Many ore deposits extend vertically, a fact which is not always known at the start of mining. If initial mining is by opencast methods and ore reserves are proven to greater depths, the pits are often planned to go deeper than originally envisaged. In such cases, surface plants and critical underground facilities -- conveyor tunnels, access ramps, ore passes, hoisting, and ventilation shafts -- are often located much closer to the pit rim and the ore body than desirable. This raises the question of stability, which may be critical for operating the mine longer term. Key considerations to evaluate are: open pit stability, shaft stability, dilution, mud rushes and air blasts, and mining method selection.
Planning Considerations: The efficient transition from open pit mining to an underground operation requires extensive planning. For a large mining operation, planning could last as long as 20 years. The main factors and activities that need to be taken into account in the planning cycle are as follows:
- Definition of the ore body
- Rock mass characterisation
- Definition of the boundary conditions
- Investigation of suitable mining methods
- Underground infrastructure
- Surface infrastructure
- Ongoing open pit factors of influence
- Underground mining and layout considerations
- Underground and surface effects of the underground mining
- Surface and groundwater effects
- Risks
- Project timing
Conclusions: Aspects of particular significance include the following:
- An economically designed pit will have slopes close to their stability limits, with little scope for extending the open pit to greater depths.
- Planning and implementing transition from surface to underground mining can take 20 years. Planning must therefore commence at an early stage.
- Surface and underground infrastructure is often at risk as pits deepen beyond planned depths.
- Transition from open pit to underground mining often risks mud rushes (if mud forming minerals are present), and from sumps and surface dams. Air blasts can result from underground collapses or mud rushes.
- The presence of an abandoned pit above can lead to greater risks of dilution and mud rushes.
- The choice of underground mining method affects the surface; and stability requirements may dictate the choice of mining method.
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