Mining Methods: Surface Mining, Haul Roads, Loading and Transport, Continuous and Highwall Mining, Bord and Pillar, Longwall, Thick Seams and Metal Stoping
1. Surface mining: layout, stripping ratio and haul roads
An opencast mine is a sequence of benches — each with a height, a face angle and a berm (bench width) — that together make the overall pit slope. With bench height H, face angle α and berm width b, each bench steps back H/tan α + b horizontally, so the overall angle is tan⁻¹[H/(H/tan α + b)]; 10 m benches at 70° with 8 m berms give an overall slope of 40.67°. The overall slope, not the face angle, is what the geomechanics of the previous section has to approve, and every degree steeper saves a great deal of waste in a deep pit.
The stripping ratio (SR) is waste removed per unit of mineral won — m³ of overburden per tonne of coal, or t/t in metal mines. The overall SR is total waste over total ore for the pit, the instantaneous SR that of the current period. The break-even stripping ratio is the most waste a tonne of ore can pay for: BESR = (value of ore − cost of mining and processing it)/cost of stripping a unit of waste. Where the SR at the pit limit exceeds the BESR, further opencast mining loses money, and that is one way to set a pit limit or the change-over depth to underground mining.
- Haul road width: a common design rule for a two-lane road is about 3.5 times the width of the widest vehicle, with safety berms (windrows) at least as high as half the largest tyre on the outer edge.
- Gradient: ramps are commonly kept to about 8–10%, since truck speed and brake duty worsen sharply beyond that; curves are flattened on ramps.
- Superelevation on a curve of radius R (m) for speed v (km/h): e + f = v²/(127R), where f is the side friction factor; with f neglected, e = v²/(127R).
- Construction and upkeep: a compacted sub-base, a crushed-rock wearing course, a crown or cross-fall for drainage, adequate sight distance, and water spraying for dust.
2. Loading, transport, continuous systems and highwall mining
The cyclic system — a loader and a fleet of dumpers — is the workhorse of surface mines. Loaders are rope shovels, hydraulic excavators (face or backhoe), front-end loaders and draglines, the last casting overburden directly into the void without trucks. A loader’s hourly output in bank volume is Q = (3600/tc) × C × F/S × E, with tc the cycle time in seconds, C the bucket capacity, F the fill factor, S the swell factor (loose over bank volume) and E the job efficiency. A 10 m³ shovel with a 30 s cycle, fill factor 0.9, swell 1.25 and 50 working minutes an hour gives 120 × 7.2 × 50/60 = 720 bank m³/h.
Matching trucks to the loader: a truck is filled in a whole number of passes, and the number of trucks that keeps the loader busy is N = truck cycle time/loading time per truck; a 24 min truck cycle with 4 min loading needs six trucks. Fewer trucks leave the shovel waiting, more leave trucks queuing — the queuing model of the systems-engineering chapter. Continuous mining systems replace the cycle with a stream: the bucket wheel excavator feeding conveyors and spreaders in soft ground; the surface miner, a cutting drum that mills a seam in layers without blasting and loads it directly; and in-pit crushing and conveying (IPCC), where a crusher in the pit turns hard rock into conveyable size and a belt replaces the longest truck haul.
Highwall mining recovers coal left in the final highwall of an opencast mine, where further stripping would cost too much. A remotely operated cutter — a continuous highwall miner with a train of conveyor modules, or an auger in thinner seams — drives parallel unsupported entries hundreds of metres into the seam from the pit floor, leaving web pillars between the entries and barrier pillars at intervals to support the highwall. No one enters the drives, and the web pillar width is designed like any other pillar.
3. Underground coal mining: bord and pillar, longwall, thick seams
In bord and pillar mining the seam is first developed by driving a grid of galleries (bords) and cross-cuts, leaving square or rectangular pillars; development alone extracts only the galleries — pillars 25.2 m square on 30 m centres (4.8 m galleries) give 1 − (25.2/30)² = 29.44%. The pillars are then depillared, extracted in a planned sequence by splitting and slicing, with the roof allowed to cave behind the working line or the goaf filled by stowing where the surface must be protected. Work is organised in panels separated by barrier pillars, so that a fire or an inrush can be sealed off in one panel. Mechanised bord and pillar uses side-discharge loaders and load-haul-dumpers, or a continuous miner with shuttle cars and roof bolters.
