An outburst cavern is defined by features such as its size, shape, location of cavern in the working face and the orientation of the cavern with respect to the axis of the mine workings (Lama & Bodziony, 1996).The size of the cavern is defined using the width, depth and height of the cavern to simplify the differences in shape. The volume obtained using these measurements is always less than the volume of material collected from the mine due to the expansion of the ejected coal. Thus, due to the irregular dimensions involved and the expansion of ejected material, the size of an outburst cavern is very difficult to define.

The shape of an outburst cavity is considered by Cis (1971) to be influenced by three factors. These are:

• Effect of gases present in the seam;

• Presence of soft layers present in the coal seam; and

• Gravitational effects.

Three factors affect the caverns and cause them to form into one of three shape groups:

• Mug or pear

• Pocket

• Cone

Mug shaped caverns are known to occur around the corner of a roadway. These caverns are elongated and run at a low angle to the face. The attached figure showing a pear shaped cavern occurred in the course of drilling a 300mm diameter hole in the face of a roadway caverns of pocket shape are so named due to the large amount of crushed coal that remains in the cavern. These are commonly seen to occur in steep seams and in seams of low dip when mining is along the dip rise. The combination of unexpected displacement of loose coal and the steep seam gas characteristics cause pocket shaped caverns. Cone shaped caverns occur as a result of small outbursts and these outbursts are stress driven. The failure occurs at right angles to the bedding plane. The cone shape is can also be seen as an outburst that is dying out (Lama & Bodziony, 1996).

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.