We have seen some cases in which an earthquake analysis with Free field lateral boundaries does not perform as expected in PLAXIS 3D 2018.
In a comparison between PLAXIS 2D and PLAXIS 3D for a few earthquake analyses using Free field elements at the lateral boundaries, we have seen that the results are not the same. When taking into account some relative parameters such as the generated mesh, a PLAXIS 2D plane strain model and a 1 m wide 3D model should give comparable results. However, the results from the investigated PLAXIS 3D 2018 models deviate from the 2D plane strain results.
It is likely that the absence of Free field corner elements to connect the free field surfaces at the lateral corners of the model contributes to this, see the images below.
Figure 1. 3D 2018 model boundaries with free field elements
Figure 2. 3D model boundaries with Free field corner elements(available now since PLAXIS 3D CONNECT Edition V20)
When comparing the PLAXIS 2D and 3D models using viscous boundaries, we see comparable results.
In PLAXIS 3D CONNECT Edition V20 (available since August 2019) we added the special corner elements when having free field elements at all four lateral sides.
For older versions, we recommend using Viscous boundaries over the Free Field boundaries to perform a dynamics analysis in PLAXIS 3D. This means in some cases you need to extend the model boundaries to ensure proper boundary behaviour.