Good to Know — Rules of Thumb (Mining)
Field-tested approximations and sanity checks for mining engineers. Every rule here is an estimate for reasonableness checks and early-stage thinking — never a substitute for the applicable standard, code, or a qualified person's judgment.
Geotechnical and ground control
- Rock mass rating (RMR) / Q-system: RMR 81–100 very good rock; 61–80 good; 41–60 fair; 21–40 poor; <21 very poor. Q > 40 exceptional; 10–40 very good; 4–10 good; 1–4 fair; 0.1–1 poor. Support pressure scales roughly inversely with Q — use the Barton support chart, not memory, for design.
- Stand-up time: an unsupported 5-m span in fair rock (RMR ~50) stands hours to days; in good rock (RMR ~70), months. When in doubt, support immediately — stand-up time is observed, not guaranteed.
- Pillar strength (hard rock): pillar strength ≈ UCS × (0.5–0.7) × width/height correction — squat pillars (w/h > 1) carry far more than slender ones; slender pillars (w/h < 0.5) are crush-prone. Tributary-area loading: pillar stress = overburden stress / (1 − extraction ratio).
- Slope angles: competent hard rock open pits: 45–55° overall; weathered/weak: 30–40°. A 1° flattening of a deep pit wall can move millions of tonnes of waste — the stripping-ratio sensitivity is the economic argument for geotechnical drilling.
- Factor of safety: slopes ≥1.3 (static), underground excavations ≥1.5 for permanent openings. Anything under analysis gets engineered, not eyeballed.
Ventilation
- Air quantity: ~0.06 m³/s per kW of diesel power (100 cfm per brake-hp is the imperial twin) — the MSHA diesel rule of thumb. Electric equipment needs far less — ventilation is usually sized by the diesel fleet.
- Air velocity: 0.5 m/s minimum in working places to prevent contaminant buildup; keep below ~4–6 m/s in travelways (dust pickup, comfort).
- Methane: <1% in working areas (MSHA action levels escalate at 1.0%, 1.5%, 2.0% — know the triggers); ventilate to dilute, never rely on luck.
- Diesel particulate: DPM is the controlling contaminant in most diesel mines — size ventilation to the DPM limit, and the gases usually take care of themselves.
Fragmentation and blasting
- Powder factor: hard rock ~0.8–1.2 kg/m³ (ANFO); soft/weak rock ~0.3–0.5. If fragmentation is coarse, powder factor is the first suspect, then timing.
- Burden ≈ 25–40 × hole diameter (in consistent units); spacing ≈ 1.15–1.4 × burden. Stemming ≈ 0.7–1.0 × burden.
- Scaled distance: vibration PPV scales with (distance/√charge); doubling distance quarters the PPV roughly (square-root scaling). USBM RI 8507 limits: 1.25 in/s at low frequency for residential structures — know the frequency dependence.
- Flyrock: the exclusion zone is set by the maximum credible throw, not the average — stemming quality controls flyrock more than charge weight.
Mineral processing
- Bond work index: Wi ~10–20 kWh/t for most ores; grinding energy ≈ 10×Wi×(1/√P80 − 1/√F80). Comminution is typically 40–60% of a concentrator's energy — the economic lever is blasting finer (mine-to-mill).
- Recovery vs. grade: pushing recovery past ~90–95% usually collapses concentrate grade — the grade-recovery curve is the plant's fundamental trade-off.
- Thickener: ~0.5–1.0 m²/t/day unit area for flotation concentrates; high-rate thickeners run 2–5× conventional flux.
- Flotation: rougher recovery 80–95%, cleaner stages trade recovery for grade; pH and Eh control selectivity more than reagent dosage does.
Mine economics and cutoff
- Cutoff grade: cutoff = operating cost / (price × recovery × payability). Below cutoff is waste by definition — reclassifying it is a reserve restatement, not optimism.
- Stripping ratio: waste:ore; open-pit economics live or die here. A pit is economic while incremental stripping ratio < breakeven stripping ratio.
- Discount rate: 8–10% real is the industry-standard base case for NPV; sensitivity at ±2% is expected in every study.
- Contingency: scoping ±35–50%, prefeasibility ±20–30%, feasibility ±10–15% (AACE classes 4/3/2). Applying feasibility-level confidence to scoping numbers is the classic promotion error.
Hydrology and dewatering
- Pit inflows: pump for the 1-in-100-year storm event in the design; size sumps for the 1-in-10 with contingency pumping.
- Drawdown: a well's radius of influence ≈ 3,000 × drawdown × √permeability (Sichardt, rough) — order-of-magnitude only; pump tests beat formulas.
- Acid rock drainage: net acid-generating if NNP < 0 (NP − AP); kinetic tests (humidity cells) predict the rate that static ABA can't.
Safety and health
- Ground control: most underground fatalities are ground-fall related — scaling and support discipline is the life-critical routine.
- Diesel: DPM is a confirmed carcinogen (IARC Group 1) — ventilation, filtration, and fleet electrification are the controls in that order.
- Silica: respirable crystalline silica OEL ~0.05 mg/m³ (MSHA/OSHA) — dust control at the source (water, ventilation, enclosures), respirators last.
- Fatigue: 12-hour shifts + long commutes + heat = the incident triangle in surface mining — roster design is a safety control.
Niche credentials worth knowing
- QP-MMSA (Qualified Person — Mining and Metallurgical Society of America): the US professional designation for taking responsibility for technical disclosures — the domestic analogue of the P.Geo/P.Eng QP roles under NI 43-101.
- MSHA instructor approvals, state blaster licenses — jurisdiction-specific tickets that gate who can do the work, covered in the mining guides.
Cross-discipline sanity checks
- Electrical: 0.06 m³/s per diesel kW (above) also sizes your ventilation power bill — fans are often the mine's largest single electrical load.
- Structural: headframe and bin loads — bulk densities (broken rock ~1.6 t/m³) for every load calc.
- Environmental: a gram of prevention in ARD characterization beats a water-treatment plant in perpetuity — the cheapest treatment plant is the one never built.
Original ETS reference writing, verified against public sources on 2026-09-30. Thin sourcing is flagged inline. Not legal advice — confirm current editions with the issuing body.