The variable nature of a coal mine roof is one of the great challenges of underground coal mining, contributing to roadway failures and roof falls. Laboratory experiments with measurement while drilling are providing a greater level of understanding of strata properties, which can help to optimise roof-support designs.
The shifting characterisation of a mine roof
A major cause of roof instability in underground coal mines is uncertain variation on strata properties of the mine roof.
Coal mine roofs are characterised using geotechnical and geological data gathered from exploration boreholes drilled at considerable distances from each other.
This however may not capture local variability and discontinuities present in the roof.
This lack of accurate information can contribute to roadway failures and roof falls.
The ability to measurement while drilling (MWD) i.e. to measure, interpret and analyse various drilling parameters during the roof bolting process, would help to reduce uncertainty and improve confidence in the development of a coal mine roof.
Large-scale laboratory drilling experiments
We undertook MWD laboratory experiments using large-scale drilling machines in synthesised and real rock-coal samples.
The experiments used large-scale drilling machine fitted with multiple sensors.
The drilling parameters, such as torque, penetration rate, weight on bit are monitored and analysed to interpret the properties of the drilled strata.
During the roadway development, we found drilling closer to the roadway face and recording drill-bit response to the strata provided geotechnical and geological property data related to the coalmine roofs.
This offered early detection and response to geotechnical hazards, such as a thinning roof coal layer, weak rocks, seam splits, discontinuities and voids.
MWD gives better data for coal-mine roof characterisation
The laboratory results on synthesised samples show relatively distinct responses of the various drilled blocks.
Depending on the material, the reaction on the monitored parameters on the drill bit changes.
The typical responses for torque, weight on bit, penetration rate and specific energy for drilling as the bit passes through are dependent on the strength and properties of the medium. The interface between the strata is also detected.
Our trial has shown the applicability of MWD on characterising a coal-mine roof; the next step is to test the concept on an actual underground mine site.