One of the Australian mines was experiencing problems with loading and the other had corrosion problems on an old ramp. Both mines wanted to know the condition of the installed bolts. DMT was contracted to apply its proprietary non-destructive test method to assess the health of the bolts.

The need to measure the performance of installed rockbolts in German deep coal mines lead to the development of the non-destructive test method. Conventionally, the performance of bolt support function is assessed by measuring deformation in the strata using methods like tell-tales and borehole endoscopy. Another method is to measure the deformation of the bolt shaft, with the strain gauge bolt for instance. Both of these measures are expensive and only monitor certain aspects of the bolt behaviour.

In particular measuring things like plastic deformation and shaft fracture is not possible with conventional methods because only the head of the bolt was visible.

The ultrasonic system is based on using sound to detect inconsistencies in materials. DMT first undertook basic laboratory trials on non-installed and non-stressed rockbolts, focusing on point anchored and fully-grouted bolts. Further tests on installed bolts found that the ultrasound measurement improved due to a lower level of random noise.

Installed bolts were then subjected to traction and shear loading which were detected by the ultrasound. This was followed by underground measurements during which the recovery of a failed bolt confirmed the accuracy of the ultrasound measurement.

"The system is capable of providing clear information on whether or not a bolt has failed as a result of fracture. It can also pinpoint the location of the break," said Ulrich Ruppel, head of rock mechanics at DMT.

DMT subsequently developed a colour-coded identification system to enable a user-friendly way of presenting results. Bolts with no fracture failure are marked green; bolts with plastic deformation are marked yellow; and fracture failures are marked red.

Ruppel said the system gave mine engineers worldwide the first easy to handle and easy to visualise non-destructive testing methods of rockbolts. The service is available in Australia through Strata Control Technology, Wollongong supported by DMT experts.