Bord and Pillar Method in Coal Mining: Advantages & Panels || Indian Minerology

Once you know a coal seam exists, the next question is how to get the coal out. In India, most underground coal comes out through one method: bord and pillar. Instead of removing everything at once, the miner first opens a network of roadways in the seam, leaving solid blocks of coal standing between them. Later, those blocks are taken out as well. That one idea explains most of the terms, rules and exam questions on this topic.

In this post we go through the methods of working in coal mines in question-and-answer form: the types of mines, bord and pillar versus longwall, development and depillaring, panels, barriers, and the legal limits on gallery and pillar sizes. We also explain how seams are classified by thickness, depth and gradient.

At a glance

Two kinds of mines: underground and opencast
Two underground methods: bord and pillar, and longwall
Bord and pillar has two stages: development (making pillars) and depillaring (removing them)
Gallery limits: up to 4.8 m wide and 3 m high
Depillaring is done by: caving or stowing, and needs DGMS permission

Contents

Types of mines and methods (Q1 to Q4)
When bord and pillar suits, with its pros and cons (Q5 to Q7)
Development and depillaring (Q8 to Q9)
Rules for shafts, rivers, railways and roads (Q10 to Q12)
Seam classification (Q13 to Q15)
Barriers (Q16 to Q17)
Panels (Q18 to Q21)
Long headings (Q22)
Quiz and FAQs

Note: The figures here (such as 4.8 m, 3 m, 13.5 m, 15 m and 45 m) are taken from the notes this post is based on, which refer to the Coal Mines Regulations of that time. The regulations have been revised since, and the Mines Act under which they were made is now part of the OSH Code, so always confirm the current requirements. Our post on Coal Mines Regulations 2017 definitions explains many of the terms used below.

Method of Working

Q1. How many types of coal mines are there?

Answer: There are two types of coal mine:

Underground mine
Opencast (open pit) mine

Q2. What are the methods of extracting coal from an underground mine?

Answer: There are mainly two methods:

Bord and pillar method
Longwall method

Figure 1: Classification of coal mining methods.

Q3. In how many ways is coal extracted in the bord and pillar method?

Answer: In two stages: development and depillaring. Depillaring itself is done in two ways: caving and stowing.

Figure 2: In caving the roof is allowed to fall. In stowing the void is filled with sand.

The sand filling in stowing is the same thing we met in our post on DGMS inspection deficiencies, where a large stowing lag was named as a leading cause of accidents.

Q4. In how many ways is development done in bord and pillar?

Answer: Development is done in three ways:

Mode	Operations
Manual	Drilling, blasting, basket loading and haulage
Semi-mechanised	Drilling, blasting, loading with SDL or LHD, haulage by tubs or chain conveyor, and belt conveyor
Mechanised	Continuous miner, shuttle car and belt conveyor. No blasting is done in this method

When Bord and Pillar Suits

Q5. Under what conditions is the bord and pillar method suitable?

Answer: It suits seams where:

The thickness of the coal seam is more than 1.5 m.
The seam is not at great depth.
The seam is not highly gassy.
The roof and floor are strong.
The coal is not too soft, so that the pillars remain stable.

Q6. What are the advantages of the bord and pillar method?

Answer:

The roadways and airways lie in solid coal, so they are cheap to maintain.
Coal production starts during development itself and continues during depillaring.
Geological disturbances are found during development, so the mine can be planned accordingly. See our post on geology for coal mining.
Few people work at one place, so their work can be tracked.
During development, work can be done below important places such as railways, buildings, roads and rivers. Only depillaring cannot be done there, unless stowing is available, in which case depillaring is also possible.

Q7. What are the problems of the bord and pillar method?

Answer:

Ventilation at the face is not as good as in the longwall method.
The coal cannot be fully extracted. Some coal is left behind in pillars.
Work goes on at many faces at the same time, which makes supervision difficult.
It is hard to work in deeper seams with this method.
Many galleries must be driven, so support costs are higher.
There is a greater chance of spontaneous heating. To understand fires and gases, read our post on mine gases and firedamp.

Development and Depillaring

Q8. What are development and depillaring in a bord and pillar mine?

Figure 3: A bord and pillar panel with galleries, pillars and a barrier.

Development: You first reach the seam by sinking an incline or a shaft (see shaft sinking in mining). Then you drive galleries in the seam (dip, level or rise), leaving some coal solid between them. This solid coal is called a pillar. The width of the gallery and the size of the pillar are fixed by law. The gallery width must not be more than 4.8 m and its height not more than 3 m. Working the whole mine in this way is called development.

Depillaring: The pillars made during development are mined from near the boundary of the mine. The coal is taken out, and the roof is either allowed to cave in or the empty space is filled with sand. This is called depillaring.

Q9. What are the legal provisions for developing a seam?

Answer:

The height of any gallery in any seam or section must not exceed 3 m and its width must not exceed 4.8 m.
Pillars in any seam or section are normally made rectangular.
The size of galleries and pillars depends on the depth of the seam. The pillar size must not be less than the size given in the table under Regulation 99 (in the regulations the notes refer to).
Pillar size is measured from the centre of one gallery to the centre of the next.

Rules for Shafts, Rivers, Railways and Roads

Q10. What is the minimum distance between two shafts or two inclines when opening a mine?

Answer: It must not be less than 13.5 m.

Q11. What are the rules for developing below a river, nallah or tank?

