Problem description

In PLAXIS 3D, default boundary conditions are automatically applied: see PLAXIS 3D Reference Manual in the section about Standard Boundary Conditions. For e.g. soil volumes this means:

• Vertical model boundaries with their normal in x-direction (i.e. parallel to the yz-plane) are fixed in x-direction (u_x = 0) and free in y- and z-direction.
• Vertical model boundaries with their normal in y-direction (i.e. parallel to the xz-plane) are fixed in y-direction (u_y = 0) and free in x- and z-direction.
• Vertical model boundaries with their normal neither in x- nor in y-direction are fixed in x- and y-direction (u_x = u_y = 0) and free in z-direction.
• The model bottom boundary is fixed in all directions (u_x = u_y = u_z = 0).
• The ‘top surface’ is free in all directions.

In some cases in the 3D 2011 and 3D 2012 versions, however these default fixities may give unexpected behaviour. Especially for  models with items “vertically sticking out” of the soil surface this may be an issue, see the following examples:

Model with (half a) volume pile sticking out of the soil surface

Model with stepwise soil surface

Solution

The algorithm for the default fixitiesis now improved since PLAXIS 3D 2013.00 to prevent the unexpected behaviour with regard to items “vertically sticking out” of the soil surface. Users are still adviced to check the boundary conditions including the automatic applied fixities, see also below.

If the default fixities are not according to your wishes you may deactivate and/or overrule the default fixities, also see the related articles below.

Alternatively you may create a “dummy” volume on top of the model such that there are no items vertically sticking out of the soil surface (i.e. the model’s top soil surface is fully horizontal now). The dummy volume should be disabled during the calculations.

As part of the normal workflow however the user should always check if the created default fixities are according to his/her expectations. You can check the fixities by going to Output and activate the Geometry > Fixities (or alternatively use CTRL+F).