Coordinating the Overlap: Open Pit to Underground Transition Planning

Transition Planning from Open Pit to Underground Mining

Transitioning from open pit to underground mining isn’t instantaneous—it’s a carefully coordinated, overlapping process that requires meticulous planning. For large-scale mines, this transition planning typically begins around ten years before open pit operations conclude.

Key Stages of Transition Planning

Several critical stages must be effectively managed:

• Deep Ore Zone Exploration: Initial exploration identifies viable underground reserves beneath the current pit.

• Determining Transition Depth and Mining Method: Choosing optimal depths and appropriate underground mining techniques.

• Design and Construction of Underground Infrastructure: Developing essential underground facilities and access points.

• Simultaneous Operations Management: Running surface and underground mining concurrently, progressively ramping down open pit activity while scaling up underground production.

Common Challenges and Solutions

A frequent oversight is underestimating the timeline for this complex transition, which can cause production gaps. A phased transition helps mitigate this risk by aligning the winding down of pit operations with the underground mine readiness.

Ensuring Pit Stability During Transition

One primary safety concern is maintaining pit stability as underground mining commences. The pit floor becomes the ceiling of new underground workings, necessitating a strategically designed crown pillar. The thickness of this pillar depends on rock strength and stress tolerance. Techniques such as stress-relief drilling, controlled blasting, or leaving an untouched ore layer help ensure stability. Pit designs can also be adjusted, such as flattening slope angles to reinforce the crown pillar and safeguard underground activities.

Coordinated Operations and Safety Measures

During overlapping operations, strict coordination is critical. Clearly defined work zones are essential to protect underground crews from surface blasting hazards. Underground development typically starts in areas where surface extraction has already concluded, often through a decline initiated 50 to 100 meters above the pit bottom, heading toward the ore body. This approach reduces risks associated with active surface blasting zones.

Managing Underground Voids and Pit Interaction

Strict rules govern underground operations beneath active pits. Underground voids directly under pit slopes are prohibited during active surface operations. Mining methods like block caving, which cause controlled collapses, are only executed after surface mining has fully ceased and all personnel and equipment are safely evacuated.

Case Study: Palabora Mine Transition

The Palabora Mine serves as a prime example: after surface mining concluded in 2004, block caving operations resulted in significant pit floor collapse, deepening it by nearly 400 meters and causing major wall failure. This case underscores a crucial lesson—never underestimate the powerful interactions between open pit and underground mining operations.