In a bord and pillar mine, developing the seam is the easy, predictable part. The real test of a mine manager and his team comes later, when the pillars they left standing have to be mined. This is depillaring, and it is where the highest risks lie: roof falls, air blasts, bumps, fires and gas.
In this post we go through depillaring in coal mines in question-and-answer form: the preparations needed before starting, caving and stowing, goaf lines, the slicing and stook methods, local falls and main falls, bumps, premature collapse, air blast and its precautions, and how to work seams lying close to each other. If you have not yet read the basics, start with our post on the bord and pillar method.
- Depillaring means extracting the pillars left during development, by caving or stowing
- Panels, isolation stoppings, a plan, stations and supports must be ready before starting
- Goaf line is the order of extraction: diagonal, step diagonal or straight. Never V-shaped
- Pillars are cut by slicing or by the stook method
- The biggest hazards are air blast, bumps, premature collapse and spontaneous heating
Depillaring
Q1. What must be given special attention before depillaring is started?
Answer:
- Panel: Pillars are grouped into a panel after deciding the number of pillars, keeping in mind production and the incubation period of the coal.
- Isolation stoppings: Except for two intakes and one return, all other roads are closed with isolation stoppings.
- Preparatory isolation stoppings: In the remaining two intakes and one return, preparatory isolation stoppings are built, and all material needed for full isolation stoppings is kept near the site so they can be built immediately if needed.
- Ventilation: Proper ventilation arrangements must be made. Our post on mine ventilation formulas covers the calculations.
- Plan: A hand plan of the district showing the method and sequence of cutting pillars must be prepared.
- Marking on pillars: Pillars that are not to be cut are marked with a 30 cm wide band of lime all around, and shown on the plan.
- Stations: A first aid station, reserve station, miners' station and an air blast protection station (shelter) must be provided.
- Support: Adequate material for fixing and withdrawing supports must be arranged.
Caving and Subsidence
Q2. Up to how many pillars behind may a splitting gallery be driven when depillaring starts?
Answer: When depillaring is started, a splitting gallery can be driven up to four pillars behind, but later splitting should be done only up to two pillars behind.
Q3. What is the caving method?
Answer: In depillaring by caving, the supports are withdrawn after the coal of the pillar has been taken out, and the roof is allowed to fall into the empty space.
Q4. What is subsidence?
Answer: When depillaring by caving lets the roof fall, the ground at the surface may sometimes sink too. This sinking of the surface is called subsidence. Whether it happens depends on the thickness of the seam, the area depillared and the depth of the seam from the surface.
Q5. What are the provisions for depillaring by caving?
Answer:
- A panel will be made.
- Splitting can be done up to two pillars from the open roof (goaf) area, and up to four pillars when depillaring is just starting.
- Pillars will be cut strictly according to the goaf line.
- After cutting a pillar (stook or slice), the support there will be removed at once, so that the roof falls quickly by itself.
- No support will be left where coal has been extracted.
- No standing stook or rib will be left where coal has been taken. If one must be left for safety, it will be blasted and punctured so the goaf falls easily.
- The method of coal extraction will follow DGMS directions.
- Support will be given according to the systematic support rule up to two pillars or 30 m behind, whichever is greater.
- Special attention will be given to support at the goaf edge.
- Special attention will be given to ventilation and gas testing.
- Signs of spontaneous heating will be watched at all times, and poisonous gas (white damp, called “khooni gas” in Hindi notes) will be checked at least once a week. Read more in our post on carbon monoxide (white damp).
- Signs of the roof becoming heavy will be watched. If a roof fall seems near, all workers will be taken to a safe place.
- A large area of open roof will not be kept. The roof must keep falling. If it does not, the danger of an air blast rises, so the roof should be brought down by induced blasting.
- When all pillars in a panel have been cut, the panel will be sealed by isolation stoppings.
Stowing
Q6. What is depillaring by stowing?
