Spoil piles – comparison of stability analysis methods

A study has been undertaken to confirm appropriate analysis methods applicable to design of granular waste materials and specifically that of “spoil piles” for open pit coal mining. The authors’ experience with spoil pile stability is that the critical failure mechanism is a two wedge mechanism in accord with that highlighted in the literature. Analysis requires careful consideration of the combination of spoil shear strengths and the angle of the “rear” scarp adopted. Moreover, the literature indicates analyses need to take account of the inclined “inter-slice” within the mechanism. To highlight these considerations an example case was assessed utilising five limit equilibrium stability packages, two “numerical based” packages and a spreadsheet developed by the author. Several analyses are based on vertical slices to pose the question: what is the magnitude of error not using an inclined “inter-slice”? The results are presented as overall spoil pile angle indicated by the use of different software, stability method and assumptions on inclination of “rear” scarp and inter-slice. The example case highlights that, provided a rigorous stability method is utilised, for a given rear scarp assumption, the results of the analyses are insensitive to the software utilised. Therefore with appropriate judgement assessment of spoil pile stability should not be predicated by use of one particular software or stability method.