Decline vs Shaft: Strategic Access Planning for Deep Underground Mining

Choosing Between Decline and Shaft for Underground Access

Transitioning from open-pit to underground mining involves a critical decision: whether to develop access through a haulage decline (inclined ramp), a vertical shaft, or a combination of both. This decision significantly affects the project’s economics, safety, and operational efficiency.

General Considerations:

• Vertical shafts are ideal for deeper deposits with high production volumes, typically more efficient when extracting thousands of tons daily at depths exceeding 300–500 meters.

• Declines (ramps or inclined tunnels) are generally better suited for shallower deposits with moderate production levels due to quicker setup, lower upfront costs, and operational flexibility.

Depth and Production Volume Relationship

According to established mining guidelines, notably from South African mining practices:

• Depth greater than 500 meters with high daily ore extraction (e.g., 5,000 tons/day or more) typically favors shaft access due to efficient ore hoisting capabilities (hundreds of tons per hour).

• Depths less than 300 meters or smaller-scale operations (less than 3,000 tons/day) often find declines more practical due to their simplicity and lower capital requirements.

However, this relationship shifts as daily production requirements increase. For example, at a production rate of about 5,000 tons per day, the cutoff depth favoring shaft access is around 500 meters. Conversely, at 1,000 tons per day or lower, declines remain practical even at depths approaching 800 meters.

International Variations and Cultural Factors

Different countries exhibit distinct historical preferences:

• South Africa: Typically employs vertical shafts even at extreme depths (up to 3,000 meters).

• Australia: Commonly uses spiral declines with truck haulage, even at depths of around 1,000–1,200 meters.

These differences highlight how local expertise, shaft-sinking traditions, and factors like electricity costs for hoisting influence the chosen method.

Geometric and Deposit Considerations

The geometry and size of the deposit significantly influence method selection:

• Wide, gently dipping ore bodies: Declines with multiple access points are optimal for managing extensive underground areas effectively.

• Narrow, steeply dipping veins: Vertical shafts provide rapid, direct access to deeper levels efficiently.

• Tabular or sheet-like deposits: Often use declines following the ore body strike.

• Vertically oriented, pillar-like ore bodies: Usually necessitate a central shaft to efficiently reach multiple underground levels.

Combined Schemes and Real-World Examples

Combining shafts and declines can optimize initial capital outlay and manage operational needs effectively. A prime example is Australia’s Jundee Gold Mine, which initially utilized declines for underground mining near the exhausted open pit, later introducing a vertical raise shaft as depth increased. This staged development reduced initial capital expenditure by spreading investments over time.

Production Rate and Haulage Method Implications

Large-scale underground operations, such as major block caving mines handling tens of thousands of tons daily, can face significant bottlenecks if using truck haulage via declines:

• Truck haulage through declines at high production rates creates congestion, increases ventilation needs, and escalates operational costs.

• Vertical shafts and conveyor systems are more effective for large-scale production, significantly enhancing ore handling capacity. Notably:

  • Palabora Mine (South Africa) employs vertical shaft hoisting to manage daily production of approximately 30,000–33,000 tons, handling around 20 million tons annually.

  • Chuquicamata (Chile) utilizes underground crushers and conveyors to efficiently manage daily ore production rates as high as 140,000 tons.

For smaller-scale operations (500–3,000 tons daily), truck haulage via declines remains economically advantageous, offering greater flexibility and lower initial investment. However, as operational depth increases, the cost per ton-kilometer rises significantly faster for declines than shaft hoisting.

Safety and Ventilation Considerations

High-traffic declines pose critical ventilation and safety challenges due to exhaust emissions and heat generated by equipment, demanding powerful ventilation systems and air shafts. Modern practices increasingly utilize electric locomotives or conveyors to minimize exhaust issues, typically restricting decline depths to about 700–800 meters for effective ventilation management.