Windblast is a term commonly used in mining to describe a sudden rush of air or gas due to the collapse of a void.

Causes

Windblast is common in longwall

Longwall mining

Longwall mining is a form of underground coal mining where a long wall of coal is mined in a single slice . The longwall panel is typically 3–4 km long and 250–400 m wide....

 coal mines, especially those whose roof strata are competent, and do not cave immediately behind the roof supports as the face advances. This results in the tendency for a large void to be created behind the roof supports in the goaf (or gob) which collapses when the overlying cantilever

Cantilever

A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...

ed strata can no longer support its own weight. When the collapse occurs, the air or gas occupying the void is displaced by rock, resulting in a pressure wave and windblast that propagates along the roadways (tunnels) of the mine. This may be followed by a "suck back" as the air pressure is equalised with the low pressure created higher up in the goaf.

Windblast can also occur in metalliferous, kimberlite

Kimberlite

Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....

 or even evaporite

Evaporite

Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...

 mines, particularly in block caving mines, as happened at the Northparkes mine in NSW, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 on 25 November 1999, killing 4.

Effects of windblast

The effects of a windblast are not limited to the physical effects of the overpressure wave. The effects of a windblast include:

• The displacement of asphixiating
  Asphyxia
  Asphyxia or asphyxiation is a condition of severely deficient supply of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which primarily affects the tissues and organs...
  
  , toxic and/or potentially explosive gases such as carbon dioxide
  Carbon dioxide
  Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
  
  , carbon monoxide
  Carbon monoxide
  Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
  
   and methane
  Methane
  Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
  
   from a coal mine goaf into the working environment
• The mixing of coal dust with air, creating a potentially explosive mixture
• The overpressure wave can throw objects into people, knock people over (and potentially throw people into stationary objects or even into automated machinery), knock miners' helmets off, pepper the flesh with gravel or small rocks, stop or alter the normal ventilation of the mine and move large items of machinery, in some cases violently.

Broken bones and fatalities are not uncommon outcomes from serious windblast events.

Windblast prevention

Windblast prevention is centred around not allowing the void to form in the first place. This can be somewhat achieved through careful mine planning. Windblast prevention also relies on monitoring to ensure the void does not form (and stopping production if one does begin to form), and the use of different methods to try to promote caving, such as hydraulic fracturing and the use of explosives.

Windblast harm minimisation

Harm minimisation seeks to minimise the damage to persons and machinery if a wind blast does occur. This may involve the following:

• The use of PPE (personal protective equipment)
• Blast-activated cut-off switches on machinery and electrical circuits (for example, in the form of a paddle that is activated by the blast)
• The use of overpressure-rated ventilation devices, of sufficient rating that they won't be destroyed by a windblast
• In hard-rock mines, harm minimisation may involve physically separating the cave (the void) from the working area by leaving an adequate thickness of muck-pile, and ensuring no other physical linkages exist between the cave and the working areas
• Providing dedicated leakage paths from the cave to atmosphere.

Confusion of windblast with outburst

Windblast should not be confused with an outburst

Outburst (mining)

An outburst is the sudden and violent ejection of coal, gas and rock from a coal face and surrounding strata in an underground coal mine. When outbursts occur, they can be very serious events, possibly even resulting in multiple fatalities....

, which is the sudden and violent ejection of rock and gas from a coal seam and the surrounding strata.