Problem description

In some cases when using a PLAXIS 3D 2013 model or using imported geometry in PLAXIS 3D AE.00, it can happen that the default boundary conditions are not applied correctly, which results in missing fixities on the boundaries.
This of course will not give the expected results of an analysis.

You can see this happening when inspecting a phase’s preview or its results:

• when parts of the model sides do not have fixities. You can visualize the fixities by activating the fixities in the menu via Geometry > Fixities, or alternatively use CTRL+F
• or, when the phreatic level is inside the model, you can see external water pressures acting on the sides. When the default boundaries are applied correctly, you should not see these external water pressures.

Figure 1. Example of missing fixities (left) and unexpected waterloads (right)

Solution

This issue has now been fixed in PLAXIS 3D AE.01.

When the above happens in a PLAXIS 3DAE.00 model, it can be solved by creating a surface prescribed displacement on the entire troubling side, and set the boundary conditions manually, for example:

• if this new surface prescribed displacement is on the Xmin or Xmax boundary: set X to fixed and Y and Z to free for the surface prescribed displacement
• if this new surface prescribed displacement is on the Ymin or Ymax boundary:  set Y to fixed and X and Z to free

Do not forget to activate this manual boundary condition (the surface prescribed displacement) in your phases.