Winning of Coal or its Products

1. Opencast method

2. Underground Method

3. Gassification of coal seams

Underground Method

1. Development

2. Depillaring

3. Special Methods

Development of Seam

Development means formation of galleries and pillars. In other words formation of skeleton

1. During development, the extraction of coal will be around 15% to 20%

2. Remaining 80% to 85% will be extracted in depillaring

Depillaring

Extraction of coal from already formed pillars.

1. Depillaring with caving

2. Depillaring with sand stowing

Caving means, allowing the roof to collapse after extraction of coal.

1. The hanging roof exerts pressures on to the pillars, stooks and ribs.

2. Roof become bad.

3. Sudden collapse of roof cause Air Blast.

Stowing: Refilling the de-coaled area with sand or other incombustibles.

What happens, if roof not collapsed.

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What is Air Blast

Displacement of air through narrow openings/galleries by sudden collapse of roof.

What happens in case of Air Blast

Due to sudden release of air through narrow openings/galleries.

1. The persons, machines and material which in path of air will be thrown away. Some times some tonnes of weights also.

2. It also sucks persons into the goaf/de-coaled area.

3. Over riding of working places.

Precautions to prevent Air Blasts

1. Bringing down the roof with induced blasting in goaf area, at regular intervals.

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1. Reduction of ribs as much as possible.

2. Keeping the openings free from obstructions.

3. Maintaining diagonal line.

Effects of caving on surface

1. Formation of subsidence basins on surface.

2. Damage to surface structures.

3. Damage to water table.

Instruments to measure convergence

1. Telescopic Convergence recorders.

2. Tel-tales

3. Bore Hole Extensometers.

4. Strain Gauges.

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