Remote Sensing Discontinuity Characterization at the Palabora Open Pit, S Africa

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Doug Stead, Matthieu Sturzenegger, Sung Lee, Andre van As,
Abstract Located approximately 390km North East of Johannesburg, the Palabora mine began open pit operations in 1964. Surface excavations ceased in 2003, and since then the mine has been operating as an underground block cave operation. Instability of the north wall associated with the block cave began in mid 2003 and eventually affected an 800m high rock slope. The failure has been modeled using a variety of techniques including 3D distinct element, FLAC3D (based on a synthetic rock mass approach) and more recently using a hybrid 2D finite/discrete element with fracture code. The main thrust of the models has been to improve the understanding of the failure mechanism and the underground open-pit interaction. Characterization of the fracture network forms a fundamental component of successful numerical modeling of the Palabora pit slope failure and this paper aims to supplement and improve existing data with particular emphasis on providing robust data, especially fracture persistence, for development of discrete fracture network models. In addition, the accurate integration of specific major photogrammetrically mapped structures into DFN models will provide an opportunity to assess the sensitivity of such models to discontinuity persistence. A description, summary and interpretation of work carried out to map geological structure in the Palabora open pit using terrestrial digital photogrammetry (TDP) and laser scanner (TLS) techniques is provided.

Fieldwork took place over 10 days in February 2008, during which TLS and TDP at 20-400 mm focal lengths was undertaken. Complete coverage of the 1.5km wide, 1.8km long, 800m deep pit was attained at all photogrammetric scales apart from f = 400m where coverage was limited to specific pit walls. The use of telephoto long focal length (400mm) imagery in rock slope characterization is shown to be an important development in photogrammetric data collection. Georeferenced 3D models have since been constructed, and to date, approximately 2500 planes mapped at a variety of ground resolutions. LiDAR data was used at selected locations in addition to conventional data to allow a detailed comparison with photogrammetric results, and in the process has identified interesting issues associated with survey data. Factors which must be considered in remote sensing data collection in large open pits are discussed including survey/model accuracy, improved data management, discontinuity mapping errors, and the time taken to construct a large quantity of photogrammetry models.

In attempting to capture large-scale structures from high resolution images, the advantages in integrating photogrammetry and mine design software are highlighted, enabling a greater number of contiguous models to be viewed at once and the importance of scale effects to be addressed. The goals of the current research are to improve both rock mass characterization and thereby 3D numerical modeling applied to the large open pit environment.
Keywords: remote sensing, Palabora, Rock mass characterization, photogrammetry, LiDAR
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