In longwall mining a single long face — commonly 150–300 m — is extracted in slices across its whole length. A shearer with ranging drums cuts a web of coal onto an armoured face conveyor (AFC), which delivers it to the stage loader and the gate conveyor; hydraulic powered supports hold the roof over the face and advance behind the shearer, letting the roof cave behind them. In retreating longwall the gate roads are driven to the panel boundary first and the face retreats towards the main roads, so the gates are abandoned into the goaf behind it; in advancing longwall the gates are formed as the face advances and have to be maintained along the goaf edge. Retreating is the modern norm: the panel is proved before extraction and no road has to be held open in the goaf. A face 200 m long taking 0.8 m webs at 3 m height in coal of 1.4 t/m³ yields 672 t a cut, so ten cuts a day give 6720 t.
| Method | How the thickness is won |
|---|---|
| Multi-slice (inclined or horizontal slicing) | the seam is worked in successive slices, descending under a caved or stowed goaf, or ascending on stowing |
| Longwall top coal caving (LTCC) | a longwall cuts the bottom section and the top coal caves behind the supports onto a rear conveyor |
| Blasting gallery | developed galleries are widened and the full height of pillar coal is broken by long-hole ring blasting and loaded by remote LHDs |
| Sublevel caving in coal | sublevels are driven and the coal between them caved and drawn |
4. Underground metal mining, stope mechanisation and ore handling
Metal-mine stoping methods are classed by what holds the ground up while the ore is removed. The choice turns on the dip, thickness and shape of the orebody, the strength of the ore and walls, the grade and value, and whether the surface may subside.
| Class | Methods | Suited to |
|---|---|---|
| Open (naturally supported) | room and pillar, sublevel (long-hole) stoping, vertical crater retreat (VCR), shrinkage stoping | competent ore and walls; flat (room and pillar) or steep (the others) orebodies |
| Supported | cut and fill, stull stoping, square-set stoping | weak walls, irregular or high-value orebodies, where selectivity and low dilution matter |
| Caving | sublevel caving, block caving (and longwall in coal) | large, massive, low-grade orebodies whose ore and cover cave well, where surface subsidence is acceptable |
- Shrinkage stoping breaks ore upward in horizontal slices and draws off only the swell (about a third), so the broken ore left in the stope is both the working floor and temporary wall support; it is drawn empty at the end.
- Sublevel stoping drills long rings or fans from sublevel drifts and blasts them into an open stope, drawn from drawpoints below; VCR loads large-diameter vertical down-holes with short spherical charges that break horizontal slices upward to a free face below.
- Cut and fill removes a horizontal slice, fills it (hydraulic, paste or rock fill) and works the next slice from the fill; block caving undercuts a large block so that it caves under gravity and its own stress, broken ore being drawn through drawpoints on an extraction level.
- Stope mechanisation and ore handling: drill jumbos and long-hole rigs, LHDs (load-haul-dump) and low-profile trucks carry ore to ore passes (inclined raises that drop ore by gravity between levels), with grizzlies to hold back oversize, chutes and loading stations to fill cars or skips, an underground crusher ahead of the skip-loading pocket, and skip hoisting in the shaft.
Key takeaways
- Overall slope = tan⁻¹[H/(H/tan α + b)]; the overall, not the face, angle decides the waste in a pit.
- BESR = (ore value − ore cost)/stripping cost; where the pit-limit SR exceeds it, opencast mining stops paying.
- Loader output = (3600/tc) × C × F/S × E in bank m³/h; trucks needed = truck cycle/loading time.
- Bord and pillar develops then depillars; retreating longwall proves the panel first; LTCC and blasting gallery win thick seams.
- Open stoping needs competent ground, supported stoping pays for weak walls and rich ore, caving suits big low-grade orebodies.
Practice questions (15)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
An opencast coal block will produce 2 million tonnes of coal and requires the removal of 5 million m³ of overburden. The stripping ratio, in m³/t, is ____.
Numerical answer — type the value.
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Answer: 2.5
SR = waste/coal = 5 × 10⁶ m³/2 × 10⁶ t = 2.5 m³/t. Inverting it gives 0.4 t/m³, which is coal per unit of waste; converting the overburden to tonnes first would give a t/t ratio instead of the m³/t the question asks for.Ore in an opencast mine is worth ₹2000 per tonne after processing, and mining and processing it costs ₹1200 per tonne. Stripping waste costs ₹100 per tonne. The break-even stripping ratio, in t of waste per t of ore, is ____.
Numerical answer — type the value.
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Answer: 8
BESR = (value − cost of ore)/cost of stripping = (2000 − 1200)/100 = 8 t/t. Dividing the value alone by the stripping cost (20) forgets that the ore itself must be paid for, and 1200/100 = 12 uses the wrong term.A pit wall is made of 10 m high benches with a face angle of 70° and 8 m wide berms. The overall slope angle, in degrees, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 40.67
Each bench steps back 10/tan 70° + 8 = 3.640 + 8 = 11.640 m for 10 m of height, so the overall angle is tan⁻¹(10/11.640) = 40.67°. Ignoring the face (10/8) gives 51.3°, and quoting 70° mistakes the bench face for the wall.A 10 m³ shovel works on a 30 s cycle with a fill factor of 0.9. The swell factor (loose/bank volume) is 1.25 and the shovel works 50 minutes in each hour. Its output, in bank m³ per hour, is ____.
Numerical answer — type the value.
