Problem description

The Fully coupled flow-deformation analysis will always consider influence of suction, even if the option Ignore suction is chosen. This means that selecting Ignore suction for a Fully coupled flow-deformation may lead to unexpected results for the stress development.

Note: Combining phases that ignore the suction with phases that will (partly) allow for suction may lead to an unexpected introduction or removal of water pressure which can create an out-of-balance in the unsaturated zone of your soil.

Solution

With the release of PLAXIS 2D 2017.01 and PLAXIS 3D 2017.01, the option to Ignore suction will be disabled when selecting the Fully coupled flow-deformation analysis.

We strongly advise not to check the Ignore suction option for a fully coupled flow-deformation analysis in PLAXIS 2D and PLAXIS 3D to allow for full development of suction in the unsaturated zone.

Practical usage
A fully coupled flow-deformation analysis will always take into account suction as suction is an indissoluble part of the coupling between deformation, pore pressure and groundwater flow.
If a project uses both fully coupled flow-deformation analysis phases as well as other types of calculation phases, it is technically possible to ignore suction in the other types of calculation phases, however, we recommend not to do so. Changing from a situation with suction to a situation without suction is physically not realistic and numerically creates an unbalance that has to be solved which then may lead to unrealistic additional deformations, excess pore pressures or stresses. When using fully coupled flow-deformation analysis as part of a project, best practice is to have all calculation phases take into account suction.

However, if for some reason the calculation must change from suction to no suction or vice versa, this change should be done by inserting a so-called plastic nil phase in between the phases where the change of suction is desired. In the plastic nil phase then only suction is respectively introduced or eliminated and the resulting unbalance is solved.

Safety phase
The most common reason to mix phases with different settings for suction is when having a Safety phase without suction following some calculation including suction. Taking into account suction in a Safety phase generally gives a higher factor of safety, hence ignoring suction in a Safety phase is more conservative. Because this is such a common situation Plaxis has automatically build in the Safety analysis that any unbalance due to changing from suctions to no suction is first solved before the factor of safety is determined. This means that in case of a Safety calculation the plastic nil step is not necessary: it is already part of the Safety analysis.

Note for older versions

If you open a project made by a previous version and you have a fully coupled flow-deformation phase with Ignore suction checked, the option to change this Ignore suction stays disabled and its status will still be checked. This means you are not able to modify this value. If you need to modify this value you either need to recreate this phase, or change the calculation type to e.g. Plastic and then set it to Fully coupled flow-deformation again. Please make sure to set the correct configuration for the phase settings and the staged construction settings again.