Answer: After the pillar coal is extracted, the empty space is filled with sand. This method of depillaring is called stowing. Our post on DGMS inspection deficiencies explains why a lag in stowing is so dangerous.
Q7. What are the advantages of depillaring by stowing?
Answer:
- No subsidence.
- No harm if the overlying seam is full of water.
- Work below rivers, buildings and railway lines does not cause them trouble.
- Thick seams can be worked easily.
- No danger of air blast.
- No danger of premature collapse or over-riding.
- No chance of spontaneous heating.
- Large panels can be made, so less coal is left in barriers and fewer isolation stoppings are needed.
- More coal can be extracted from the seam safely.
- Depillaring is easy in water-bearing strata.
- The chance of poisonous or hot gas is almost nil.
Q8. Under what conditions must depillaring be done by stowing?
Answer:
- A seam with heavy water seepage and water in the strata.
- A seam under old workings full of water that cannot be drained.
- A seam under surface features such as railways, roads, rivers, water tanks, ponds or buildings that cannot be removed.
- A fiery seam.
- A seam whose coal catches fire quickly by itself.
- A highly gassy seam.
- A seam thicker than 5 m where all the coal must be extracted.
- A seam whose roof rock is so hard that it does not fall easily.
Goaf Lines
Q9. What is a goaf line?
Answer: The line formed by the order in which pillars are extracted during depillaring is called the goaf line.
Q10. How many types of goaf lines are there?
Answer: Three: diagonal, step diagonal and straight.
Q11. What is a diagonal goaf line?
Answer: This goaf line makes an angle of 45 degrees with the pillars on the dip side of the seam. It is very common in our mines and is used where the seam has a normal roof.
Q12. What is a step diagonal goaf line?
Answer: In this goaf line, whenever two pillars are cut, one pillar is left standing between them to support the pillars being cut. It is used in seams with a very bad roof.
Q13. What is a straight goaf line?
Answer: The goaf line is straight, along the dip, rise or level line. It is used where the roof is very strong. Keeping it along the level makes it easier to set up chain conveyors and loading machines, and it is convenient for stowing too.
Q14. Why should a V-shaped goaf line not be allowed to form?
Answer: A V-shaped goaf line puts pressure on the pillars, and the pillars, roof and supports can suddenly give way, which raises the danger of accidents. Our post on DGMS inspection deficiencies lists failure to follow the line of extraction as a common problem.
Slicing and Stook Methods
Q15. In how many ways can pillars be cut?
Answer: In two ways: the slicing method and the stook method.
Q16. What is the slicing method?
Answer:
- Each pillar is divided into two parts (three if it is large) by a level splitting gallery, which is not more than 3.6 m wide.
- In the dip-side half of the pillar, slices wider than 4.5 m are not driven. The exposed roof area should not exceed 60 square metres.
- Only one slice is driven in a pillar at a time. After all the coal of the dip-side half has been extracted, slices are driven in the upper half.
- In depillaring by caving, a rib of at least 1.8 m is left on the goaf side of the slice. In stowing, the rib is at least 1.5 m thick.
- The coal of the rib is taken out safely. If it cannot be extracted, it is punctured (blasted) in caving, and made thinner and left in stowing.
Q17. Where is the slicing method used?
Answer: Slicing is good for a poor or weak roof. Today most Indian coal mines use the slicing method for depillaring.
Q18. What is a stook?
Answer: When a pillar is to be cut, it is divided into 4 or 6 parts by driving galleries. Each such part is called a stook.
Q19. In how many ways can a stook be cut?
Answer: There are three ways. First the pillar is divided into 4 or 6 stooks according to its size, and then extraction starts with the stook on the goaf side.
- For a strong roof: Stooks are cut in the order 1, 2 and 3. A rib about 2 m thick is left on the goaf side. Finally this rib is safely made thinner, or its coal is fully extracted.
- Half-moon method: Stooks are cut in order 1, 2 and 3, and then the remaining 2 m rib is made thin and left. This is suitable for a normal roof.
