The 'Guidelines for Open Pit Slope Design' developed as part of the Large Open Pit (LOP) project was published in 2009. The guidelines were focused on hard rocks in open pit mines so did not address design issues for pit slopes in materials generally classed as weak rocks. Research currently being undertaken by the LOP sponsors is directed at filling this gap. Canvassing of the LOP sponsors and their affiliates resulted in identifying pit slope issues in five general categories of weak rocks:

more »

... Cemented transported sediments. 2. Saprolites (residual soils and weathered rocks). 3. Leached rocks/soft iron ores (leached, creating a rock with a high void ratio). 4. Mudrocks. 5. Hydrothermally altered rocks. Establishing the strength of hard rocks is traditionally approached using rock mass classifications and the Hoek−Brown failure criterion. However, there is ample evidence that these approaches do not capture the strength characteristics and the failure processes evident in weak rocks. In this paper the authors describe some of the processes that should be considered when attempting to establish the strength and failure envelopes input into slope designs for open pits in weak rocks. 'Guidelines for Open Pit Slope Design' was produced in 2009 (Read and Stacey, 2009). These guidelines were focused primarily on open pit slopes in hard rocks and did not address the challenges that are often associated with open pits in weak rocks. While open pits in weak rocks tend to be shallower than those in hard rocks, pit slopes several hundred metres deep have been developed in weak rock and hence failures can still be significant. Weak rocks typically require flatter slope angles than are used in hard rocks and these flatter slopes can also have significant economic impacts. It is therefore critical that the presence of weak rocks is recognised early in the design process, since they often cannot be mitigated in the same way as hard rocks. Because weak rocks were not addressed in the original guidelines, the LOP is in the process of compiling guidelines for open pit slope design in these weak rocks. The first step in this process was to canvass the mine operators to identify the material that they classed potentially as weak rock, and particularly the materials that presented operational challenges. All the results from that survey are summarised in Figure 1 . Each of the material types reported in Figure 1 has a unique geological origin and can behave in distinctly different ways in an excavated slope. While the initial survey results reported a broad range of rock types the focus for this study was restricted to weak rocks and hence the case histories for volcanic breccias, quartzites and kimberlites were excluded. The purpose of this paper is to provide an overview of the behaviour of slopes in weak rocks that has been observed in large open pits and provide a framework for characterising, analysing and designing in these materials.