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:
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.
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.
PLAXIS 2D Tutorial 09: Stability of dam under rapid drawdown [Tutorials]