- Two-side splitting: Used when the roof is weak. Galleries are driven into the stook from two sides, and a sentinel pillar (chowkidar, marked M) is left. A rib 2 m thick is left on the goaf side. If caving is used, this rib is extracted or punctured, and if stowing is used the rib is made thinner. At the end, the coal of the sentinel pillar is taken out safely as far as possible.
Q20. Why are pillars sometimes left after splitting? (Splitting as a final operation of depillaring)
Answer: Sometimes the surface, or the strata above the seam, must be protected. The seam above might have water-filled workings, or the surface may hold a river, nallah, road, railway line or building, and cracks must be avoided. In such cases depillaring should be done by stowing. But if stowing material is not available and caving is not allowed, the pillars are split: each pillar is divided into 2 to 4 parts, and small pillars are left. These small pillars should be not smaller than 6 m by 6 m. This method is called splitting as the final operation of depillaring.
Falls, Bumps and Collapse
Q21. What is a local fall?
Answer: When the supports are withdrawn from the goaf area, the roof begins to sag and some layers of the roof break and fall. This is called a local fall. Its effect does not reach the surface.
Q22. How long after the supports are removed does a local fall occur?
Answer: It depends on the roof. If the roof is sandstone, a local fall occurs within 24 to 48 hours of removing the support. If the roof is weak, the local fall can come even earlier.
Q23. What is a main fall?
Answer: As depillaring goes on, when the extracted area becomes equal to or greater than the square of the depth of the seam from the surface at that place, the falls spread upward until the ground starts sinking or cracks appear at the surface. This is the main fall. It comes much later than a local fall, and it comes sooner if depillaring is done near a fault, or if the upper seam has already been depillared. See our post on faults and other geological disturbances.
Q24. What is a bump? How many types are there?
Answer: A bump is the sudden release of pressure in the strata. There are two types: pressure bump and shock bump.
Q25. What is a pressure bump?
Answer: When the weight of strata on a coal pillar becomes excessive, the pillar cannot bear the pressure and fails suddenly.
Q26. What is a shock bump?
Answer: When there is a strong bed of strata in the roof, or heavy roof hangs over the goaf, a sudden break of this strata produces a shock. The coal pillars cannot bear this shock and fail.
Q27. What is premature collapse?
Answer: In depillaring by caving, the roof sometimes falls right after a pillar is cut and before the support is withdrawn. This is premature collapse. Many things can cause it: an exposed area that is too large, poor support, sagging of the roof or a weak roof. When it happens, the supports are crushed, there is a chance of an accident and extra load falls on the pillars.
Q28. What is over-riding of pillars?
Answer: When there is hard sandstone or a sill in the roof during depillaring by caving, the roof does not fall in time and stays hung over a wide area. When it finally does fall, its weight on the pillars is so great that they cannot bear it and are crushed. This is called over-riding of pillars.
| Event | Cause in brief | Result |
|---|---|---|
| Local fall | Roof sags after support is withdrawn | Some roof layers fall. Does not reach the surface |
| Main fall | Extracted area equals or exceeds the square of the depth | Falls extend to the surface with sinking or cracks |
| Pressure bump | Excess weight on a coal pillar | Pillar fails suddenly |
| Shock bump | Sudden break of strong strata or hung roof | Shock crushes pillars |
| Premature collapse | Roof falls before the support is withdrawn | Supports crushed, accident risk |
| Over-riding | Hung strong roof falls late | Pillars crushed by the sudden weight |
Air Blast and Roof Warning Signs
Q29. What is an air blast?
Answer: In some mines the roof is strong rock, hard sandstone or a sill. When depillaring by caving, the goaf does not settle and a very large area stays open and full of air. When the roof finally settles all at once, the air in this area is forced out at high pressure and speed by the pressure of the falling rock. This is called an air blast.
