The strength reduction method can be used to establish a slope stability safety factor. In this approach the safety factor is obtained by progressively reducing the shear strength of soil elements until the slope “fails”. In a finite element analysis this happens when the algorithm does not converge, and the deformations become excessive.
An important question that arises is if the traditional limit-equilibrium based slope stability and the shear reduction methods should produce the same value. Experience has shown that in many cases they will and in many cases, they will be relatively close.
When running a strength reduction approach sometimes one can get illogical results. Because such algorithms do not recognize the real stability mechanism always as the most critical, in many cases a shallow failure surface or element might be controlling the produced strength factor.
It is generally advised that an engineer considers both the limit-equilibrium slope stability as well as the strength reduction method. When looking at the traditional approach we should examine a wide range of potential circular and non-circular sliding surfaces. Automatic algorithms might reveal more complex slope stability mechanisms but might not identify the most critical sliding surface.
The following images present some cases where the limit-equilibrium slope stability safety factor is compared against the finite element strength reduction safety factor in our DeepEX software:
