When the following conditions are met the external water pressures are not applied on a specific surface:
This will lead to wrong calculation results.
To investigate if this is relevant in your model, open the preview:
Figure 1. Example of missing external water pressures, as shown on the phase’s preview
When the interface is defined above the surface, or when the interface is active, the external water pressures are correctly determined.
The easiest solution is to activate the interface. In order to still have the correct load acting on this surface with inactive interfaces, please add a distributed surface load to add the external waterloads manually. Make sure to only activate this load when the interfaces are inactive. Then in the preview, check if the external waterloads are really missing to avoid adding a double load on this surface.
This issue has been fixed in PLAXIS 3D 2013.01