The pressure and speed of the air can throw people, tubs, supports and sheets out of its way, and bring poisonous gas from the goaf into the workings. This can cost lives and property. According to the source notes, an air blast has killed 14 workers together in one mine (West Chirimiri colliery) and 5 workers together in another (Parascole West mine).
Q30. What precautions should be taken in a depillaring district to avoid an air blast?
Answer:
- Do not keep a hung goaf. As soon as a slice is taken, the roof should fall there. If it does not, bring it down by induced blasting.
- To avoid a hung goaf, extract the coal of the rib. If the rib cannot be extracted, puncture it.
- After taking the rib, remove all standing support completely. No standing support should be left there.
- Carry out depillaring strictly according to the goaf line.
- Keep as many galleries open in the district as possible.
- Have a “safety valve” of 1.5 m by 1.5 m built in the isolation stoppings, with a brick and mud wall in the middle. In an air blast, this part breaks, reducing the air pressure and saving the isolation stopping from damage.
- Keep the preparatory isolation stopping material, such as door stopping, lying ready.
- Build shelters to protect people from an air blast, and make sure they are not in the path of the blast.
- Keep an experienced mining sirdar on duty in this district.
- Mining sirdars and overmen should carry a whistle. When there is a danger of an air blast, they blow it, and everyone leaves work and goes to the shelter at once. If an air blast occurs before reaching the shelter, lie down on the ground where you are, or go into the gallery that is likely to be less dangerous.
- If an air blast seems likely, cut the electric power to the section, stop the winding and remove the standing tubs from the line or sprag them.
- Install convergence recorders in the district to track the rate at which the roof is sagging.
- Hold mock rehearsals from time to time, so that everyone knows what to do if an air blast occurs.
Q31. What are the signs of the roof becoming “hot” (heavy)?
Answer: A “hot” roof means weight is coming on the roof. The signs are:
- Tapping the roof with a stick gives a dull, thudding sound.
- Tapping a standing support gives a heavy sound.
- The supports in the working take weight. The lid on a prop splits, or the prop breaks or bends.
- Noises start coming from the goaf.
- Coal of the rib or sentinel pillar begins to break and fall.
- Small pieces start falling from the roof in the goaf.
- Cracks in the goaf start letting in water.
- The amount of firedamp in the workings increases. For gas basics, see our post on mine gases and firedamp.
Working Two Seams Together
Q32. What precautions must be taken for working the upper seam after the lower seam has been goafed?
Answer:
- Give the goaf of the lower seam at least 5 years to settle well before working the upper seam.
- The parting between the two seams should be at least 30 m.
- The upper seam is disturbed by the goaf below, so coal cutting machines and loading machines cannot run well. Hence they are not used.
- Because the floor is disturbed, main and tail haulage should be used.
- More support should be given.
- All precautions against water should be taken.
- Gas from the lower seam can rise, so all precautions against gas should be taken.
Q33. What is a contiguous seam?
Answer: If two seams are separated by a parting of 9 m or less, they are called contiguous seams.
Q34. What are the rules for working in a contiguous seam?
Answer:
- DGMS permission must be taken.
- The parting between two seams for working should not be less than 3 m. A thick seam may be worked as contiguous seams in two sections, with at least 3 m parting between them. If the parting between two seams or sections is more than 9 m, they can be treated as separate seams and worked separately, and there is no need to work them as contiguous seams.
- Pillar size and gallery width are kept the same in both seams.
- Pillar over pillar and gallery over gallery are kept everywhere. If the gradient of the seam is more than 30 degrees, this is not required.
- The thickness of the parting is checked by a bore hole at every second junction.
- During development, the lower seam is driven one pillar ahead.
- The main fan is fitted in the upper seam so that intake air comes through the lower seam.
- To connect the two seams, a staple shaft or incline is made, which must be 4 to 5 pillars behind. As development advances, the two seams are connected every 4 to 5 pillars behind.
- If a staple shaft is made, a ladder is fitted so that people can go up and down.
- One overman is in charge of both seams, but the mining sirdars are separate.
