While we almost always take it for granted this type of analysis is the most common liar. Most of the times it lies a little, sometimes it lies a lot, but sometimes it hides the truth. It might come as a surprise, but slope stability analysis is the most common liar in our arsenal. The reasons might surprise you:
a. First, and foremost, all traditional slope stability analyses are upper bound solutions. This means that there will almost always be a critical surface/sliding mechanism that is more critical than the one we are computing.
b. Many engineers only consider circular sliding mechanisms, even in sands. While the circular mechanism is mostly applicable for clays, in sands a log-spiral, combined wedge surface is more common.
c. Sometimes your automatic search will get trapped in a local minimum. In such cases engineers might need to force a user defined initial search.
d. While we mostly run 2D analyses, but all failures are typically 3D.
e. The interslice force function is always an unknown.
f. Slope stability analysis will not predict stress concentrations on stiffer elements. It simply assumes that the element resistance is applied beyond the sliding surface. This can lead to significant under-design if only slope stability is used without additional methods.
This does not mean that we should not carry slope stability analysis, it simply means that we need to be more aware about the limitations, and as a general advice leave some extra room in your safety factor. Some of the images below demonstrate some of these cases:
Figure: Seismic Finite Element Analysis showing higher stress concentrations in lower anchors
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