https://communities.bentley.com/products/geotech-analysis/f/forum/219774/free-field-boundary-condition-with-undrained-a-condition-for-the-soil-materialsDear All, I am performing some local seismic response analysis with Plaxis 3D. I would like to know if the boundary conditions of type &quot;Free field&quot; can be implemented in case the materials are in Undrained condition ( A or B). In fact, inspectingen-USTelligent Community 12https://communities.bentley.com/thread/674862?ContentTypeID=1Fri, 01 Oct 2021 08:50:32 GMT6dad98f5-dbc9-4c4d-a9ba-e9da8dc6aa8e:a0e20175-6878-4635-9f98-4870b92eaddfDennis Waterman<p>Dear Fausto,</p> <p>In principle it&#39;s fine to use Free field boundaries in an Undrained analysis. It could indeed be that close to the boundary there are locally larger deformations that occur if for a moment the effective stresses were very low there. But since the boundaries should be far away from your project area it is not a problem and it does not effect the results for your project.</p> <p>If you would get more than just local deformations emerging from the free field boundaries then a solution could be to model next to the free field boundaries a small zone of Drained material (1-2 elements thick is enough). However, please note that that would create a transition from Undrained to Drained material in your soil that may reflect waves due to a stiffness difference. Hence, it would make your free field boundaries less effective. Therefore we only do this is there is a major deformation effect at the free field boundaries. In practice, such major effect very occasionally occurs when large scale liquefaction occurs directly next to the free field boundary.</p> <p>With kind regards,</p> <p>Dennis Waterman</p><div style="clear:both;"></div>