Metal mining is a different world from coal mining. The ore body is not a neat flat seam. It can be a thin steep vein, a thick lens, or a massive body of low-grade rock, and every one of those shapes asks for a different way of getting the ore out. That is why the topic has so many named methods, and why it keeps appearing in exam papers.
This post walks through the basics of underground metalliferous mining in a simple order: the key terms, how raises are driven, how sampling works, how to calculate the average grade of an ore block, the twelve common stoping and caving methods, and finally the rules about who may manage a metal mine. The material follows the chapter on mining methods in A Complete Guide for Mining Engineers.
1. Key terms to know first
Gestation period
This is the time between the start of mining work and the point where ore production actually begins. In metal mines it is usually very long, because shafts, levels and raises must be developed before the first tonne can be drawn.
Cut-off grade
The grade below which the mineral can no longer be processed at a profit. Ore richer than this is worth mining; ore poorer than this is treated as waste.
Break-even cut-off grade
The grade at which the net value is exactly zero: income equals cost, so you neither gain nor lose.
2. Raises and winzes
A raise or winze is a drivage that connects one level to another during the development stage. A raise is driven upwards; a winze is sunk downwards from a level. They give you ventilation, ore passes, manways and the start of stopes.
Raising methods
- Open raising by drilling, blasting and mucking: suitable for moderate lengths (up to about 8 m), inclinations of 40° to 60°, and hard strata or country rock.
- Two or three compartment raising: for vertical and very steep raises, limited to about 15-20 m.
- Jora raise lift: a mechanised method that suits raises up to 100 m long, but it is not used much now.
- Long hole drilling: practical limit is about 15-50 m, with raises up to 35° from vertical. This method gives the lowest cost per metre of drivage and the highest speed.
- Alimak raise climber: for inclinations greater than 40° from horizontal, with raise length not more than about 150 m.
- Raise borers: for limited diameters (roughly 1 to 3.7 m). A pilot hole of 230-300 mm is drilled first and then back-reamed to the required size.
- Drop raising: suited to raises of 50-60 m that are nearly vertical. Holes are charged and blasted using Vertical Crater Retreat (VCR), with spherical charges having a length to diameter ratio of about 6:1.
3. Sampling in mines
Sampling means judging a large mass of rock by studying only a small part of it. In mines there are four main types: channel or groove sampling, chip sampling, grab sampling and bulk sampling.
Channel sampling is the one most often used when sampling underground openings such as drifts, tunnels, adits and cross cuts, and also trenches dug in rock. It can be hard work, depending on the hardness of the rock. The best approach is to cut a channel roughly half an inch to one inch deep and about six inches wide along the full length of the wall being sampled. The chips are collected as the rock is removed and stored in sample bags, with each bag representing a regular sample interval. If the rock type changes, the sample interval should follow the change in rock type instead.
4. Average grade of an ore block
Real ore blocks are rarely uniform, so we divide the block into sections, measure the length, width and grade of each, and take a weighted average.
- L = total length of the block
- W = average width of the block
- gi = grade of the i-th section
For a block split into three sections, this becomes:
5. Twelve metal mining methods
Choosing a method is mostly a matching exercise: dip of the deposit, thickness, strength of ore and walls, and value of the ore. Here is each method in plain terms.
Supported methods
1. Overhand stoping
- Work proceeds from the lower main level towards the upper main level.
- Used in steeply dipping narrow veins, or bedded deposits 2-3 m thick.
- Needs strong ore and stable wall rocks; ore is drilled with compressed air drills.
2. Underhand stoping
- Work proceeds from the upper main level towards the lower level, starting from a winze.
- Suited to thin, steeply dipping veins (more than 40°) with strong wall rocks.
- Drawbacks: ore transport is difficult and blasting efficiency is poor.
3. Breast stoping
- For horizontal or mildly dipping deposits up to about 3 m thick (more than 3 m needs two lifts).
- Ore and hanging wall must be strong.
- Low cost and preferred for low-grade ore; lends itself to large-scale mechanisation of drilling, blasting, loading and transport.
- The worked-out space is supported by irregular pillars, timber cribs and props.
4. Sublevel stoping
- A sublevel is a level between the two main levels. Work proceeds from lower to upper level.
- Needs a steeply dipping, thick ore body with stable hanging wall and foot wall, competent ore and regular boundaries.
- Long-hole drilling of 50-200 mm diameter and up to 90 m length in a ring-hole or fan pattern.
- A draw point system is used: loading cross-cuts with finger raises and cones up to the undercut.
5. Long-hole (blast-hole) stoping
- For steeply dipping (more than 50°) or vertical deposits.
- Large diameter holes (150-200 mm) up to 60-70 m long, drilled from the upper level to the lower main level.
- Holes are blasted by the VCR (vertical crater retreat) method.
6. Room and pillar mining
- Ore and hanging wall should be strong.
- For flat or mildly dipping ore bodies (less than 30°), with thickness of about 3-20 m.
- In thicker deposits, excavation starts with the top section and the rest is recovered by benching in one or more steps.
7. Shrinkage stoping
- For steeply dipping deposits with free-flowing ore.
- The ore must not be affected by storage in the stope (some sulphide ores oxidise and decompose when exposed to air).
- Suitable for ore bodies 3-12 m thick with comparatively strong hanging wall and foot wall.
