Mine waste dump stability analysis
Mine waste facilities are an essential part of any mining process. In several cases such as Los Frailes(1) the design failure of the tailings facility caused massive financial damage to the mining company. More recently, the breach of the Mount Polley Dam in British Columbia caused a 45% drop in the value of the company, and at the time of writing is expected to cost the company $ in clean up costs (link). Every waste facility is unique, since the geological factors differ at each mine. Understanding of the geological conditions is paramount as they will dictate the location, size, and constructability of the waste facility.
The most important part of the stability analysis is determining what you are trying to model (or: What problem are you trying to solve?). This requires having a detailed knowledge of the site and project background, as the foundation conditions will heavily dictate the design.
ASD vs LRFD
One important part of stability analysis is determining what standard of design to use between Allowable Stress Design (ASD) and Load Resistance Factor Design (LRFD). The current state of geotechnical engineering is such that since LFRD is probability based, it is typically used only in soil- structure interactions (such as MSE walls and Pile design).
When using ASD, it is important to choose an appropriate Factor of Safety (FS) for the project conditions. Generally, a higher factor of safety is required for situations where there is more uncertainty, but a higher factor of safety typically means an increase in cost for the mine. There is a requirement of the designer to cover high and low values – or was that specific to sheared/unsheared shales?
In order to decide how to model stability, the likely failure mode and stress conditions must be understood. Some models include (in increasing complexity):
• Limit equilibrium (Rocscience Slide, Geo-Studio SlOPE/W)
• Finite element (Rocscience Phase2, GeoStudio SIGMA/W, Plaxis)
• Finite Difference (Itasca FLAC)
• Distinct element (Itasca UDEC)
In the case of mine waste facilities, there are usually relatively low stresses, and the structures are usually constructed from soil like materials. Therefore limit equilibrium analysis is likely going to be the model chosen as it is simple and relevant. Mine waste facilities are typically processed material such as blast rock or milled material, which is placed above ground. Thus, both Finite Element and Distinct Element Models are rarely used except for specific cases.
A limit equilibrium analysis, although the simplest of the models, is applicable to many slope design problems.
Finite Difference and Finite Element
Finite Difference/Element models are vastly more complex than Limit Equilibrium models. This section will explain the general principles used in the two models types, as well as the differences between the two.
List of Other Important Considerations
Another key design parameter is what building materials are available and how much? This is very important especially when it comes to construction of the facility.