Open cut mining operations are continually faced with the risk of rock slope failure. Such collapses pose a significant risk to personnel, resulting in damage to mining equipment and disruption to the mining process. However, slopes rarely fail without precursor movements. This thesis describes the development of a new technology, the Slope Stability Radar (SSR) which can accurately detect these precursor movements. The SSR utilises microwave interferometry to enable sub-millimeter displacements to be detected remotely, without the need for reflectors placed on the slope. It is not dependent on highly stable footing. Dust, atmospheric changes and vibration from mining equipment also have little effect on its operation.
Currently available slope monitoring techniques include laser EDM with reflectors, reflectorless laser scanners, wire extensometers, visual spotting of the face and visual crack monitoring. Reflectorless laser scanners are the most similar to the SSR, but their displacement measurements are less precise (by at least an order of magnitude), they have a shorter range and require more stable footing. Laser EDM from reflectors has greater range but is limited by the availability of reflectors on the slope. Wire extensometers can have similar accuracy, but produce only a limited number of point measurements and are subject to a high incidence of false alarms. Visual spotting has the drawback of only being useful with adequate lighting. The spotter can also be easily misled by unrelated localised rilling and gives only a short warning time before failure. Crack inspection relies on the cracks being both visible and accessible.
The SSR originated from applying differential interferometric radar techniques to a ground based radar. The system has a mechanically scanned beam produced using a physical aperture. Such a system is tolerant of beampointing errors and capable of displacement precision better than 1/100th of a wavelength for stable targets with high SNR. Spatial phase unwrapping is not necessary as temporal unwrapping is sufficient for wall movements. The low sidelobes of the physical aperture are sufficient to reject clutter, particularly reflections from mining vehicles. The effect of surface changes induced by rain is small, with most of the signal disturbances attributed to weathering of the face.
Standard interferometric processing has been augmented by a number of algorithms to ease interpretation of the data. Atmospheric correction based on assuming part of the wall remains stable reduces the effect of refractive index changes to submillimeter levels. These changes would otherwise produce apparent displacements of the order of ±5 mm for targets at 300 meters range. Masking algorithms reject areas such as sky and foreground obstructions through their weak return, and vegetated areas are detected through statistical analysis of the signal coherence. A spurious target rejection algorithm allows the removal of sudden changes caused by trucks from the dataset.
A new interferometric filtering technique has been developed. Due to the high SNR of the data, current interferometric filtering algorithms are insufficient, as they do not use the full interferometric signal or lack adjustability of the filter shape. The new technique produces a significant improvement in the mean square error on both real and simulated data, while maintaining resolution. This new filter should also be applicable to other interferometric data.
The SSR is highly accurate, producing displacement measurements of a corner cube at 40 m with an accuracy of 12 μm. This accuracy, corresponds to an error of less than 0.001 of the system's wavelength, with more than 200 wavelengths between the system and the target. In the field, it could not be guaranteed that the system was accurate, where the measured displacements are the same as the actual displacements, only precise, with each of the measurements are similar to each other. This is because high quality reference measurements could not be obtained and only the component in the direction of the radar is measured. For failure detection, this was not seen as a problem, as it is acceleration of movement that is critical.
The thesis covers the trial of the SSR at two open cut coal mines. At Drayton coalmine (NSW, Australia), a previously unstable slope was monitored over a three week period. The radar operated reliably over this period, giving precise measurements to the face (standard deviation of 0.12 mm). However, very little movement occurred, with only a 2mm movement of part of the slope being detected. Problems with the quality of the power supply and the radar's footing were also solved during this trial. For the second field trial at Moura (Queensland, Australia), the system's abilities were more adequately tested. The SSR detected wall movements that were strongly correlated with the highwall mining operation.
The experience gained in these field trials indicated where improvements could be made to the system. These include the addition of a digital camera to allow registration of the deformations, improved signal processing and a telemetry link to access the data off-site. This new ground based interferometric system has shown great potential to deliver the required data to geotechnical and mining engineers, allowing improved prediction of slope movements. The SSR should improve the safety of coal extraction near unstable slopes, reduce mine downtime due to unpredicted failures and, most importantly, stop injuries and fatalities. It is anticipated that the system will be equally applicable in other open cut mining operations such as nickel mines, gold mines and quarries. There is also the potential of applying the technique to underground mining operations such as in open stopes. While the SSR will not completely displace other monitoring methods, its high precision, broad area coverage without reflectors, rapid scene updates and high automation, makes it a strong addition to the tools available.