Have you defined boundary conditions for eighth DOF at this surface ? If you leave it without any BCs it should be impermeable by default since this is natural BC in pore fluid flow analyses. Thus the fluid shouldn’t be able to flow neither into nor out of this surface in such case.
Then, if the surface is not involved in contact interaction, it should be impermeable.
Check the following examples from Abaqus documentation for reference:
- The Terzaghi consolidation problem
- Calculation of phreatic surface in an earth dam
There should be some additional problems in the documentation as well, perhaps in the benchmarks/verification guides but I am not sure (I have not touched Abaqus in years so I don't remember well).
Pore pressure element has additional DOFs - read/understand the element formulation discussed in one of the guides. As implicitly suggested above, at the contact interface, there will be continuity in the fluid flow - you should see that in your results.
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