FE mesh is not correct

Question

Dear Community, 
 
 I am running a 3D transient seepage analysis. Orginally the model consisted of an upper an lower surface and could be analyzed. I then added an intermediate surface so that I can add specific boundary conditions above and below that particular surface. However, when I tried to run the analysis, I get an error message saying that the "FE mesh is not correct." I tried to decrease the mesh element size but this did not work. 
 
 Any ideas as to what might be the cause?

Faseel Khan · Answer

It's best not to try to follow the outside of the surfaces with a region unless you include every surface node on the boundary of the mesh in the region. There's too much room for imprecision otherwise. So, use a region that is slightly inside the surface mesh boundary. 
 The area of the region may include intersections with the surface mesh elements in the region points. This is not needed and can cause issues with imprecision. The algorithms in the meshing code will properly add these intersection points. Adding them yourself can sometimes lead to problems in the meshing algorithms. So, simplifying the area of the region to have fewer points and not add the intersection points with the surface mesh can also help. There could also be thin tetrahedral elements being generated. It is possible to see where these elements are located in the mesh by turning on the "Show Only Poor Shape Quality Elements" feature on the Format tab of the Mesh Settings dialog. To see internal elements it helps to set the opacity to say 1% for the region/layers on the Material Layers dialog. This will allow viewing of the inside of the model. 
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