4 bolt primary roof support patterns

A recent fall of ground at a Bowen Basin underground coal mine has prompted the Inspectorate to review similar incidents that have occurred in this region.

Some of these incidents include:

• a fall of roof and rib which resulted in a fatality
• a recent fall of ground in a freshly supported development roadway
• a fall in the gate road development panel
• a fall of ground in the tailgate of a longwall panel
• a fall of ground in a gate road development panel which resulted in coal mine workers being trapped.

The root causes of these falls are often complex and vary from mine to mine, however the 4 bolt primary roof support pattern appears to be a consistent theme in the investigations into these incidents.

It is recognised that some mines have very competent roof material and a low stress regime that may justify the use of a 4 bolt pattern. However, it should also be noted that the impact on support effectiveness, when reducing from a 6 bolt pattern to a 4 bolt pattern, is far greater than a simple ratio between the numbers of bolts installed.

It is strongly advised that a proper geotechnical assessment is carried out by a duly qualified geotechnical engineer as part of any proposal to reduce support patterns down to a 4 bolt pattern. This assessment should be based on a recognised methodology supported by field measurements and numerical modelling with realistic input parameters.

The Inspectorate will continue to review their data base of incidents of falls of ground in the Bowen Basin underground coal mines and will provide further direction to industry when the findings are finalised.

Authorised by Andrew Clough - Chief Inspector of Coal Mines

Contact: Russell Albury, Deputy Chief Inspector of Coal Mines , +61 7 4936 0119

Issued by Queensland Department of Natural Resources and Mines