Longwall Mining Method

In bord and pillar mining, you leave pillars behind and then come back to mine them. Longwall mining takes a completely different approach. Instead of leaving blocks of coal standing, a long straight face is opened across the seam, and the whole block of coal is mined in one continuous operation. A machine travels along the face cutting coal, steel supports hold up the roof just where people work, and the roof behind is allowed to fall or is filled.

In this post we go through the longwall mining method in question-and-answer form: what it is, longwall advancing and retreating, where it is suitable, the layout of a longwall face, how the shearer and power supports work and the terms such as gate road, main gate, tail gate and stable. If you want to compare it with the other method, read our post on the bord and pillar method first.

At a glance

Longwall method: coal is extracted from a face about 60 to 200 m long
Two types: longwall advancing and longwall retreating
Suited to: seams 0.6 to 3.5 m thick, deep and gassy seams, weak roof and floor, and where high output is wanted
Main equipment: double-drum shearer, armoured flexible conveyor (AFC) and self-advancing hydraulic power supports
Gate roads: the main gate (intake) and the tail gate (return)

Contents

What is longwall? (Q1 to Q2)
When is it suitable? (Q3)
Face layout and working (Q4)
Gate roads and stables (Q5 to Q7)
Longwall vs bord and pillar
Quiz and FAQs

Note: The figures here (such as the 60 to 200 m face length, the 200 to 760 tonne power supports and the 30 m of gate road support) are taken from the notes this post is based on. Actual face length, supports and layout depend on the mine's approved scheme, so always follow the current regulations and the plan approved for your mine.

Longwall

Q1. What is the longwall method?

Answer: In an underground mine, the method of extracting coal by driving a face 60 to 200 metres long is called the longwall method.

Q2. What are the types of the longwall method?

Answer: There are two types:

Longwall advancing
Longwall retreating

Longwall advancing

In this method, two galleries are driven a set face length apart, and they are joined by a third gallery. One side of this third gallery is the longwall face. The coal is cut from this face and the face moves toward the boundary of the mine, that is, it advances, which is why the method is called longwall advancing. The space left by the extracted coal is either stowed or allowed to cave. While advancing toward the boundary, both roads are kept safe.

Figure 1: Longwall advancing. The face moves toward the mine boundary with the gates kept open behind it.

Longwall retreating

Here, two galleries are first driven to the boundary of the mine, a set face length apart. Near the boundary, a third gallery is driven to join them. One side of this third gallery is the longwall face, and as it works, the face moves backward toward the shaft or incline, that is, it retreats. This is the longwall retreating method. It can be worked with caving or with stowing.

Figure 2: Longwall retreating. The face works back toward the shaft or incline.

Q3. Under what conditions is the longwall method suitable?

Answer: The longwall method is suitable for:

A seam thicker than 0.6 m and thinner than 3.5 m.
A seam deeper than 500 m.
A seam with a high chance of spontaneous heating.
A highly gassy seam. Read more in our post on mine gases and firedamp.
Where both the roof and the floor are weak.
Where the coal seam itself is weak.
Where a high output is wanted from an underground mine.

Compare these with the conditions for bord and pillar in our post on methods of working. Deep seams are reached by shafts, which you can read about in shaft sinking in mining.

Q4. Explain the layout of a longwall retreating face.

Figure 3: Layout of a longwall retreating face with a 140 m face.

Face support: Hydraulic chocks, called power supports, are fitted tightly side by side along the face. They have a load capacity of 200 to 760 tonnes, chosen according to need. They can advance by themselves (self-advancing), and they are pushed forward with the help of the armoured flexible conveyor (AFC) that runs along the face.

Gate road support: The gate roads are strongly supported for 30 m from the face. In this, girders are placed every metre on hydraulic props and tightened to the roof with sleepers.

Method of working at the face:

Figure 4: The double-drum shearer on the AFC, with the power support behind it.

A Double Ended Ranging Drum shearer (DERD) is mounted on the chain conveyor (AFC) along the face. It has two drums. The first drum cuts the face from the bottom and the second cuts from the top.
The shearer travels from the rise side toward the dip, cutting coal. The cut coal falls directly onto the chain conveyor.
The chain conveyor carries the coal to the stage loader, where it drops onto the gate belt and leaves the panel.
When the shearer has moved 15 to 20 m ahead while cutting, the work of advancing the power supports from behind begins.
As the shearer keeps cutting forward, the power supports and the chain conveyor are moved forward toward the face behind it.
When the coal on the face has been cut up to the dip, the shearer is brought back toward the rise. The roof exposed behind the power supports is allowed to fall, or it is stowed.

