While setting up a dynamic model you may choose from the following dynamic boundary conditions:
When choosing “none” only the standard fixities are applied to this boundary so no special dynamic boundary conditions are used. To create a so called “fixed base” this option should be used along the base (i.e. y_min for PLAXIS 2D and z_min for PLAXIS 3D) of the model in combination with a line prescribed displacement. A fixed base is a fully reflective boundary.
A viscous boundary consists of viscous dampers applied in x, and y, and z (for 3D) direction along the boundary. Viscous boundaries absorb the incoming wave energy. The viscous boundary conditions are in general used for problems where the dynamic source is inside the mesh.
The compliant base boundary condition is only available for the baseof the model: y_min for PLAXIS 2D and z_min for PLAXIS 3D. The compliant base is made up of a combination of a line prescribed displacement and a viscous boundary. Internally the prescribed displacement history is transferred into a load history. The combination of a load history and a viscous boundary allows for input of an earthquake motion while still absorbing incoming waves. This option is in general preferred for earthquake analysis.
The free field boundary conditions are only available for the lateral boundaries, i.e. x_min / x_max, and y_min / y_max (3D). The free field boundaries are made up of a combination of a load history and a viscous boundary. The load history is the load coming from the free field motion at this level. The combination of a load history and a viscous boundary allows for an input of an earthquake motion while still absorbing incoming waves. This option is in general preferred for earthquake analysis.
The tied degree of freedom are only available for the lateral boundaries (i.e. x_min and x_max). This option will tie the nodes of the left and right model boundaries such that they will undergo the exact same displacements. This option allows for example for modelling a perfect one dimensional soil column to perform a site response analysis. Also see this related article for further information on the practical use of this feature.
Tied degrees of freedom is not available for PLAXIS 3D.
Note that in order to use the options compliant base and free field the manual creation of interface elements along the full model boundary in Structures mode is required. By creating an interface we create a so called node pair which allows for the necessary decoupling to be able to apply the input motion (load history) while also being able to absorb incoming waves. Also see this related article on interfaces for more information on node pairs.