Room & Pillar Mining

Room-and-pillar mining leaves a grid of pillars to hold up the roof of flat-lying deposits (coal, potash, salt, limestone). Learn tributary-area pillar stress, pillar strength formulas (Salamon–Munro), factor of safety, extraction ratio trade-offs, and retreat (pillar robbing).

In room-and-pillar mines, the overburden weight that rested on the whole area now rests only on the pillars. Pillars too small → collapse (Coalbrook, 1960: 435 deaths). Pillars too large → ore left behind. Pillar design balances safety and extraction.

An analogy

The pillars are like legs of a table holding up a heavy load: fewer, thinner legs carry more stress each.

Layout

Rooms (entries) and crosscuts are driven in a regular grid leaving square or rectangular pillars. Continuous miners or drill-and-blast cut rooms; shuttle cars or conveyors remove ore; roof bolts support the roof.

Pillar stress and strength

Tributary-area theory assumes each pillar carries the overburden above its share of the area. Pillar strength rises with width and falls with height (width-to-height ratio effect). Wide, squat pillars are much stronger.

Extraction

Typical first-pass extraction is 40–60 %. On retreat, pillars can be partly or fully extracted (robbing) to allow controlled caving — increasing recovery but requiring careful sequencing.

Beyond tributary area

Numerical models (LaModel, FLAC) capture load transfer to barrier pillars and abutments. Probabilistic design and floor/roof failure checks (weak floors can punch) complete the analysis. Salt and potash creep over time.

Formulas

• Tributary area stress: σ_p = σ_v (w + B)² / w² = σ_v / (1 − e)
• Extraction ratio: e = 1 − w² / (w + B)²
• Salamon–Munro strength (coal): σ_s = 7.176 w^0.46 / h^0.66 (MPa)
• Factor of safety: FoS = σ_s / σ_p (≥ 1.6 for coal)

Worked example: Pillar stress and FoS

Given: Depth 300 m, pillars 20 m, rooms 8 m, height 3 m

Find: σ_p, strength, FoS, extraction.

• σ_v = 0.025 × 300 = 7.5 MPa
• σ_p = 7.5 × (28/20)² = 14.7 MPa
• σ_s = 7.176 × 20^0.46/3^0.66 = 13.8 MPa → FoS = 0.94
• e = 1 − (20/28)² = 49 %

Answer: FoS 0.94 — unsafe!

Worked example: Redesign

Given: Same conditions, try w = 36 m

Find: FoS and extraction.

• σ_p = 7.5 × (44/36)² = 11.2 MPa
• σ_s = 18.1 MPa → FoS = 1.61
• e = 1 − (36/44)² = 33 %

Answer: Safe at FoS 1.6 but extraction drops from 49 % to 33 %

Common mistakes

• Wrong: Ignoring the width-to-height effect. — Right: Tall pillars are weaker — check w/h.
• Wrong: Using surface depth at a valley for stress. — Right: Use the full overburden depth at each panel.

Applications

• Coal: Bord-and-pillar in South Africa, India, USA.
• Potash & salt: Large rooms in evaporites; creep considered.
• Limestone: Underground aggregate mines with big pillars.

Key takeaways

• σ_p = σ_v (w + B)²/w².
• Strength rises with width, falls with height.
• FoS ≥ 1.6 for coal pillars.
• Safety vs extraction trade-off.

Going further

• LaModel: Displacement-discontinuity modelling of coal pillars.
• Pillar retreat: Controlled robbing sequences and mobile roof supports.
• Probabilistic design: Probability of failure from strength variability.