Problem description

If a volume is set to dry in the stage definition in PLAXIS 3D2011.00 for a groundwater flow analysis, this setting is ignored for the groundwater flow calculations: the volume is not taken to be dry for the groundwater flow analysis. This will result in a groundwater flow through this set-to-dry volume based on the groundwater flow boundary conditions, and it will result in the generation of pore pressures in this set-to-dry volumes.
In the deformation analysis, the pore pressures will be used according to the results of the groundwater flow calculations, including the pore pressures that are a result of the groundwater flow inside this set-to-dry volume. 

The above will have effects on:

• the groundwater flow field: it can pass trough the dry volume;
• the stress distribution in these dry volumes: it may contain pore pressures, and so the total and effective stresses may not be equal.

Note that Output will not show pore pressures in these volume defined to be dry.

Solution

In order to exclude the volume from the groundwater flow calculation to get the correct flow field and the correct, dry, stresses in this dry volume, the following trick could be used:

1. create a copy of the material that is assigned to the volume
2. in this copy of the material, select the drainage type to be non-porous and select the correct unsaturated soil weight
3. assign this non-porous copy of the material to the dry volumes in the appropriate phases

With the above trick users can obtain the desired flow field and the desired stresses in the set-to-dry volume. Please check your results carefully.

This is fixed since PLAXIS 3D 2011.01