From Pit to Depth: Engineering the Transition to Underground Mining

Understanding the Optimal Transition Depth

Successfully transitioning from open-pit to underground mining hinges on balancing economic benefits with engineering constraints. The ideal depth for transition occurs when the incremental cost of deepening an open pit equals or surpasses the cost of underground mining for the same ore volume. Proper planning for this transition should begin 5 to 15 years before reaching the pit’s economic limit. Identifying this point early ensures the implementation of critical infrastructure, slope stability, and maximum reserve recovery, ultimately maintaining continuous, profitable mining operations throughout the life of the deposit.

Geomechanical Factors Shaping Mining Method Selection

Geomechanical conditions play a crucial role in determining the appropriate underground mining method following the open pit phase. Key considerations include:

• Rock Strength and Fracturing: Rock quality is assessed using classification systems such as RMR, Q, GSI, or strength criteria like the Hoek-Brown criterion, which assist in developing limit state diagrams to anticipate potential failures.

• Pit Slope Stability: The strength and stability of pit slopes significantly affect the feasible depth of an open pit. Stronger, uniform rock types, such as intrusive formations with Uniaxial Compressive Strength (UCS) exceeding 150–200 MPa, allow steeper slopes (45–60°), enabling deeper pit operations. Conversely, weaker, highly fractured rocks (UCS below 50 MPa) require gentler slopes (under 40°), limiting pit depth.

Selecting the Right Underground Mining Method

Choosing the appropriate underground mining method based on rock strength is vital for operational safety and efficiency:

• Strong Rock Masses (UCS >100 MPa): Methods involving large stable chambers, such as room-and-pillar or sublevel stoping, are optimal.

• Moderately Strong or Mixed Rock Masses: Methods employing backfill or controlled caving, like cut-and-fill or sublevel caving, are preferred. For instance, sublevel caving effectively operates in iron ore deposits in Sweden and Ukraine, leveraging strong magnetite ore (UCS ~200 MPa) with weaker surrounding host rock naturally filling voids post-blast.

Advances in Block Caving Technology

Historically, block caving was limited to weaker rock masses (UCS 6–60 MPa). However, technological innovations, such as hydraulic fracturing and preconditioning blasting, have enabled its application in significantly stronger rocks. Projects such as Cadia East and Northparkes in Australia, Grasberg in Indonesia, El Teniente in Chile, and Palabora in South Africa have successfully implemented block caving techniques in rocks with UCS values ranging from 140 to over 300 MPa—strength levels comparable to granite.

Stress Management at Depth

At greater depths, typically beyond 800–1,000 meters, stress management becomes crucial. High hydrostatic stress combined with brittle rocks increases rock-burst risk, rendering large excavation chambers impractical. In these scenarios, smaller stopes or controlled caving methods are recommended to relieve stress effectively. Pillar-based methods like room-and-pillar become unfeasible at such depths due to pillar failure, necessitating methods that either backfill excavations to provide support or utilize controlled caving techniques to manage stress.

Impact of Surface Operations on Underground Stability

The long-term effects of surface mining significantly influence underground stability. Prolonged open-pit operations can create zones of tension and fractures around pit walls. Underground excavation beneath these weakened zones can trigger instability, collapses, or water inflows through relaxed fractures. A notable case is Palabora, where underground ore extraction reactivated movement along fractured zones of the pit’s northern slope, causing subsidence into the caving zone. Hence, methods that reduce seismic risk, like gradual caving, are often preferred when an unsupported pit remains above underground workings.