By Bongani Dlamini

Australia’s mining sector has been encouragingly proactive in implementing safety initiatives that have resulted in a significant reduction in underground coal mine roadway failures and instabilities, but there is still room for improvement using the measurement while drilling (MWD) concept for characterising the coal mine roof.

Operational experience has shown geotechnical variability of the coal mine roof is a key driver for roadway instabilities and failures; and while the characterisation of the coal mine roof relies heavily on surface exploration borehole data, this data is almost always inadequate.

The result has been a growing reliance on underground roof borescoping and, occasionally, upholes to supplement exploration boreholes in order to fully characterise the geotechnical and geological variability in the roof.

While providing some valuable geotechnical information, these techniques generally involve a substantial time lag between identifying a geotechnical hazard and the data being received to allow for monitoring and risk management.

In a paper published with CSIRO in January 2020, we looked at the work being done using the measurement while drilling (MWD) concept and found it offers significant promise.

Large-scale drilling experiments in synthesised sandwiched rock samples without interfaces were carried out, the “Preliminary Investigation into Measurement While Drilling as a Means to Characterize the Coalmine Roof” paper explained.

“The drilling response data were analysed to identify whether the drill data differentiates the various strengths associated with the rock samples. The initial results show that the drilling data can differentiate the synthesised rock samples.”

The paper is uncategorical.

“A major cause of roof instability in underground coal mines is the uncertain variation (for example, thickness, competence, discontinuities) of the coal mine roof,” the paper continued.

Normally, the geotechnical and geological data gathered from exploration boreholes, which are drilled at considerable distances from each other, are used to characterise the thickness and quality (including strength) of the coal mine roof. This limited data cannot capture the local variability presented in the coal mine roof.

MWD technology can be used for real-time monitoring to better predict hazards such as “weak” lithology, discontinuities and changes in key geotechnical units in real-time, allowing for earlier response and improved management of geotechnical risks.

It has been long recognised that drilling parameters such as penetration rate, torque, rotary speed and vibration can offer valuable insights on geotechnical properties. MWD technology provides exactly these insights facilitating earlier detection of geotechnical hazards.

MWD has been used in open-cut drill and blast, and in some underground hard rock mines, however, various technical reasons have resulted in the technology not being fully integrated in the roadway development process.

This is despite the simple fact that the more we understand the characterisation of the coal-mine roof, the more can be done to mitigate and manage risks.

The continuous miner mounted drill rig is a valuable under-utilised real-time underground “exploration” and geotechnical characterisation tool. Experienced underground drill operators can intuitively provide a qualitative classification of roof “strength” variability based on factors such as vibration and penetration rate of drill steels.

The next logical step in the safety management process is to capture these cues through MWD technology, analyse them and develop predictive geotechnical indices for improved real-time coal-mine roof characterisation.

This can be done by instrumenting drill rigs with sensors to capture MWD parameters while drilling through various coal mine roof geotechnical units.

As MWD technology continues to evolve, so too does its potential to save lives, improve site safety and reduce the financial and reputational costs of safety incidents to a company, it is a growing area of focus for Australia’s coal mining sector.

Bongani Dlamini, is an associate geotechnical engineer at Cartledge Mining and Geotechnics.

*Article published in the April-June 2020 issue of The Asia Miner