A mining engineer gets a phone call about a copper-gold prospect. The resource report says 42 million tonnes at 0.9% copper equivalent, and the geologist is enthusiastic. The first question the engineer asks is not about grade. It is: how deep does the ore start, and how much waste sits on top of it? Those two numbers — depth and waste volume — decide whether the project becomes an open pit, an underground mine, or nothing at all. Get them wrong and a project that looked like a $600 million asset turns into a liability nobody will finance.
This is the part of mining that trips up newcomers and, sometimes, seasoned investors: the assumption that one extraction method is inherently better. In reality, open pit vs underground mining is an economic comparison, and the deciding tools are the strip ratio, the deposit depth cutoff, geometry, grade, and recovery. Below is how a practitioner actually walks through that decision — with numbers, the order of steps, and where people go wrong.
The number that decides everything: strip ratio
Strip ratio is the tonnes of waste you must move to extract one tonne of ore. A 3:1 ratio means three tonnes of waste for every tonne of ore. It sounds like a simple accounting figure. It is actually the single most powerful lever on a project's economics, because waste removal is pure cost with no revenue attached.
Imagine a deposit with 20 Mt of ore and 60 Mt of waste overlying it. Strip ratio is 3:1. Now imagine the same ore under 120 Mt of waste — 6:1. The ore is identical. The grade is identical. But the second project must move twice as much material, and every tonne of that material is diesel, tyres, blasting, haul-truck hours, and a larger fleet. At a haulage cost of roughly $2–4 per tonne moved depending on distance and equipment, the difference between 3:1 and 6:1 on a 20 Mt orebody is tens of millions of dollars over the mine life — before you even reach the ore.
This is why strip ratio is the first filter, not the grade. A high-grade deposit buried under a mountain of waste can lose to a lower-grade deposit that sits near the surface.
Breaking-even strip ratio
The practical version of the question is: at what strip ratio does open pit stop being viable? You compare the cost of extracting a tonne of ore by open pit (including its share of waste stripping) against the cost of extracting that same tonne underground. If open pit cost per ore tonne climbs above underground cost, the pit loses — assuming recovery is comparable.
For example: suppose underground mining costs about $55 per tonne of ore hoisted, while open pit ore mining costs $18 per tonne plus $3 per tonne of waste. At 3:1, open pit costs $18 + (3 × $3) = $27 per tonne. At 10:1, it costs $48. At 12:1, it crosses $54 — roughly the underground figure. That crossing point is the break-even strip ratio, and it is deposit-specific because every cost line differs.
Deposit depth cutoff: where the pit runs out of room
There is a physical limit that no spreadsheet can override: a pit can only get so deep before the walls will not stand up. As you deepen an open pit, the walls must be flattened to stay stable — a pit wall at 45° near the surface may need to be laid back to 35° or less at depth, and every degree of flattening means removing more waste to reach the same ore. The strip ratio climbs steeply with depth, and at some point it becomes uneconomic.
The deposit depth cutoff is that depth — the point below which open pit mining is no longer the cheaper option. In practice, most open pits struggle to justify depths beyond roughly 300–500 metres, and many economic pits are far shallower. Below that cutoff, the ore is a candidate for underground methods.
But here is the trap: the cutoff is not a fixed number you look up. It depends on:
- Ore grade and metal price — a high-grade deposit can justify deeper, more expensive pits.
- Wall rock competence — weak, altered rock forces flatter walls and kills the pit earlier.
- Recovery — if underground recovery is poor, a deeper pit may still win.
- CAPEX — a shaft or decline costs hundreds of millions; sometimes the pit wins simply because the underground alternative cannot be financed.
A common mistake is to treat 400 metres as a universal rule. It is not. A narrow, high-grade vein might go underground at 150 metres because the pit would be all waste. A wide, low-grade porphyry might justify a 600-metre pit because the ore is continuous and the walls are competent.
Geometry matters as much as depth
An orebody's shape decides which method can even physically access the ore. A flat, tabular seam near the surface is a natural open pit. A steeply dipping vein that plunges to 800 metres is a natural underground target, because a pit would have to remove enormous amounts of waste to chase a narrow ore zone downward.
Picture two deposits, both 10 Mt of ore. The first is a 200-metre-thick blanket starting 30 metres below surface. The second is a 4-metre-wide vein dipping 70° and extending to 700 metres depth. The first screams open pit — low strip ratio, simple geometry, cheap access. The second screams underground — a pit chasing a 4-metre vein would have a strip ratio in the dozens, and the ore is narrow enough that a decline and stopes make far more sense.
Geometry is often the deciding factor before economics even enters the room. If the shape does not fit the method, no grade will save it.
