AN underground mine has identified numerous strata support roof bolts that appear to have manufacturing defects, according to Queensland Regional Inspector of Mines Central Michael Downs.
The problem was discovered during an installation and inspection.
Initial investigation has identified that steel bolts appear to have been rolled with an oval rather than round cross-section at a steel manufacturing plant, Downs said.
As the thread has been machined onto the bolts, ‘flats’ on the thread section seem to have occurred on the flatter face of the oval cross-section of the bar.
During the installation process when the nut is screwed up the bar to tension it against the rock strata, it appears that the nut has locked on the bolt when in fact it is jammed when the damaged portion of the bolt thread was encountered.
As a result of locking in this manner, it can appear that the bolt is correctly tensioned.
Downs recommended all mines to thoroughly check their strata support bolts for manufacturing defects that may inhibit correct installation.
Checks may include and be not necessarily limited to supplier QA processes and level of statistical assurance; mine store receipting procedures and delivery checks; and operator checks during installation with appropriate awareness of potential problems.
This issue should be brought to the attention of mineworkers involved in ground support activities, together with an awareness of possible problems that could arise in manufacture, Downs said.
District Inspector of Mines
07 4967 0869