Knowledge Base

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.