Underground » Strata Control and Windblasts
Underground roadways in coal mines are critical to maintain continuous production of coal. Roadways need to be stable, reliable and functional. One cause of roof instability in underground coal mines is inadequate roof support due to insufficient geological and geotechnical information. Normally, geotechnical and geological information are gathered from exploration boreholes but due to the huge cost involved in drilling exploration boreholes, these are drilled at considerable distances from each other, which is inadequate to capture the variability in roof quality. Conveniently, during roadways development, thousands of holes are drilled into the exact roof horizon to install roof bolts, cable bolts and extensometers. A method to capture the roof quality data from this drilling in real time, for example, Measurement While Drilling (MWD), would be an asset. Such captured detailed roof quality information enables to optimise roof support and prevents roof instability in locally poor ground conditions.
MWD is comprised of hardware and software components. Hardware components are various sensors that are either built-in to the drilling system or added during operations and software components include data extraction, analysis and interpretation methodologies.
Laboratory and field experiments were designed and conducted in this project. For laboratory experiments, samples were made from scratch and drilled in a controlled environment. Field experiments were carried out at an underground longwall mine and an open cut mine. Mechanical sensors, (displacement, torque and weight on bit) and non-mechanical sensors (geophones and accelerometers) were used in classifying the drilled layers.
From laboratory experiments, it was found that the drill signals have a capacity to identify relative strength of drilled medium including interfaces up to 1 mm. Even for irregular and heterogeneous real coal-rock samples, the drill signals showed a potential to characterise them based on their relative strength. However, the mechanical sensors data showed limitations on identifying a clear demarcation between the interfaces of the strata. A preliminary relationship between MWD signals and relative mechanical properties of the drilled medium was established.
Two field experiments were conducted, one in an underground roadway for roof bolt drilling and another in an open cut mine for blast hole drilling. During the roof drilling, the MWD seismic signals could identify fine changes (thickness < 2cm) of rock mass types. Similarly, at the open cut mine, the amplitude of geophone signals showed a close relationship with drilling activities and were able to represent the density anomaly, which was recorded from the geophysical logging.
This project is a step towards utilising machine provided MWD signals in association with a newly conceptualised Seismic MWD technology. The results demonstrate that the seismic MWD signals can be used to back-analyse the strength of rock strata, which provides a sound platform for this technology to be further developed into a practical and commercial system.