Answer: No work is done below a river, nallah or tank, and none within 15 m of their boundary. If work is needed there, DGMS permission must be obtained.

Q12. What are the rules for developing below a railway line or road?

Answer: Work cannot be done within 45 m of the boundary of a railway line or road without DGMS permission.

Other precautions from the same notes: If subsidence of the surface is likely, fencing must be put up. And before depillaring is started, permission must be taken from DGMS.

Seam Classification

Q13. Into how many types are seams divided according to thickness?

Answer: Four types:

Thin seam: thickness less than 1.5 m
Moderately thick seam: 1.5 m to 4.5 m
Thick seam: 4.5 m to 9 m
Very thick seam: more than 9 m

Q14. What are seams called according to depth?

Answer:

Shallow seam: less than 150 m deep
Moderately deep seam: 150 m to 450 m deep
Deep seam: more than 450 m deep

Q15. What are seams called according to gradient?

Answer:

Less than 1 in 11: Flat seam
1 in 11 to 1 in 3: Inclined seam
1 in 3 to 1 in 2: Steep seam
Steeper than 1 in 2: Very steep seam

Figure 4: Seam classification by thickness, depth and gradient.

For a deeper look at how seams occur, read our post on the form of a coal deposit or seam.

Barriers

Q16. How big should the barrier between two districts be?

Answer: The barrier between two districts should be equal to one pillar size.

Q17. How thick should the barrier between one mine and another be?

Answer: A barrier of at least one pillar thickness must be left between one mine and another. The barrier size is taken from the table under Regulation 99, according to the maximum depth of the seam and a gallery size of 4.8 m. Half of the pillar is left by one mine and half by the other.

Panels

Q18. What is a panel?

Answer: During development, groups of 5 to 6 galleries are taken together to make separate sections. These sections are separated from each other by barriers of solid coal. Each such section is called a panel.

Q19. How big is a panel made?

Answer: The number of pillars in a panel is fixed so that all the coal can be extracted by depillaring within the incubation period, the time in which coal can catch fire by itself. The number of pillars depends on the incubation period and the daily production.

Q20. What are the advantages of working in separate panels?

Answer:

If spontaneous heating occurs, that panel can be sealed off without damaging other panels or losing much coal.
If an explosion happens in one panel, other panels or parts are not damaged.
Pillars are less likely to crush or fail.
Fresh air can be sent to each panel from the main intake.
Because the panels are small and surrounded by coal barriers, surface subsidence is less.
Depillaring can start soon after development, without waiting for the whole mine to be developed. Pillars are not left for a long time, so problems such as dust, ventilation, spontaneous heating and pillar failure are reduced.
Fewer isolation stoppings are needed during depillaring.

Q21. What are the disadvantages of working by panels?

Answer:

About 20 percent of the coal is left behind in the barriers.
Many air crossings have to be built. For airflow basics, see mine ventilation formulas.
Machines and coal transport have to be arranged separately for each panel.

Long Headings

Q22. How long can a gallery, drift or shaft be driven without a connection?

Answer: A shaft, incline, other outlet or gallery (a long heading) that is being driven can be extended to any length without a connection, provided that proper ventilation is arranged and the number of persons working at one time along the whole road is less than 20.

Quick Revision Points

Two kinds of mine: underground and opencast. Two underground methods: bord and pillar, and longwall.
Bord and pillar: development, then depillaring (caving or stowing).
Suits seams thicker than 1.5 m, not deep, not highly gassy, with strong roof and floor.
Gallery: not more than 4.8 m wide and 3 m high. Pillars normally rectangular, measured centre to centre.
Two shafts or inclines: at least 13.5 m apart. Within 15 m of a river, nallah or tank and within 45 m of a railway or road: DGMS permission needed.
Thickness: below 1.5, 1.5 to 4.5, 4.5 to 9, above 9 m. Depth: below 150, 150 to 450, above 450 m.
Gradient: flat below 1 in 11, inclined 1 in 11 to 1 in 3, steep 1 in 3 to 1 in 2, very steep above 1 in 2.
Barrier between districts: one pillar. Barrier between mines: at least one pillar, half from each mine.
Panels limit fire and explosion damage but leave about 20% coal in barriers.

Check Your Knowledge

1. What are the two stages of the bord and pillar method?

A. Stowing and caving
B. Development and depillaring
C. Drilling and blasting
D. Opening and closing

Show Answer

B. Development and depillaring

2. The maximum width of a gallery under the provision given in these notes is:

A. 3 m
B. 4 m
C. 4.8 m
D. 6 m

Show Answer

C. 4.8 m

3. A seam that is 2.5 m thick is classed as:

A. Thin
B. Moderately thick
C. Thick
D. Very thick

Show Answer

B. Moderately thick

4. A seam with a gradient of 1 in 5 is called:

A. Inclined seam
B. Flat seam
C. Steep seam
D. Very steep seam

Show Answer

A. Inclined seam

5. The minimum distance between two shafts or two inclines is:

A. 5 m
B. 8 m
C. 10 m
D. 13.5 m

Show Answer

D. 13.5 m

6. Which of these is a disadvantage of working by panels?

A. Fire cannot be sealed off
B. Explosions spread to other panels
C. About 20 percent of the coal is left in the barriers
D. Fresh air cannot reach each panel

Show Answer

C. About 20 percent of the coal is left in the barriers