If you are preparing for the DGMS or Mining Sirdar exams, opencast safety is one of those chapters where a single number can decide the mark. Is it 1.5 m or 3 m? 45° or 70°? Most students lose marks right there, not on the concepts.
The good news is that this chapter follows a clean pattern. Benches, dump yards, haul roads, dumpers, machines and, the tricky one, working an opencast above old underground galleries. We will take them one at a time, with diagrams and a number sheet at the end. For the wider picture of why these rules exist, you can also read our post on strict adherence to safety regulations in open-cast mining.
1. Bench Safety in Opencast Mines
Q. What precautions must be taken for the safety of benches?
A. Soft soil, murrum, gravel, clay or loose debris
- Sides are sloped safely, and the slope must not be more than 45° from the horizontal.
- Or the sides are cut as steps (benches). Each bench is not more than 1.5 m high, and its width is not less than its height.
B. Coal
Coal faces are either sloped at not more than 45° or cut in benches. Here the bench height may go up to 3 m, and the width must still be at least equal to the height.
C. Rules that apply everywhere
- No tree, loose boulder or debris is allowed within 3 m of the edge of any bench.
- Nobody may undercut a bench so that it forms an overhang, and nobody may allow it to be done.
D. Mechanised opencast mines
- Bench height must not exceed the boom height of the loading machine. If the machine digs or picks up material from below its own level, the height must not exceed the digging depth the boom can reach.
- The bench slope must not be steeper than 70° from the horizontal.
- Bench width is worked out like this:
- not less than the bench height;
- if dumpers run on the bench, not less than 3 times the dumper width;
- the width of the largest machine on the bench plus 2 m;
- take whichever of the above gives the greatest width.
2. Dump Yard Rules
Q. What are the legal provisions for a dump yard in opencast mines?
- The overburden dump must not be higher than 30 m.
- If the height has to go beyond 30 m, the bench slope must not exceed 37.5°.
- A berm is built at the dumping point so that a dumper cannot slip over the edge while tipping.
- The berm must be at least 1 m high and at least half the height of the dumper’s tyre. Whichever is greater is the height you build.
- The whole dump yard is fenced so that nobody walks in by mistake.
3. Haul Roads
Q. What is a haul road? In an opencast mine, it is the road on which dumpers and other HEMM (heavy earth moving machinery) travel.
Q. What legal provisions apply to haul roads?
- Haul roads must be properly constructed and well maintained.
- Where possible, keep separate roads for loaded and empty dumpers (in and out). If that is not possible, the road must be at least 3 times the dumper width. Otherwise, provide a proper place for dumpers to turn and stop.
- On a curve, the driver must see clearly 30 m ahead. If that cannot be done, make separate roads for incoming and outgoing dumpers.
- The gradient must not be steeper than 1 in 16.
- Wherever a dumper could fall into a depth, build a 1 m high parapet wall along the edge.
- Where dumper traffic is heavy, two-wheelers and pedestrians need a separate path. It can run beside the haul road, but it must be fenced off.
4. Stripping Ratio
Q. What is stripping ratio? It is the volume of overburden, in cubic metres, that has to be removed to get one tonne of coal. If you remove 4 m³ of overburden for every tonne of coal, the stripping ratio is 1 : 4.
Q. Which machines suit which stripping ratio?
The pattern is easy to remember. The more overburden you have to move per tonne of coal, the bigger and more specialised the machine must be.
5. Preventing Dumper Accidents
Q. What precautions prevent accidents involving dumpers? Dumpers are the biggest accident source in a mechanised opencast, so this one is asked often. If you want to see just how large these machines can get, have a look at the world’s largest mining dump trucks.
- Driver selection: a board assesses a person’s fitness before appointing them as a dumper driver.
- Dumper inspection: only a fitter or mechanic holding a competency certificate may check and clear a dumper.
- Unauthorised driving: prohibit it. The ignition or cabin key stays with the driver or a responsible person.
- Haul roads: build and maintain them as per the legal provisions.
- Loading and unloading: do not load and unload dumpers at the same spot.
- Entry of other vehicles: trucks and vehicles not belonging to management stay out unless a responsible person has allowed entry.
- Unauthorised riders: nobody except an authorised person may ride on a vehicle.
- Condition of dumpers: a competent person examines the dumper and permits work only if it is in good condition.
- Audio-visual alarm: every dumper must have a working audio-visual alarm.
- Traffic rules: frame rules for the mine and explain them to the drivers.
- Sign boards: put up signs wherever the road needs them.
- Training: train drivers at a training centre, covering safety and traffic rules.
- Bench width: the bench on which dumpers run must be at least three times the dumper width.
- Berm: at least 1 m high in the dump yard so that dumpers do not go over the edge.
