While working in the Classical mode the Suction cut-off should be activated by default since we cut off all (suction) steady state pore pressures at (approximately) zero. Note that in the Classical mode we work with Terzaghi stress definition and we do not take the unsaturated zone into account.
Default setting in Classical mode
While working in the Advanced mode by default the Suction cut-off should not be activated (but editable) since we do want to take the suctions in the unsaturated zone into account. Note that in the Advanced mode we work with Bishop stress definition.
Default setting in Advanced mode
Now when switching between the two modes the Suction cut-off may be unexpectedly activated in the Advanced mode and unexpectedly deactivated in Classical mode (while the input field remains non-editable). Effect in the latter situation may be that the full steady state pore water suctions are taken into account in the calculation leading to too high effective stresses and thus wrong results. Additional problem here is that these suctions now cannot be seen in Output since in the Classical mode suctions are assumed to be zero and thus not displayed.
This issue is now resolved in PLAXIS 2D AE: in this version the different calculation modes are integrated, and now you can switch the suction cut-off on and off for each phase.
For PLAXIS 2D Classic and PLAXIS 2D2012 (and earlier):
When working in the Calculation window and switching between Classical and Advanced mode be sure to explicitly check the Suction cut-off setting. When switching from Classical to Advanced mode and finding the suction cut off activated: simply switch it off if desired. When switching from Advanced mode back to Classical mode and finding the suction cut off deactivated (and the field non-editable): switch to another Calculation type (i.e. Plastic or Consolidation) and then back to the original Calculation type. The Suction cut-off field should be activated again.