Grade, recovery, and the CAPEX/OPEX trade-off
Once depth and geometry point to a method, the economics close the case. Two cost structures compete:
| Factor | Open pit | Underground |
|---|---|---|
| CAPEX | Lower upfront, scalable fleet | High — shaft, decline, ventilation, ground support |
| OPEX per ore tonne | Lower if strip ratio is low | Higher — drilling, blasting, hoisting, ventilation |
| Recovery | Often higher, simpler ore handling | Can be lower; dilution is a real risk |
| Ramp-up time | Faster | Slower — development takes years |
| Selectivity | Low — must mine waste with ore | High — can chase high-grade zones |
Recovery is the quiet variable. Underground mining introduces dilution — waste rock mixing into ore from blasting and stoping — which lowers the grade delivered to the mill. If dilution drops your head grade by 15%, the mill processes the same tonnes for less metal, and the underground project's economics can collapse. Open pit has its own dilution, but it is usually more predictable.
The trade-off is rarely clean. Underground offers selectivity and access to deep, high-grade ore but costs more per tonne and takes longer to build. Open pit offers scale and speed but demands low strip ratios to stay competitive. The right method is the one whose cost structure matches the deposit's geometry and grade, not the one that sounds more modern.
The walkthrough: a deposit in five steps
Here is how a practitioner actually works through mining method selection on a real prospect. Take a hypothetical porphyry: 80 Mt ore at 0.6% copper, top of ore at 25 metres, bottom at 450 metres, moderate wall rock, metal price around $9,000/t copper.
- Estimate the pit strip ratio. Model the pit shell at the chosen metal price. Suppose the shell gives a 2.5:1 strip ratio. Waste is 200 Mt. That is a lot of material but a manageable ratio.
- Check the depth cutoff. The orebody bottoms at 450 metres. A pit to that depth is feasible but wall angles will flatten near the bottom — re-model with flatter walls and the strip ratio may climb to 3.5:1. Still viable.
- Compare to underground. An underground block cave could access the deeper, higher-grade core. But the CAPEX is high and the ore is low grade — the underground option likely loses on cost per tonne.
- Test recovery and dilution. If underground dilution drops head grade to 0.5% Cu, the underground case weakens further.
- Decide and stress-test. Open pit wins here. Then re-run at a 20% lower copper price — does the pit still clear its break-even strip ratio? If not, the project is price-sensitive and the plan needs a higher-grade starter pit.
The most common failure is skipping step 2 — assuming the pit can go as deep as the ore does. The second most common is ignoring dilution in the underground case, which flatters the underground option and leads to a plan that disappoints in production.
Where self-study runs into a wall
The mechanics above are learnable. The hard part is that mining method selection is a calculation chain: resource estimation feeds the pit shell, the pit shell feeds the strip ratio, the strip ratio feeds the method comparison, and the method comparison feeds the NPV. Get one link wrong and the whole chain misleads you. Self-study through scattered papers and YouTube lectures gives you the concepts but not the chain — and no one checks whether your strip ratio is calculated on the right tonnes or whether your dilution assumption is realistic.
This is where structured learning pays off. A course that walks through the full chain — geology, resource estimation under JORC/CRIRSCO, mining methods, economics — gives you the order of operations and someone to check your numbers against. The Geology & Mining Engineering course does exactly this, with Excel calculators for deposit evaluation and worked examples of the method comparison. It is one path among several: a university degree gives depth but takes years; an internship gives practice but rarely the full economic picture; a course sits in between — faster than a degree, more structured than self-study.
If your interest is the safety and containment side of deep geological work, the repository-versus-storage comparison in Nuclear Energy & Radiation Safety (IAEA, NRC) covers a parallel decision framework for subsurface engineering. More on the topic: Industrial Safety.
Who this decision framework suits — and who it does not
This way of thinking fits people who need to make or evaluate a mining decision: junior geologists and mining engineers moving from theory to project work, investors and analysts assessing a feasibility study, and technical professionals in adjacent fields who need to read a mining report without being fooled by a glossy strip ratio. It also suits students preparing for a resource estimation or mine planning role, where the interview question is often exactly this: why open pit and not underground?
It does not suit someone looking for a shortcut to operating a mine. Method selection is a planning skill, not an operating one — it tells you what to build, not how to run it. And it does not replace site experience: no course will teach you the feel for a wall that is about to fail or a ventilation circuit that is underperforming. Those come from the ground.
What the framework does give you is a checkable criterion. Instead of arguing about which method is better in the abstract, you calculate the strip ratio, find the depth cutoff, compare the cost per ore tonne, and let the numbers decide. That is the difference between an opinion and an engineering decision.
If you want to work through that chain with worked examples and calculators rather than assembling it from scattered sources, the Geology & Mining Engineering course covers the full path from deposit geology to the economics that decide the method — and you can start with the first module today.