Stoping Methods

Metal mines extract ore from openings called stopes. Learn open stoping (sublevel open stoping, longhole, VRM), filled stoping (cut-and-fill, paste fill), shrinkage stoping, their selection criteria, dilution and ore loss, stope stability (Mathews stability graph) and backfill.

Most hard-rock underground metal mines use stoping methods. The choice depends on orebody width, dip, rock strength and grade. The key economic factors are dilution (waste mixed with ore), ore recovery and productivity.

An analogy

A stope is like a room carved out of a building: the walls (hanging wall and footwall) must stand long enough to empty the room, or you must fill it as you go.

Open stoping

Sublevel open stoping (SLOS/longhole): drill long fans or rings from sublevels, blast into an open void, muck from drawpoints at the bottom with remote LHDs. High productivity, requires competent rock and regular geometry. Vertical crater retreat (VCR) and VRM are variants.

Filled stoping

Cut-and-fill: mine a horizontal slice, fill the void (hydraulic, rock or paste fill), then mine the next slice from the fill — flexible, selective, suits irregular or weak orebodies. Paste fill (tailings + 3–7 % cement) also disposes of tailings underground and allows mining adjacent stopes.

Dilution and recovery

Overbreak of hanging wall and sloughing fill dilute ore; pillars and poor draw leave ore behind. Unplanned dilution of 10–20 % is common. Cavity monitoring surveys (CMS) measure actual stope shapes.

Stope stability

The Mathews stability graph plots N' (from Q', stress, joint orientation and gravity factors) against hydraulic radius to classify stope surfaces as stable, transitional or caving. Engineers size stope spans and cable-bolt walls accordingly.

Formulas

• Ore recovered: T_ore = T_s × R
• Total mined with dilution: T_mined = T_ore / (1 − D)
• Diluted grade: g_mined = g × (1 − D) (zero-grade dilution)
• Hydraulic radius: HR = area / perimeter

Worked example: Stope reconciliation

Given: Stope 30 × 25 × 60 m, ρ 2.8, grade 2.0 % Cu; Recovery 90 %, dilution 10 % (zero grade)

Find: Tonnes mined and grade.

• In situ = 30 × 25 × 60 × 2.8 = 126 000 t
• Ore recovered = 113 400 t
• Mined = 113 400/0.9 = 126 000 t at 1.8 % Cu

Answer: 126 kt at 1.8 % Cu — same tonnage, lower grade, less metal

Worked example: Hydraulic radius

Given: Hanging wall 40 m long × 60 m high

Find: HR.

• HR = 2400/(2 × (40 + 60)) = 12 m

Answer: 12 m → check against Mathews graph for the wall's N'

Common mistakes

• Wrong: Ignoring dilution in reserves. — Right: Apply planned and unplanned dilution to every stope.
• Wrong: Oversized open stopes in weak rock. — Right: Check HR against the Mathews stability graph.

Applications

• Base-metal mines: Sublevel stoping with paste fill (Mount Isa, Kidd).
• Narrow gold veins: Cut-and-fill and longhole with tight dilution control.
• Tailings disposal: Paste fill returns tailings underground.

Key takeaways

• Open, filled and shrinkage stoping.
• T_mined = T_ore/(1 − D); grade × (1 − D).
• HR = area/perimeter for stability.
• Paste fill enables high recovery.

Going further

• Cavity monitoring: Laser scans of stopes measure overbreak.
• Stope optimisation: MSO/Deswik SO generate optimal stope shapes.
• Fill design: Paste strength requirements for exposure.