Hi
Such mesh is not possible for Telemac has there is no free surface under the structure.
According to your needs, there is some turn around possibilities but which needs some fortran adaptation...
One of them is to mesh only the pile to represent the permeable part
Then on the upper layer, which you defined in your 3D mesh, you could create an artificial atmospheric pressure to place the free surface at the bottom level of the impermeable part...
There was already a discussion on this subject in the forum with some ideas and comments in Telemac3D parts (see
#13917)
Regards