[EDIT]
The issue is solved, as I figured out how to make it work. I'll link to the discussion in case anyone having this problem needs help with getting it to work:

https://steamhost.cn/steamcommunity_com/app/346110/discussions/0/2281582783354420881/
[/edit]

[This'll be a direct copy-paste from Ark discussion thread I'd made, but it's probably better to be here anyway.]

Hey.

So I'm trying to do an arch under a bridge like so:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2111948692

but alas, I am left with the foundations sticking like a sore thumb there and I'm struggling to find out how to make this happen. I've seen this having been done on screenshots of the CKFR mod, so it should therefore be possible.
I'm new when it comes to building stuff with this mod, so it's possible I'm just missing something potentially obvious.