Underground » Strata Control and Windblasts
A prototype tool has been developed to allow testing for instability and zones of weakness in a mine roof. The tool measures the velocity of a seismic wave passing through the rock mass. In general terms, the slower the measured velocity, the weaker the rock. The wave is generated by a mechanical source within the tool which impacts the bore hole surface.
The use of seismic techniques for estimating rock strength is well established. The challenge in this work has been to develop a tool that would fit into a nominal 28 mm diameter roof bolt hole. These holes would be dry so the source and the receivers must be in physical contact with the walls of the hole. The tool also needs to be certifiable intrinsically safe.
Compressed air is used to fill bladders which couple the tool to the walls of the hole and to drive an air cylinder which forms the hammer source. Tests in various materials in the laboratory and in drill holes underground at the University of Queensland Experimental Mine confirm the operation of the tool. Results are presently captured on a laptop computer and analysed by hand, but development of an intrinsically safe PC controller which will perform these tasks is almost completed.
It is envisaged that the tool will be able to be operated by a geotechnical engineer, mine deputy or other mine staff. Current estimates are that it will take approximately 30 minutes to fully log a hole of 3 m length. The tool could be used for designing roof support requirements and for the on-going monitoring of a roof to check for deteriorating conditions.
At completion of the report, it was recognised that further trials in underground coal mines and development of a final prototype were required before commercialisation could take place. With the assistance of the ACARP monitor, Mr Bob Butcher, tests were subsequently undertaken at Baal Bone Colliery. These confirmed that the tool was able to distinguish between intact roof and roof which had undergone degradation.