Safety analysis and undrained behaviour

Safety analysis in Plaxis uses a so-called phi/c reduction method in which the strength of the soil materials will be reduced with a factor ΣM_sf until either failure is reached for a stable value of ΣM_sf,  or the maximum number of calculation steps is reached. In case a project has one or more material sets specified as either Undrained A or Undrained B such reduction of strength may lead to a change of excess pore pressures in the model. This change of excess pore pressures can be prevented by selecting the option “Ignore undrained behaviour” for the Safety analysis phase. In this case the excess pore pressures already present at the beginning of the Safety analysis phase will remain, but no change of excess pore pressures will occur during the Safety analysis.

A point of discussion can be whether excess pore pressures should be generated during a Safety analysis or not. Unfortunately, no straightforward answer can be given to this problem as one cannot compare with what would happen in reality as safety analysis through strength reduction is a non-physical process. In order to choose whether excess pore pressures should be allowed to change during Safety analysis the following aspects can be taken into account:

• For loading problems (for instance embankments) allowing excess pore pressures to change often leads to an increase of excess pore pressures. This would therefore lead to a lower factor of safety compared to the case where no change of excess pore pressures would be allowed.
• For unloading problems (for instance excavations) allowing excess pore pressures to change often leads to a decrease of excess pore pressures and sometimes even to pore tensions (suction). This would therefore lead to a higher factor of safety compared to the case where no change of excess pore pressures would be allowed.

Based on the aspects above one could choose to use or not use the option “Ignore undrained behaviour” such that the lowest factor of safety is obtained, hence the most conservative solution.
Another argument heard is to never allow change of excess pore pressures because the factor of safety should be determined from a specific situation with specific (fixed) excess pore pressures. In other words, the factor of safety should be determined from a “snapshot” situation in the design and analysis process and therefore the excess pore pressures should remain constant during Safety analysis being the excess pore pressures resulting from the project analysis.

In the end the choice for using the option “Ignore undrained behaviour” in a Safety analysis is something for the user to decide. However, it may also be there is a recommendation with respect to the use of the option “Ignore undrained behaviour” in for instance the local building code.