S Coetsee Reutech Mining, South Africa
The empirical derivation of the minimum berm width and the formulae presented by the various design criteria are different due to the input parameters selected, thus resulting in a range of minimum berm widths that can be calculated for the same set of rock mass and geometric conditions.
When compiling a berm design model, in assigning a derived (calculated) friction angle and cohesion value, the Factor of Safety (FoS) and Probability of Failure (PoF) for a suite of kinematically identified plane or wedge sliding instabilities may be overestimated and may not representative of true mining conditions.
Rather than applying dataset cutoff criteria to the FoS or PoF for plane or wedge instability spill volume, the use of all data regardless of the FoS or spill volume is required in order to accurately assess the minimum berm width that can be calculated and then recommended for the design. This is based on the assessment of all spill volumes, spill radii and the criteria accepted and applied.
As shear strength properties changeover the life-of-mine, so does the FoS. Assuming that the FoS is static and the derived PoF is indefinitely valid will result in a design that may be insufficient in maintaining the berm retention factor assigned.
Therefore, attaining and utilising derived FoS and PoF allows for all plane and wedge shape iterations compiled during the modelling process to be utilised. This methodology allows for the assessment of the minimum berm widths calculated using the published criteria and advanced dataset assessment techniques.