Outburst size

The size of an outburst is defined using a combination of the mass or volume of the material ejected from the outburst cavern and the material remaining within the cavern together with the volume of gas released from the outburst.

The volume of material ejected from the outburst is usually measured as loads of shuttle cars when the roadway is being cleared following an outburst. It is difficult to accurately measure the volume of material ejected from the cavern as the volume collected in the shuttle cars rarely equals the measured volume of the outburst cavern due to smaller particles not being included in the measurement. Also, the cavern dimensions are difficult to measure accurately due to inconsistencies in the sides of the cavern. Therefore as an approximate method it has been suggested that the total volume of material ejected is equal to twice the volume of material collected outside of the cavern.

Measurement of the volume of gas liberated from the outburst can be made using gas monitoring systems present in the mine. This is usually the case in known gassy mines where methane monitoring equipment is available throughout the mine. However, mines that experience carbon dioxide outburst encounter trouble measuring the gas volume as carbon dioxide monitors are only placed in the return shafts.

Classification of the size of an outburst has been proposed by Majcherczyk and Kobiela (1987). The classification placed an outburst into a 5 group system based on the mass of material ejected in the outburst as follows:

Group I – Outburst of very small size: 0.5 to 10 tonnes
Group II – Outburst of small size: 10 to 50 tonnes
Group III – Outburst of medium size: 50 to 400 tonnes
Group IV – Outburst of large size: 400 to 1,000 tonnes
Group V – Outburst of very large size: >1,000 tonnes