilation demand, and backup systems?
Mine plans now incorporate energy availability into both the design and operational strategies. Shift scheduling should be based on power availability to allow for high-energy activities to utilise off-peak periods. Equipment deployment should also be staggered to avoid simultaneous startups at once. Development scheduling should also allow for ventilation as it goes especially in shaft sinking or decline systems.
Ventilation management can integrate the use of Ventilation on Demand (VoD) i.e., use of sensors and schedules to control underground airflow. Fan selection should include the use of high-efficiency axial fans or the use of variable speed drives (VSDs) on main or booster fan installations.
Use of backup and hybrid energy systems such as diesel generators and/or solar systems, which lower base demand and stabilise power during peak outages.
Plans should allow for buffering of headings and the creation of stope inventories for flexibility and maintenance of the production schedules.
What role do software and digital mine planning play in your daily work, and which tools have proven most reliable under local conditions?
The core roles of digital mine planning tools are for 3D geological and resource modelling, stope and development design, scheduling and production planning, and reporting and communication. Various tools are utilised in different mining operations in Zimbabwe, but the Deswik Suite comes highly recommended under local conditions. Other software packages include Datamine Studio UG/Studio RM, Leapfrog Geo (for geological modelling), Maptek (Vulcan and Evolution), and AutoCAD/MineSight, although less flexible, but are useful for most legacy systems.
How are sustainability and ESG considerations shaping the future of underground mine planning in Zimbabwe?
Sustainability and Environmental, Social and Governance (ESG) are increasingly redefining the future of mining in Zimbabwe, driven by both regulatory and economic necessity. Companies are now under scrutiny from investors, the community, and the government to align with the Sustainable Development Goals. Environmental considerations are being made on mind design to cater for water management, energy efficiency, carbon reduction and environmental rehabilitation at the closure of operations. Social impacts and community expectations are being met with access designs being planned away from community zones, employment of locals and progressive rehabilitation for future land use. Planning Teams are increasingly required to document ESG risks with technical studies and mine design documentation in alignment with the Environmental Management Act and other guiding principles such as the Global Reporting Initiative (GRI). Technology also now plays a part in monitoring key elements such as water quality, emissions and noise utilising the Internet of Things (IoTs). In future, ESG compliance will become central to financing as investors and Banks will increasingly demand transparency in ESG-integrated feasibility studies.
How do you calculate the economic cut-off grade for an underground operation, considering factors like mining cost, recovery, and metal price?
This calculation integrates several key variables, i.e., mining cost, processing, and General and Administration (G&A) costs, metal prices, recoveries, and payability. Your economic cut-off grade in an underground mine distinguished between ore and waste.
A simple equation for economic cut-off grade for a single metal mining operation is given as:
COG=C/(R*P*F)
Where:
- C = Total cost per tonne (Mining + Processing _ G&A) in $/t
- R = Recovery rate (given as a decimal e.g. 0.85 to 0.95 this is based on lab test and plant performance.
- P = Metal price in $/oz or ($/g or $/t)
- F = Payability factor (if selling concentrates) (if selling to a smelter or refinery, you apply deduction factors, e.g. 98%).
For multi-metal (polymetallic) deposits, the Net Smelter Return (NSR) approach is used and the NSR/t is compared to the cost per tonne. COG is applied to mine planning for block modelling and generation of ore/waste classifications.
Can you share insights into how mine planning aligns with compliance, such as Mine Survey regulations, Safety Standards, or Environmental Management Plans?
Cross-functional planning teams involve Mine Planners, Geologists, Geotechnical Engineers, Surveyors, Environmental Officers and SHEQ Personnel, which ensures legal obligations, risk management and stakeholder expectations are built into the mine plan from the onset. Misalignment risks legal penalties, operational delays and the loss of social license to operate.
Mine Survey Regulations stipulate how mine plans should comply with the Mining (Management and Safety) Regulations SI109 of 1990 and subsequent updates, giving accurate spatial data for legal boundaries, certified survey outputs and stope tonnage reconciliation and void tracking.
Safety and Occupational Health Standards guide how mine designs comply with ground support requirements, establishment of escape routes and refuge bays, ventilation planning, blasting and explosives regulations.
Environmental Management Plan (EMP) integration stipulates for designs that minimise surface disturbance, meet progressive rehabilitation/reclamation goals and plans for water management.
Regulatory submissions and reviews require quarterly production reports where the mine plan provides the stope tonnage estimates, survey submissions reflecting actual developments, environmental audits require plans showing the disturbed areas, water use and backfilling (if any) and safety files showing drawings of escape routes, support designs and ventilation plans.
Lastly, what advice would you give to young Zimbabwean professionals aspiring to become mine planners in a rapidly evolving mining sector?
I will close off this interview with a quote by an Abstract African, which says, “The future belongs to those who believe in the beauty of their dreams.”
Success in the Mine Planning field does not depend only on technical competence but the ability to adapt, lead and think systematically in an evolving and sometimes resource-constrained environment. Proficiency in software is a non-negotiable advantage in modern planning. Pursue certifications and continuous improvement through learning new skills. Get mentorship and field exposure, and be bold to explore new approaches.
“The future of Zimbabwe’s mining landscape requires Mine Planners who can think like Engineers, act like Economists and speak like Strategists.”
You can get in touch with Eng Mukonoweshuro at [email protected]
