Bedded coal and iron ore deposits in Australia are usually hosted in complexly jointed, faulted or folded, highly anisotropic rock masses. In coal, these often comprise moderately strong siltstones and sandstones with weak coal seams, siltstones and shales. For iron ore, these comprise strong banded iron formations discretely interbedded with very weak shales. Slope failure mechanisms typically involve sliding along bedding (anisotropy) planes combined with joints or faults acting as release planes or forming step-path failure mechanisms.
Slope stability modelling techniques have evolved over the years and increased in complexity with continuous improvements in computing capability and available software. Less than 20 years ago, basic kinematic analysis was the primary means of designing large rock slopes. In the 2000s, the use of two-dimensional limit equilibrium analysis and numerical modelling rapidly increased with faster computing. As we approach the 2020s, three-dimensional limit equilibrium and finite element analysis software are readily available and offer a range of options to model the behavior of complex, anisotropic rock masses. The results obtained by these different modelling approaches, for example, isotropic vs. anisotropic, or 2D vs. 3D can vary significantly depending on the geological conditions.
This paper presents case studies from both open pit iron ore and coal mines that compare the factor of safety (FoS) obtained from 2D and 3D limit equilibrium modelling approaches. The case studies clearly show the limitations of 2D modelling when the rock mass being excavated is highly anisotropic in nature and when large-scale geological structures are present whose geometry cannot be adequately represented in plane strain. Results further indicate that modelling solely in 2D can lead to either the over-estimation or under-estimation of FoS, by failing to locate the section of slope with the lowest FoS or failing to adequately model the anisotropic conditions under which failure is likely to occur. The tools are now readily available to facilitate 3D modelling techniques alongside existing 2D techniques to complete a comprehensive review of slope stability. This will allow both the optimization of slope designs to be completed, and increase design reliability by identifying the sections of slope more susceptible to failure in true 3D space.
In 2014–15, income from sales and services from coal and iron ore mines in Australia was approximately $115 Billion (AUD), which accounted for approximately 80% of total mining and quarrying sector (ABS, 2016).
The Pilbara Region of Western Australia hosts the majority of economically extractable iron ore deposits in Australia. Hundreds of open cut mines are operated by major mining companies near the townships of Newman, Paraburdoo and Tom Price with single operations often having access to several individual pits in similar ground conditions. Due to the broad regional expanse of the operations, particularly in the iron ore sector, a very high extraction rate is achieved despite vertical development rates remaining relatively low (typically one to three benches or 10m to 30m per year in a single iron ore pit). Final pit depths or total rock slope heights range from less than 100m to 350m.
The Hunter Valley Region of New South Wales and the Bowen Basin Region in Queensland host the main coal deposits used for energy and steel manufacturing. Mining typically begins with an initial excavation, called a box cut, and then progresses down dip in a series of strips, with the coal closest to the surface extracted first. Pit geometry is dictated by equipment capabilities, the location of economic coal seams, and geotechnical constraints to achieve an acceptable design. Individual bench heights typically do not exceed 60m and overall excavated slope heights rarely exceed 150m.