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Use of photogrammetric mapping techniques for slope stability

For pit slope design, geotechnical data is typically collected by mapping the benches to supplement data collected from boreholes. Where an open pit has been in operation for some years, bench faces are often difficult to access and, increasingly, access is being restricted to reduce the risk of exposure to rockfall hazards. In order to address these issues, photogrammetry is being used more frequently.

SRK Consulting (UK) Ltd has acquired the expertise to undertake photogrammetric surveys using the Sirovision® system developed by CSIRO Australia. Besides purchasing the data manipulation and analysis software, SRK has invested in the photographic and survey equipment needed to collect the data.

SRK has previously used Sirovision® successfully on a number of open-pit projects. In Colombia, South America, SRK’s brief was to carry out a slope design optimisation study for a cut back of an open-pit coal mine that would increase the height of the highwall from 150m to 250m. As part of this study, during a five-day site visit, Sirovision® was used to:
• Map large-scale geological structures on the existing highwall. For this the camera set up was positioned at the footwall slope crest about 400m from the highwall
• Capture bench scale structural data for use in detailed kinematic analysis from close range (4m-10m)

In conjunction with the large-scale mapping, detailed physical properties of over 1200 individual discontinuity occurrences were obtained from selected bench face locations. These properties included:
• Three dimensional location of features
• Orientation (dip/dip direction)
• Trace lengths/plane area (also used to calculate joint persistence)
• Large-scale joint surface roughness
• Termination indexes (to ascertain feature chronology)
• Fracture set spacing

Sirovision® allowed the rapid collection of a large data set of new structural information, saving the client time and money over more traditional methods of geotechnical data collection. The structural data collected, together with the incorporation of newly measured faults into the geotechnical model, provided for robust and statistically-sound kinematic analysis of the proposed new highwall pit slope design. The 3D positional information of the coal seams and other geological boundaries allowed the geological model to be verified and updated.

Xander Gwynn:

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