Problem description

When having multiple surfaces on the same location in PLAXIS 3D’s structure mode, the program will merge these surfaces when going to one of these modes: mesh mode, water mode and staged construction mode. When conflicting items are defined, PLAXIS 3D takes the settings from the first defined surface: this can include different surface load values on these overlapping surfaces, or when both surfaces have been defined as a plate and the assigned plate material is not the same.
Usually this is not a big issue since it can be changed easily in the phases: e.g. change a material assignment to a plate, or change the load values in the appropriate phase.

However, one item that cannot be changed in the staged construction mode, is the definition of the local axis direction. For any local axis dependent parameter, this can become a problem then. Some examples when this can go wrong:

• interface definition. A positive interface is defined on the side of the surface in which the positive direction of local axis 3 (perpendicular to the surface) points. If there is a conflict between two surfaces, it could mean that the positive interface and/or negative interface can flip sides. The direction of the local axis 3 is by default defined by the drawing direction of the surface (clockwise or counterclockwise). So having drawn multiple surfaces and/or having decomposed a volume into surfaces could already lead to a conflicting direction of the local axis 3.
• local axes dependent parameters for anisotropic material datasets for plates and geogrids:  E1 and E2.
• surface contractions. When having an incremental distribution set, this is dependent on the direction of the local axis 1. If the direction of local axis 1 should flip, this means the incremental contraction value will follow this too.

Users are advised to check the direction of the local axis after the intersection and in the staged construction when defining the phases.

Solution

This has been fixed since PLAXIS 3D AE.00: when surfaces overlap with a conflicting local axis definition, the program will give a warning to the user when going from one of the blue modes (Soil/Structures) to one of the green modes (Mesh/Flow conditions/Staged construction). Now it is easy to inspect these differences and judge if the local axis definition is still correct.

To avoid this in PLAXIS 3D 2013 or earlier:

• Try not to use multiple surfaces on the same location, but add several features (surface load, plates, etc) to the same surface. This can easily be done by just right clicking the surface and add the appropriate feature from the pop-up menu. To merge multiple surfaces, please use the intersect tool by selecting these surfaces and then right click the selection and choose to intersect.
• Or, when there are overlapping surfaces, merge these surfaces prior to intersection using the intersect tool by selecting both surfaces and then right click the selection and choose to intersect. In that case, the items will be intersected and when possible merged so that there are no overlapping surfaces anymore.