Free field boundary condition with Undrained A condition for the soil materials.

Question

Dear All, 
 I am performing some local seismic response analysis with Plaxis 3D. 
 I would like to know if the boundary conditions of type "Free field" can be implemented in case the materials are in Undrained condition ( A or B). In fact, inspecting the results I notice strange concentrations of deformation at the edges of the soil domain. If it is not possible to use the free field at boundary in undrained condition how can I model them in Plaxis 3D where the Tied Degree of Freedom condition does not exist? 
 Thank you

Dennis Waterman · Answer

Dear Fausto, 
 In principle it'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. 
 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. 
 With kind regards, 
 Dennis Waterman