- About 40% of the broken ore is drawn during mining; ore recovery is 75-90%.
8. Cut and fill stoping
- Works in steeply as well as mildly dipping ore bodies with firm ore, and in small as well as large deposits.
- Offers selectivity and low dilution; the operation is cyclic.
- Provides permanent support to surface structures.
9. Square set stoping
- Walls and back of the stope are supported by regular timber frameworks called square sets.
- Used for very weak country rock and ore, and for recovering fracture remnants.
- The ore must be high grade to pay for such a costly and labour-intensive method.
10. Resuing
- A method of stoping for narrow veins in steeply inclined deposits.
- Involves blasting a sizeable quantity of barren wall rock in contact with the vein.
Caving methods
11. Block caving
- For steeply dipping deposits or massive ore bodies.
- The ore must be weak enough to cave under its own weight.
- Used for low-value, low-grade ore, and the surface is allowed to cave.
12. Sub-level caving
- The overburden and part of the ore are induced to cave in, so both should be weak in nature.
- For steeply dipping deposits; fan drilling is suitable for extraction.
- Dilution varies from 10 to 35%, and ore loss is between 5 and 20%.
Quick comparison table
| Method | Deposit type | Key point |
|---|---|---|
| Overhand stoping | Steep narrow veins, 2-3 m bedded | Lower to upper level; strong ore and walls |
| Underhand stoping | Thin veins, more than 40° | Upper to lower level; poor blasting efficiency |
| Breast stoping | Flat or mild dip, up to 3 m | Low cost, mechanisable, low-grade ore |
| Sublevel stoping | Steep, thick, competent | Ring/fan holes, 50-200 mm, up to 90 m; draw points |
| Long-hole stoping | More than 50° or vertical | 150-200 mm holes, 60-70 m; VCR blasting |
| Room and pillar | Less than 30°, 3-20 m thick | Thick bodies benched from the top |
| Shrinkage stoping | Steep, 3-12 m thick | 40% drawn during mining; 75-90% recovery |
| Cut and fill | Steep or mild dip, firm ore | Selective, low dilution, cyclic, surface support |
| Square set | Very weak rock and ore | Timber frames; costly, high-grade ore only |
| Resuing | Narrow steep veins | Blast barren wall rock beside the vein |
| Block caving | Steep or massive, weak ore | Low-grade ore; surface allowed to cave |
| Sub-level caving | Steep, weak ore and cover | Dilution 10-35%, ore loss 5-20% |
6. Competent persons in underground metal mines
The Metalliferous Mines Regulations (MMR) link the qualification of key officials to the number of persons employed, either below ground or in the whole mine.
Manager (Table 1.13)
| Persons employed (X) | Qualification |
|---|---|
| More than 150 below ground, or more than 400 in the whole mine | First Class Manager's Certificate |
| Between 75 and 150 below ground, or between 150 and 400 in the whole mine | First Class or Second Class |
| Any other case | First Class, Second Class or Manager's Permit |
Assistant Manager / Under Manager (Table 1.14)
The number of assistant managers also depends on the strength of the workforce. For larger mines, one additional Assistant Manager is required for every additional 300 persons below ground, or 800 in the whole mine, or part thereof. Where the workforce is above 1200 below ground, or above 3200 in the whole mine, at least one Assistant Manager must hold a First Class Manager's Certificate.
Winding Engineman (Table 1.15)
| Type of engine | Qualification |
|---|---|
| Electric engine of 75 h.p. or more, or a steam or compressed air winding engine with cylinder diameter above 30 cm | First Class Engine Driver's Certificate |
| Any other case | Second Class Engine Driver's Certificate |
Representative Factor (RF)
The representative factor is the ratio of the actual leasehold area to the area shown on the mine plan. The values given are:
- RF = 200 : 1 for mica mines and other mines with small-scale workings below ground.
- RF = 2000 : 1 for mines with large opencast workings, and for surface plans of large leasehold areas.
- RF = 1000 : 1 in other cases.
My take
If you are new to metal mining, do not try to memorise twelve methods in one go. Start with the dip of the deposit: flat, steep or massive. Then ask whether the ore and the walls are strong or weak. Most of the answers fall out of those two questions. The numbers (holes of 150-200 mm, recovery of 75-90%, dilution of 10-35%) are the part to revise at the end.
Related posts to read next
More topics: Mining Methods, Mine Development, Rock Mechanics and Blasting & Explosives.
Frequently asked questions
What is the difference between a raise and a winze?
Both connect levels. A raise is driven upwards from a lower level, while a winze is sunk downwards from an upper level.
Which raising method is cheapest and fastest?
Long hole drilling, with a practical limit of about 15-50 m and an inclination up to 35° from vertical.
Which stoping method allows ore recovery of 75-90%?
Shrinkage stoping, where about 40% of the broken ore is drawn while the stope is being mined.
Which methods are used for weak ore?
Square set stoping (for very weak rock and high-grade ore) and caving methods such as block caving and sub-level caving.
What is VCR?
Vertical Crater Retreat. Large-diameter holes are blasted with spherical charges, in lifts from the bottom upwards, retreating along the hole.
Source note: This is a simplified explanation of the underground metalliferous mining chapter in A Complete Guide for Mining Engineers. It is meant for learning and exam preparation. For legal requirements, always refer to the current Metalliferous Mines Regulations and DGMS circulars.
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