The Layer Method and the Method of Slices generally produce very similar results. Differences between the two methods are summarized in the table below. The standalone version of the USDA-ARS BSTEM model uses the Layer Method unless it has to compute tension cracks or cantilever failure, which cause it to switch to the Method of Slices. The choice mainly involves a trade-off between a theoretical consideration (the normal force distribution) and a practical consideration (run time).
Method of Slices computes a somewhat more realistic distribution of normal stresses along the failure plane (below). The Layer Method computes larger normal stress for larger layers, which tend to be along the top of the failure plane while the Method of Slices computes larger normal stresses for larger slices which tend to exert their forces at the base of the failure plane (which is a more realistic assumption). Therefore, without tension cracks, the Method of Slices will generally compute a slightly higher FS for the same data set. However, because the Method of Slices allows tension cracks, which tend to remove more resisting forces than driving forces, the Method of Slices often returns a lower FS, and more failures.
Table: Method selection criteria
Layer Method
Method of Slices
Customized for bank failure applications.
Closer to the comparable geotechnical analyses.
Higher normal stresses along the failure plane generally computed for the higher layers.
Higher normal stresses along the failure plane generally computed for the deeper layers.
Non-Iterative. More computationally efficient.
Apportions normal stresses according to more physically based assumptions.
Switches to method of slices if tension cracks or overhanging banks form.
Computes tension crack and cantilever failures.
However, because the Method of Slices is iterative and the Layer Method is not, the Layer Method is more computationally efficient and can decrease run times. Since there are already two iterative computations in BSTEM outside of the FS computation (e.g., analysis for several nodes up the bank face and the selection of the critical failure plane for each node) bank failure analysis can be computationally expensive for big projects or long runs. The Layer Method may reduce those run times.
