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Rapid characterisation of slope instability using LiDAR

Following a multi-bench structural instability along the primary haul road for an operating open-pit copper mine, SRK was tasked with rapidly evaluating geologic structure in the area and subsequently assessing pit slope stability. It was important to determine whether significant pit slope design modifications were necessary for further mining and whether safe transport through the area would be possible.

Given the critical time frame and high risk associated with field mapping near the area of instability, SRK employed remote data collection techniques to characterise geologic structure from a safe distance. Pit walls were scanned using an I-Site 4400CR laser (LiDAR) scanner, automatically integrating high-resolution digital images with detailed 3-dimensional point clouds. Approximately 50,000m2 of pit wall were scanned from three primary setups covering four mining bench levels.. The total time required to complete the scanning, including data post-processing, was approximately 8 hours.

Using the Vulcan Geotechnical Module, details of rock structure were digitised directly into an electronic database exported for analysis. The parameters collected in the customised geotechnical database for each of the approximately 400 discontinuities digitised included discontinuity type, coordinates (easting, northing and elevation), orientation (dip and dip direction), length, spacing and termination (single, double or none). The use of these techniques also allowed accurate modelling of several major (pit wall scale) structures. Structural characterisations based on the LiDAR scanning were ultimately confirmed with field reconnaissance.

A sufficiently large sample of the discontinuity population acquired from the LiDAR scans enabled SRK to develop a representative geotechnical model. Statistical datasets were analysed using stochastic simulations of discontinuity orientation, strength and persistence to determine the probability of similar instabilities occurring within the existing bench design and wall orientation as excavation progressed. Approximately 3 days were necessary to digitise discontinuities and analyse datasets.

Given the variability in geologic structure, SRK determined that bench scale stability could be significantly improved by implementing relatively minor changes to the pit design. Specifically, rather than continuing with a curved wall, which oriented a significant portion of the wall in the newly identified adverse direction (relative to rock structure), the design was modified to include two linear walls with an obtuse “corner,” effectively eliminating the portion of the wall oriented in the adverse direction and, consequentially, reducing the likelihood for reoccurrence of the same mode of instability. Mining of the remaining pit wall has been completed without further such instances of instability.

In this application, LiDAR provided several advantages over conventional field sampling techniques including the ability to acquire comparatively large and accurate data sets quickly and with limited exposure of personnel in areas that would otherwise be unsafe and inaccessible. Directly digitising the data into electronic format also eliminated data input steps and their potential source of error. Above all, LiDAR proved a useful and efficient tool for collecting objective rock mass information which can supplement the subjective observations and assessments of the geotechnical engineer.

Michael Levy:

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