Abstract
Rio Tinto Iron Ore has identified that the high wall slope angles in its Pilbara open pits to date are generally conservative, as much of the wall exposure is built to plan and is without major wall performance issues. This design conservatism has largely been due to gaps in orebody knowledge and layers of conservatism in the design approach. As mining footprints and crests become more constrained and more mining is taking place below the groundwater level, robust methodologies to steepen and safely manage slopes have become necessary - to support waste movement reduction, increase ore delivery, and identify high-risk areas. A project was commenced to identify the methodologies required to confidently steepen pit walls using selected open pits as initial cases for assessment and trial. The project leveraged off industry best practice to identify points of conservatism and explore levers in the design pathways that would allow for significant increases in inter-ramp angles to be achieved without undue risk.
An extensive set of generic analyses was undertaken to understand the sensitivity of inter-ramp slope stability in the pits to various factors, the main ones being: the presence of relatively strong layers in the weaker stratigraphic units and the proportions of strong materials within the stratigraphic unit; the impact of inter-ramp slope height in various materials, bedding dip angles and obliqueness of bedding strike relative to the slopes; the definition of the disturbance zone behind the pit slope; the use of various slope stability analysis methods in 2D and 3D; and the radius of curvature of pit slopes. This allowed for a robust assessment and design approach to be developed and for preliminary indication of potential optimum slope angles to be achieved. A detailed re-assessment of rock mass domains and properties at the test sites was then carried out and stability analyses conducted for the purpose of identifying optimum slope angles and the batter-berm geometries that would support these. This resulted in the inter-ramp angles in the initial focus pits being significantly steepened. This paper presents an overview of the approach developed and the key levers identified for slope steepening in these Pilbara Iron Ore pits.
Authors
- Ian de Bruyn, SRK Consulting, Australia
- Jaya Mylvaganam, SRK Consulting, Australia
- James Jung, Rio Tinto Development & Technology, Australia
- Kim Le Roux, Rio Tinto Iron Ore, Australia
- Darryl Young, Rio Tinto Iron Ore, Australia
- Deb Casparis, Rio Tinto Iron Ore, Australia