- Inspection: a responsible person inspects all dumpers carefully and regularly.
6. Machines Used in Opencast Mines
Q. Name the machines used in an opencast mine and their functions.
| Machine | What it does |
|---|---|
| Shovel | Loads soft or blasted rock and coal into dumpers. Electric or diesel, available in many sizes. |
| Dumper | Carries coal and overburden. Common sizes: 25, 35, 50, 85, 120 and 170 t. |
| Backhoe | Cuts soft material and loads it into dumpers, or lifts blasted material. Can dig below the level it stands on. |
| Dragline | The largest machine in opencast. Stands on top of the bench, lifts material from below and dumps it directly. Used for overburden only, and dumpers are not loaded by it. Boom lengths of 70, 90 and 96 m, electrically driven. |
| Drill machine | Makes blast holes in coal and overburden. Hole diameters range from 100 to 350 mm. |
| Dozer | Pushes material in the dump yard, clears and levels the bench top, and builds haul roads. |
| Scraper | Cuts soft surface soil, loads it in its own bucket, carries it and spreads it in the dump yard. |
| Motor grader | Maintains the grade of the haul road and keeps it clean. |
| Bucket wheel excavator | For soft overburden. Toothed buckets on a wheel cut the material and load it on a conveyor, which feeds the dump yard or loads dumpers. |
| Surface miner | Cuts coal without blasting. Runs on 3 or 4 crawlers and has a drum with bits that cuts coal or rock. It cuts a strip about 15 cm deep and 2 to 3 m wide, and a conveyor reaching 5 to 6 m behind loads the dumper. In India it is used at the Lakhanpur opencast mine (Mahanadi Coalfields). |
7. Opencast Extraction of Underground Pillars
Q. What dangers arise when pillars of an underground mine are extracted by opencast methods?
- Coal dust may explode.
- A fire can start.
- Galleries can cave in, and machines or workers may fall into them.
Q. What precautions are taken against coal dust explosion in this case?
- Cleaning and dusting: where the galleries can be reached, clean out the coal dust and apply stone dust in larger quantity.
- Isolation stoppings: where underground work is also going on, separate the opencast and underground areas with explosion-proof isolation stoppings. If that has not been done, take everyone out of the mine during blasting. After the blast, send people underground only after inspection, which includes checks for noxious gas and fire.
- If the galleries cannot be reached, do stone dusting from the surface:
- On the first overburden bench above the coal seam, drill holes in a grid of 18 m × 18 m to join the underground galleries.
- Put 2 tonnes of stone dust into the galleries through each hole.
- Push the drill rod down to the gallery floor and blow 20 m³ of compressed air at 3.5 kg/cm² for 45 minutes, so the dust spreads all around.
- After pulling out the drill rod, add another 2 t of stone dust and spread it the same way. If the gallery is larger than 14 m², increase the quantity.
- Drill holes: holes for blasting on the first bench above the galleries must be placed so that no hole sits directly above a gallery.
- Proper parting: keep 3 m of parting between the coal seam and the overburden. After every blast, drill pilot holes to keep checking the parting thickness.
- Sand filling: if a crack below the hole is likely, fill 2 m of sand at the bottom before charging. If cracks are not likely, 0.6 m of wet sand or water capsules is enough before explosives go in.
- Delay detonators: not to be used in coal benches without DGMS permission.
- Shelter: nobody, including the shotfirer, may take shelter within 100 m of the bench being blasted. The blasting shelter must be an approved one.
- Management: if an opencast and an underground mine work together, both stay under one manager or agent.
For more on the blasting side, read our posts on blasting safety regulations in open cast mining and blast design parameters in surface mining.
Numbers to Memorise
| Item | Value |
|---|---|
| Soft ground: slope / bench height | 45° / 1.5 m (width ≥ height) |
| Coal: slope / bench height | 45° / 3 m (width ≥ height) |
| Clear zone at the bench edge | 3 m |
| Mechanised bench slope | 70° maximum |
| Dumper bench width | 3 × dumper width |
| Dump height / slope above 30 m | 30 m / 37.5° |
| Berm height | 1 m or ½ tyre height, whichever is more |
| Haul road gradient / curve visibility | 1 in 16 / 30 m |
| Parapet wall at drop | 1 m |
| Grid for stone dust holes | 18 m × 18 m, 2 t per hole |
| Parting above coal seam | 3 m |
| Sand filling (crack likely / unlikely) | 2 m / 0.6 m |
| Shelter distance during blasting | 100 m |
Test Yourself
Reading is only half the job. Once you have gone through this chapter, try our Mining Safety MCQ Questions – Part 01 and see how many answers you get right without looking back.
If this post helped, share it with a friend who is preparing for the same exam, and tell us in the comments which chapter you want next.
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