Problem description

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:

1. for the case with a plate without an interface during plastic calculations: the external waterpressures are taken into account twice on the plate resulting in too high effective stresses beneath the plate (see the image below, left);
2. for the case with a plate with an interface during the K0 procedure: the external waterpressures are not taken into account resulting in too low effective stresses beneath the plate (see the image below, right).

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.

Solution

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:

• if possible lower the external phreatic level to the surface of the model, or;
• detect the situation where the error occurs and then apply a positive or negative surface load on the plate to compensate for the wrong external waterpressures. Where necessary also switch from the K0 procedure to a gravity loading to be able to take this surface load correctly into account;

This is fixed since PLAXIS 3D 2011.01