> Seems the bottom side face still lives !
> Is that normal?

Yes because in this case it wasn't selected. If a face is not selected, it will survive and have a wall thickness on it, it doesn't matter if it is on the side or the top or the bottom, it only depends on the selection.

Only the selected faces will become openings.

If you want to have a hole going all the way from the top to the bottom, then you could get that by selecting both the top and the bottom faces before doing the shell.

But you don't have to only select top or bottom pieces - for example here I selected the top face and two sides:


The other thing is that it is possible to just select the whole solid object instead of any faces, and this will create a hollow cavity within the solid.

- Michael