- A telephone is provided for communication between the two seams.
- A large-scale plan (1:240) of both seams must be kept.
- Depillaring: (a) the upper seam is depillared one or two pillars ahead; (b) the Systematic Support Rules are the same for both seams, and as far as possible chock over chock and prop over prop are set in both seams; (c) support withdrawal is done in both seams together.
Quick Revision Points
- Before depillaring: panel, isolation stoppings, preparatory stoppings, ventilation, plan, lime-band marking (30 cm), stations and support material.
- Splitting gallery: up to 4 pillars behind at the start, later 2 pillars. Support up to 2 pillars or 30 m behind, whichever is greater.
- Goaf lines: diagonal (45 degrees), step diagonal (bad roof), straight (very strong roof). V-shaped is not allowed.
- Slicing: split gallery up to 3.6 m, slice up to 4.5 m, exposed roof up to 60 m², rib 1.8 m (caving) or 1.5 m (stowing).
- Stook: pillar in 4 or 6 parts, rib about 2 m on the goaf side. Final splitting leaves pillars not smaller than 6 m by 6 m.
- Local fall: 24 to 48 hours in sandstone roof. Main fall: extracted area equal to or more than the square of the depth.
- Air blast safety: induced blasting, safety valve 1.5 m by 1.5 m, shelters, whistle, convergence recorders and mock rehearsals.
- Upper seam after lower seam goafed: wait 5 years, with at least 30 m parting. Contiguous seams: parting 9 m or less, DGMS permission, minimum 3 m parting.
Check Your Knowledge
1. Which goaf line must never be allowed to form?
A. Diagonal goaf line
B. Straight goaf line
C. Step diagonal goaf line
D. V-shaped goaf line
Show Answer
D. V-shaped goaf line
2. The maximum width of a splitting gallery in the slicing method is:
A. 2.4 m
B. 3.6 m
C. 4.5 m
D. 6 m
Show Answer
B. 3.6 m
3. In depillaring by caving with the slicing method, the rib left on the goaf side is at least:
A. 0.6 m
B. 1.0 m
C. 1.8 m
D. 3.0 m
Show Answer
C. 1.8 m
4. A local fall occurs within how long after support is withdrawn in a sandstone roof?
A. 24 to 48 hours
B. 1 to 2 hours
C. 7 days
D. 30 days
Show Answer
A. 24 to 48 hours
5. What size is the 'safety valve' built into an isolation stopping?
A. 1 m by 1 m
B. 1.5 m by 1.5 m
C. 2 m by 2 m
D. 3 m by 3 m
Show Answer
B. 1.5 m by 1.5 m
6. Two seams separated by a parting of 9 m or less are called:
A. Parallel seams
B. Lower seams
C. Contiguous seams
D. Upper seams
Show Answer
C. Contiguous seams
Frequently Asked Questions
What is the difference between caving and stowing?
In caving, the supports are withdrawn and the roof is allowed to fall. In stowing, the void left after extracting the pillar is filled with sand.
What is a goaf line?
The line formed by the order in which pillars are extracted during depillaring.
What is an air blast?
The sudden, forceful rush of air out of a goaf when a large hung roof falls all at once. It can throw people and equipment and bring poisonous gas into the workings.
What is the difference between local fall and main fall?
A local fall is a fall of roof layers near the goaf soon after support is withdrawn and does not reach the surface. A main fall comes later and extends up to the surface.
What is a contiguous seam?
Seams separated by a parting of 9 m or less.
Related Posts
- 👉 Bord and Pillar Method in Coal Mining
- 👉 DGMS Inspection Deficiencies and Remedial Measures
- 👉 Geology for Coal Mining: Coal Formation, Faults and Dykes
- 👉 Coal Mines Regulations 2017: Definitions Explained
- 👉 Carbon Monoxide (White Damp)
For more notes and practice sets, visit the Indian Minerology homepage, browse the mine gases posts or try our Mining Legislation MCQs.
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