Gate Roads and Stables

Q5. What is a gate road?

Answer: To work a longwall face, two roads are made, one for intake and the other for return. These two roads are called the gate roads.

Q6. What are the main gate and the tail gate?

Answer:

Main gate: The intake road. A belt conveyor is installed in this road to carry coal. It is usually the gallery on the dip side, so that ventilation and coal transport are easy.
Tail gate: The return road. Haulage lines are laid in it for transporting materials, and haulage is installed. It is usually the gallery on the rise side.

For how the air travels along these roads, see our post on mine ventilation formulas.

Q7. What is a stable?

Answer: In the longwall advancing method, to move the face forward, the galleries on both sides are driven 8 to 10 metres ahead of the face. These galleries are called stables.

Longwall vs Bord and Pillar

Point	Longwall	Bord and pillar
How coal is extracted	From a long face of about 60 to 200 m, mined in one continuous operation	By driving galleries to make pillars, then extracting the pillars (depillaring)
Roof support	Hydraulic power supports at the face, with the roof behind them caved or stowed	Supports in galleries and during depillaring
Suitable seam	0.6 to 3.5 m thick, even deep (over 500 m) and gassy seams, weak roof and floor	Seams thicker than 1.5 m, not too deep, not highly gassy, strong roof and floor
Ventilation at the face	Good, with a clear intake and return along the face	Face ventilation is not as good
Output	High	Lower
Equipment	Shearer, AFC, power supports, stage loader	Continuous miners, loaders, shuttle cars or conventional equipment

To see how pillars are removed in the other method, read our post on depillaring in coal mines.

Quick Revision Points

Longwall face length: about 60 to 200 m.
Two types: advancing (face moves toward the boundary) and retreating (face moves back toward the shaft or incline).
Suitable for seams 0.6 to 3.5 m thick, deeper than 500 m, gassy, spontaneous heating prone, with weak roof and floor.
Power supports carry 200 to 760 tonnes, are self-advancing and are moved with the AFC.
Gate roads are supported strongly up to 30 m from the face.
DERD shearer: one drum cuts from the bottom and one from the top. Supports are advanced when the shearer is 15 to 20 m ahead.
Main gate: intake with the belt conveyor, usually the dip side. Tail gate: return with haulage, usually the rise side.
Stables: galleries driven 8 to 10 m ahead of the face in longwall advancing.

Check Your Knowledge

1. What is the approximate face length in the longwall method?

A. 10 to 30 m
B. 60 to 200 m
C. 300 to 500 m
D. 1 to 5 m

Show Answer

B. 60 to 200 m

2. In longwall retreating, the face moves:

A. Toward the mine boundary
B. Upward only
C. Back toward the shaft or incline
D. Sideways only

Show Answer

C. Back toward the shaft or incline

3. The intake road of a longwall panel is called the:

A. Main gate
B. Tail gate
C. Stable
D. Face

Show Answer

A. Main gate

4. The galleries driven 8 to 10 m ahead of the face in longwall advancing are called:

A. Gate roads
B. Splits
C. Goaf lines
D. Stables

Show Answer

D. Stables

5. What does AFC stand for?

A. All face conveyor
B. Armoured flexible conveyor
C. Automatic face cutter
D. Advanced face chock

Show Answer

B. Armoured flexible conveyor

6. Which of these seam conditions suits the longwall method?

A. A very thin seam under 0.3 m
B. A shallow seam with strong roof
C. A deep, highly gassy seam with weak roof and floor
D. A seam thicker than 10 m

Show Answer

C. A deep, highly gassy seam with weak roof and floor

Frequently Asked Questions

What is the difference between longwall advancing and retreating?

In advancing, the face moves away from the shaft toward the mine boundary. In retreating, the gates are driven to the boundary first and the face works back toward the shaft or incline.

What does a longwall shearer do?

It travels along the face on the AFC. Its two drums cut the coal from the top and the bottom, and the coal falls onto the AFC.

What are power supports?

Self-advancing hydraulic roof supports placed side by side along the face. They protect the people working at the face and are moved forward as the face advances.