Outburst Cavern
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 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 images opposite 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 can also be seen as an expression of an outburst
that was dying out (Lama & Bodziony, 1996).
