For models with an external phreatic level (i.e. a generated waterlevel above the model surface) and a plate element and/or interface element specified on the model surface the resulting (vertical) effective stresses should be carefully checked in all cases. In a number of situations the external waterpressures are not taken into account correctly, resulting in wrong effective stresses.
Typical examples with plates:
In the image above you can see a plate without interface (left) and a plate with an interface (right). In both cases, the waterlevel is 1 m above the surface, and the plate (located at the left side of the model) has no weight. In the left image, you can see a small increase of effective stresses underneath the plate due to a doubling of the external waterpressures.In the image at the right side, you see a plate with an interface. Here the external water pressure is not taken into account at all with reduced vertical effective stresses. In some cases you can even see a part of the phreatic level missing.
Users who encounter the above issues and/or have doubts on model results are requested to contact the Plaxis support department. Plaxis will help users to find the optimum workaround for their specific situation.
Some general tips which might help to proceed in a number of situations:
This is fixed since PLAXIS 3D 2011.01