Problem description

When you want to assess the Factor of Safety after a fully coupled flow-deformation analysis, you may not want to include the increased strength due to suction in the soil’s unsaturated zone.
In the Safety analysis you want to calculate, this can be achieved by activating the Ignore suction option.

Two things happen in this case when using PLAXIS 2D 2018, PLAXIS 2D 2019 and PLAXIS 2D CONNECT Edition V20:

• When you introduce an unbalance at the start of a Safety phase, the calculation can only vary the ΣM_sf value to find and solve the equilibrium and convergence to a solution. This is not exactly the same equilibrium solution strategy as in a Staged construction loading type of calculation where the out-of-balance forces are gradually solved via the ΣM_stage multiplier.
  When the out-of-balance forces are large at the start of the Safety analysis, the calculation can only adjust the ΣMsf value to further solve the out-of-balance. In some cases, you may see an initial drop of the ΣM_sf value below 1, before it picks up. Due to this, the model could show some erratic numerical behaviour.
• In a Safety analysis, the pore pressures including the suctions (when present) are always read from the previous phase. Using the Ignore suction for this Safety phase will only set the degree of saturation to 1.0, therefore the effective suction (i.e., the multiplication of total suction by the degree of saturation) in fact increases. This increase in effective suction will increase the effective stresses.
  When using a frictional material (φ ≠ 0°) and the effective stresses increase, it will result in a higher strength of the soil. In case of loading problems, like embankments, this will lead to a higher safety factor. Next to that, these suction values are not shown in the results in the Output program.

For these two issues, the first one can lead to inaccurate results, while the second one will likely overpredict the factor of safety in the case you have a large unsaturated zone.

Solution

This issue is solved since PLAXIS 2D CONNECT Edition V20 Update 1 (V20.01.00): the suction is now properly ignored in the safety analysis, but still, it is better to insert a plastic nill phase between the fully coupled flow-deformation analysis, and the Safety phase in which you ignore the suction. This will overcome the issue with the inaccurate results due to larger initial out-of-balance forces.

For older versions, you are strongly advised to insert a Plastic nil phase between your calculation phases in which suction is taken into account and your safety phases, e.g., a fully coupled flow-deformation analysis, and the Safety phase in which you ignore the suction. This is to prevent both issues (inaccurate results and possible overprediction of the factor of safety)

To do this, just insert a phase between the Fully coupled flow-deformation phase and the Safety phase. For this new phase, make sure it is a Plastic phase that takes pore pressures from the previous phase and activate the Ignore suction option. Then, calculate at least this new Plastic phase and the Safety phase.
The Plastic phase will now take care of resetting the suctions to zero and solve the out-of-balance forces due to this change, after which the correct safety factor without the effects of suction is calculated.