Solution collapse breccia pipes (SCBPs), that form under the influence of gravity, and sinkholes were studied on the Colorado Plateau in Arizona, USA to identify preserved evidence of their formation mechanisms. Close range photogrammetry was employed for data acquisition and reconstruction of the threedimensional (3D) scene of the outcrops. Key geological features were accurately mapped within their true 3D space using photogrammetry. Interpretations of the results suggest that the mechanisms controlling the collapse, and its debris distribution within the pipe, are influenced by in situ discontinuities in the host rocks at every scale from the regional context to the size and shape of the collapsed blocks.
A rising demand for metals has forced the mining industry to target deeper orebodies. With increasing depth, lower grade orebodies become uneconomical because of higher extraction and production costs. Hence, the industry is transitioning to more cost-effective underground mass mining methods such as block caving (BC). In BC, the ore is undercut by blasting to create a void and then drawing off the broken rock to induce failure and subsidence of the overlying rock mass (cave back). This initiates an upward progression of the cave either due to failure along natural discontinuities (gravity caving) or due to stresses induced in the cave back (stress caving). Detailed knowledge of the geology and structure is required at all scales to manage the mine caving processes. This involves predicting the caveability and fragmentation of the rock mass and the effects of subsidence-induced deformation mechanisms on the surrounding rocks, and at surface. BC is a 'blind' mining method and direct observations of the mined zone are limited. To enhance the understanding of the processes operating in gravity induced cave systems, solution collapse breccia pipes (SCBPs) and sinkholes were studied on the Colorado Plateau, USA, as potential analogues to mining induced caving. SCBPs form under the influence of gravity, when rock masses progressively collapse into a developing void produced by water flow and the dissolution of rocks such as limestone. These processes result in an upward stoping process through the overlying rocks, which eventually forms a vertical, pipe-shaped column of broken rock. After lithification, this structure becomes an SCBP. Recent erosion that formed the Grand Canyon system has exposed many SCBPs in section on canyon walls.This study employed close-range photogrammetry and photo-interpretative mapping to study the features of well exposed but mostly inaccessible SCBPs and sinkholes. Georeferenced 3D models allowed accurate data to be collected for structural analysis, inferences to be made about rock mass behaviour, and the identification of processes that operated during subsidence. This was the first approach to study SCBPs by applying close-range photogrammetry. The results enhance the understanding of SCBP geological evolution over time.Four zones were identified in typical SCBPs, with each exhibiting different deformation and collapse mechanisms and/or flow processes. From the centre to the periphery, typical SCBPs are comprised of, (1) the breccia body, a clast to matrix-supported breccia with angular to sub-rounded clasts, (2) the pipe margin, a zone of intense fracturing and shearing, (3) the deformation zone, a region of faulted and displaced rocks surrounding the breccia body, and (4) the undeformed wallrocks. The mechanics of SCBP collapse are most strongly influenced by lithology and both preexisting and stress-induced discontinuities. Placing studied SCBPs in a regional context showed that their location and shape is controlled by basement faults and joint patterns.iii The analysis of clasts in ...
This paper presents the monitoring and management of a large-scale toppling failure that was first observed in 2005 at the Ok Tedi mine. Ok Tedi is a large open-cut copper gold mine in the Western Province of Papua New Guinea and is one of the wettest mines in the world (>10 m annual rainfall). The management of large-scale failures in open-pit mines expresses geotechnical challenges in terms of balancing economical designs and safe operations. The failure located at the southern end of the West Wall was reactivated in June 2019 due to continuous mining and prolonged rainfall events. Erosion, cracking, and rock toppling associated with lateral displacements, evolved rapidly. Hence, ground control and groundwater management plans, including a Trigger Action Response Plan (TARP) and measurements to monitor movement across the wall, were implemented. This monitoring system, associated with daily visual inspections, enhanced the understanding of the necessary controls to maintain a safe mining operation. Furthermore, the water management plan was crucial in managing this failure. The successful risk-control of this large-scale toppling failure at Ok Tedi ensured continuous mining with a functional balance between economic and safety during operations.
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