Selecting a Mining Method

Surface or underground? Caving or stoping? Learn the families of mining methods, the factors that drive selection (depth, geometry, grade, rock strength, economics), stripping-ratio limits, and how to decide when an open pit should transition to underground.

The mining method is the single biggest decision in mine design — it sets costs, safety, recovery and production rate for decades. Shallow, large, low-grade deposits suit open pits; deep, high-grade or narrow ones suit underground methods.

An analogy

Choosing a mining method is like choosing how to eat a cake: slice from the top (open pit), dig a tunnel to the filling (underground stoping), or let it collapse into a spoon from below (caving).

Method families

Surface: open pit, strip mining (coal), quarrying, placer/dredging, in-situ leaching. Underground — supported: room-and-pillar, stope-and-pillar; artificially supported: cut-and-fill, shrinkage; unsupported open stopes: sublevel stoping, VRM; caving: longwall, sublevel caving, block caving.

Selection factors

Geometry (thickness, dip, depth, shape), grade distribution, rock-mass strength of ore and host, groundwater, required production rate, dilution tolerance, labour and capital, environmental and surface constraints. The Nicholas method scores these to shortlist methods.

Surface to underground transition

As a pit deepens, each extra tonne of ore requires more waste. When the incremental stripping ratio exceeds the BESR, underground mining becomes cheaper. Many mines run both (e.g. a pit above and a block cave below — Grasberg, Chuquicamata).

Novel methods

In-situ recovery (uranium, copper) pumps solutions through the orebody; deep-sea nodule mining; mechanised narrow-vein mining; continuous rock cutting instead of drill and blast.

Formulas

• Stripping ratio: SR = waste tonnes / ore tonnes
• OP vs UG breakeven: BESR = (C_UG − C_OP,ore) / C_w
• Rule: Mine by open pit while incremental SR < BESR

Worked example: Transition depth decision

Given: Underground cost $45/t ore; Open-pit ore mining $3/t, waste $2.5/t

Find: BESR.

• BESR = (45 − 3)/2.5 = 16.8

Answer: Mine by open pit while the incremental SR < 16.8 t/t

Worked example: Stripping ratio

Given: Pit contains 30 Mt ore and 120 Mt waste

Find: Overall SR.

• SR = 120/30 = 4.0

Answer: 4 : 1

Common mistakes

• Wrong: Using the overall SR for transition decisions. — Right: Compare the incremental (marginal) SR with the BESR.
• Wrong: Selecting by cost alone. — Right: Include safety, recovery, dilution and rock mechanics.

Applications

• Greenfield projects: Method selection in scoping and pre-feasibility.
• Pit-to-underground transitions: Timing crown pillars and underground development.
• Coal: Strip mining shallow seams, longwall deeper seams.

Key takeaways

• Surface for shallow/large; underground for deep/high-grade.
• SR = waste/ore; BESR from cost difference.
• Rock strength decides supported vs caving.
• Selection balances cost, safety, recovery.

Going further

• Nicholas & UBC methods: Scoring systems for method selection.
• Crown pillars: Rock left between pit floor and underground stopes.
• Continuous mining: Mechanical cutting for hard rock under development.