Webinar: Digging Deeper – The Shift to Underground Mining

Determining the Optimal Transition Depth: Open Pit to Underground Mining

The shift from open-pit to underground mining marks a critical strategic decision driven by technical, geotechnical, and economic considerations. Typically, the transition occurs when deeper open-pit mining no longer makes economic sense or becomes unsafe due to wall stability risks. Common methods like Lerchs–Grossmann calculations help define the pit’s final outline. Key factors influencing the transition depth include the ore-to-waste ratio, equipment productivity, wall stability, and operational costs.

Economic Analysis of the Transition Zone

To identify the optimal transition point, mining companies use combined economic and mathematical modeling. The transition depth is set when the combined Net Present Value (NPV) of an open-pit followed by underground operation exceeds the NPV of open-pit mining alone. Studies show this approach can boost overall project NPV by approximately 2%. Critical elements like crown pillar thickness directly impact the available underground reserves and overall viability.

Transition Depth by Deposit Type and Geomechanical Properties

Optimal transition depths vary significantly based on deposit characteristics. Small gold mines often transition around 200–400 meters, whereas large porphyry copper mines can economically operate open pits down to 700–1000 meters. Factors like reserve structure, metal price forecasts, ventilation, and dewatering greatly affect these decisions. Strategic adjustments, such as steepening pit slopes or stabilizing walls, can extend pit life and improve overall project economics.

Geomechanical Factors in Method Selection

Geomechanical conditions heavily influence the choice of underground mining methods. Rock strength, fracturing, in-situ stress, and pit slope stability determine feasible mining methods. Systems such as RMR, Q, GSI, and Hoek–Brown criteria help evaluate rock quality and predict stability. In strong rock masses, methods like room-and-pillar or sublevel stoping are effective. In weaker or fractured rocks, caving methods with backfilling, like sublevel caving or block caving, are preferred.

Selecting Underground Mining Systems

Choosing the correct underground mining system depends heavily on rock strength and geotechnical conditions. High-strength rocks (UCS >100 MPa) suit large-chamber mining like sublevel stoping, while weaker formations (UCS <50 MPa) require caving methods. Technological advancements, such as hydraulic fracturing, now enable block caving even in stronger rocks, expanding viable options for deep mining. Prominent examples include Cadia East, Northparkes, DMLZ, El Teniente, and Palabora, showcasing successful block caving implementations even at UCS around 300 MPa.

Managing Geostatic Stress and Stability

Increased depth amplifies geostatic stress, leading to rockburst risks and instability. At depths beyond 800–1000 meters, pillar-based methods become unviable, and preference shifts to stress-relief caving or backfilling techniques. Transitioning underground beneath an existing open pit introduces unique geomechanical challenges. Previous open-pit operations alter rock mass behavior, creating tension zones prone to collapses or water inflow during underground mining. Case studies like Palabora highlight these risks, emphasizing cautious method selection to minimize seismicity and maintain surface stability.

Underground Access Methods: Shaft vs. Decline

Post-open-pit access to deeper reserves involves selecting between vertical shafts, inclined declines, or hybrid systems. Shafts efficiently handle deep deposits with high ore output (over 5000 tonnes daily), while declines are practical for shallower mines or lower production rates (under 1000 tonnes/day). Country-specific approaches vary widely—South Africa predominantly uses shafts for very deep mining (up to 3000 m), whereas Australia favors spiral declines even at depths of 1200 m, highlighting localized expertise and operational preferences.

Combined and Tailored Access Systems

Combining vertical shafts and declines offers operational flexibility, improved safety, and phased infrastructure investments. Mines often start with declines for initial access, gradually adding vertical shafts for increased production. For instance, Jundee Gold Mine in Australia utilized a phased approach, initially developing declines before constructing auxiliary shafts to sustain deeper mining. Proper portal placement and crown pillar design are crucial for stable and efficient transitions.

Safety and Practical Considerations in Underground Access

Ensuring safety in underground operations requires reinforced shafts, emergency evacuation systems, and ventilation management. International standards mandate secondary egress for long declines, prompting hybrid setups like Cuiabá mine’s combined shaft-decline arrangement in Brazil. These systems, though costlier initially, significantly enhance operational flexibility and safety.

Phased Transition Strategies for Continuity

Transitioning from open pit to underground is inherently phased, often beginning up to 15 years before open-pit closure. Key steps include deeper exploration, infrastructure design, simultaneous operations, and gradual scaling of underground production. Proper phasing avoids production gaps, ensures slope stability, and optimizes crown pillar management through techniques like stress-relief drilling and slope reinforcement.

Coordinating Concurrent Surface and Underground Operations

Managing simultaneous surface and underground activities demands strict coordination. Underground development typically begins away from active blasting zones to ensure safety. In block-caving scenarios, cave-induced collapses occur only after surface mining ends, as exemplified by Palabora mine’s experience, which resulted in extensive pit floor collapse and slope failures, underscoring the importance of careful coordination.

Ventilation, Dust, and Dewatering Challenges

Effective ventilation and dewatering systems are crucial from the initial underground construction stages. Managing surface dust intrusion, especially during blasting, requires dedicated ventilation shafts and advanced filtration. Water control is equally vital, as uncontrolled seepage can trigger dangerous mudrushes. Integrated dewatering systems, like those employed at Argyle mine, effectively mitigate such risks by proactive groundwater management.

Real-world Transition Case Studies

Successful transitions, such as Grasberg and Palabora mines, emphasize advanced planning and phased infrastructure investments. Despite meticulous planning, production dips often occur, managed effectively through stockpiling high-grade ore or extending surface mining operations. Debswana’s Damtshaa mine and Freeport’s Grasberg demonstrate these strategies, underscoring the importance of flexibility and preparedness.

Economic and Financial Optimization

Economic viability during transitions relies on integrated financial modeling to maximize NPV. Optimized production schedules and phased infrastructure investments balance upfront costs and ongoing revenues. Utilizing mixed-integer programming models helps determine optimal ore block allocation between open-pit and underground mining, significantly enhancing economic outcomes.

Cut-off Grades and Economic Strategies

Transition planning demands careful consideration of cut-off grades. Higher underground costs typically increase these thresholds. Stockpiling strategies, plant optimization, and cost controls help manage transitional economic impacts. Dilution, especially with block caving methods, must be factored into revenue forecasts to ensure realistic economic projections. Gradual capital expenditures, often financed by ongoing open-pit profits or external investments, mitigate financial impacts and enhance long-term viability.

The transition from open-pit to underground mining represents a complex interplay of technical planning, geomechanical analysis, safety considerations, and economic optimization. With careful preparation, robust infrastructure planning, and strategic operational management, mines can smoothly navigate this critical transition, maximizing resource value over the mine’s lifespan.