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Answer: 720
Cycles per hour = 3600/30 = 120; bank volume per cycle = 10 × 0.9/1.25 = 7.2 m³; output = 120 × 7.2 × 50/60 = 720 bank m³/h. Multiplying by the swell factor instead of dividing gives 1125, and leaving out the efficiency gives 864.A dumper takes 4 minutes to be loaded and its complete haul cycle, including loading, is 24 minutes. The number of dumpers that keeps the shovel continuously busy is ____.
Numerical answer — type the value.
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Answer: 6
N = truck cycle/loading time = 24/4 = 6: while one truck loads, five are travelling, dumping or returning. Using the travel time alone (20/4 = 5) forgets that the cycle already includes loading, and five trucks would leave the shovel idle.A haul road curve has a radius of 100 m and a design speed of 36 km/h. Neglecting side friction, the superelevation required (as a ratio), to three decimal places, is ____.
Numerical answer — type the value.
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Answer: 0.102
e = v²/(127R) = 36²/(127 × 100) = 1296/12 700 = 0.102. The 127 already converts km/h to m/s with g = 9.81; using v = 10 m/s in the same formula gives 0.0079, a unit error, and v²/(gR) with v in m/s gives the same 0.102.In highwall mining, coal is extracted by:
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Answer: B — a remotely operated cutter driving unsupported entries into the seam from the pit floor, leaving web pillars
Highwall mining sends a remotely controlled miner or auger into the exposed seam; no one enters the drives, so they are unsupported, and web pillars between them carry the highwall. Stripping further is exactly what it avoids, and a dragline moves overburden, not coal.Which of the following is a continuous surface mining system?
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Answer: C — Bucket wheel excavator with conveyors
The bucket wheel excavator digs continuously and discharges onto a belt that runs without interruption. Shovel–dumper and loader–truck systems work in cycles, and a dragline, although it needs no trucks, still digs, swings and dumps in discrete cycles.A coal seam is developed on the bord and pillar system with pillars on 30 m centres and galleries 4.8 m wide. The percentage extraction during development, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 29.44
Pillar side = 30 − 4.8 = 25.2 m, so extraction = 1 − (25.2/30)² = 1 − 0.7056 = 0.2944 = 29.44%. Using 4.8/30 = 16% counts galleries in one direction only; the area left in pillars is 70.56%.A longwall face is 200 m long and the seam is extracted to 3 m height. The shearer takes a 0.8 m web and the face completes 10 cuts a day. The coal density is 1.4 t/m³. The daily production, in tonnes, is ____.
Numerical answer — type the value.
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Answer: 6720
Per cut = 200 × 0.8 × 3 × 1.4 = 672 t, and 10 cuts give 6720 t/day. Leaving out the density gives 4800 m³, a volume not a tonnage; real faces lose a few per cent to face-end work and stoppages, which the question does not ask for.Which of the following statements about longwall mining are correct? (Select all that apply.)
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Answer: A — In retreating longwall, the gate roads are driven to the panel boundary before extraction begins; C — The armoured face conveyor carries the cut coal along the face to the stage loader; D — Powered supports advance behind the shearer and the roof caves behind them
Retreating longwall drives the gates first and abandons them into the goaf; the AFC runs along the face into the stage loader; and the supports step forward behind the shearer, letting the goaf cave. It is advancing longwall that forms its gates as the face moves on and must keep them open along the goaf edge.A 9 m thick coal seam is to be worked in one pass by cutting its lower part with a longwall and recovering the upper part as it caves behind the supports. The method is:
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Answer: B — Longwall top coal caving
LTCC cuts the bottom section with a normal longwall and lets the top coal cave through windows in the supports onto a rear conveyor — one pass for the full thickness. Multi-slice working takes several passes, and the blasting gallery method breaks pillar coal by long-hole blasting from widened galleries.In which stoping method is most of the broken ore left in the stope to serve as the working floor and as temporary support for the walls?
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Answer: B — Shrinkage stoping
Shrinkage stoping draws off only the swell after each slice, keeping the stope full of broken ore to stand on until the stope is finished. Sublevel stoping keeps an empty open stope, block caving draws caved ore continuously, and square-set stoping holds the ground with timber sets.Which of the following statements about metal-mine stoping methods are correct? (Select all that apply.)
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Answer: B — Block caving suits large, massive, low-grade orebodies that cave readily; C — Cut and fill suits steep, irregular orebodies with weak walls; D — Sublevel stoping requires competent ore and wall rock
Caving relies on the ore breaking under gravity and pays only at large tonnage; cut and fill supports weak walls and follows irregular ore selectively; an open sublevel stope stands only in competent rock. VCR uses large-diameter vertical down-holes with spherical charges, not short horizontal holes.In an underground ore-handling system, the main purpose of a grizzly at the top of an ore pass is to:
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Answer: B — hold back oversize lumps that would block the pass
A grizzly is a grid of bars that passes ore below a set size and retains boulders, which are broken by a rock breaker before they can hang up in the pass. It does not weigh, concentrate or ventilate; density separation belongs to the mineral processing plant.