Water tank ring foundation partial removal

Do you have the original calculations?

Good chance the foundation is designed for hoop stresses, which cutting away a portion of would compromise.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

Without knowing the details, my initial thought is "There's got to be a better way".

Most commonly, tank ringwalls are poured as one pour, no joints. However, it is not uncommon for them to be poured in quadrants or otherwise segmented with construction joints, and a few engineers insist on them being built that way. As Brian notes, primary loading is hoop stress. If you can remove a section and manage to tie back in to the existing hoop steel, you should be able to match original design. It would simplify things if there were no lap splices in that vicinity.

If the existing pipe is concrete-encased, it may be easier to abandon it in place and put a new underground pipe to one side of it.

Also consider abandoning the underground pipe and adding a shell nozzle (and insulation if required).

If tank is short and squat the foundation may have been designed as "self anchored" for seismic. And ring foundation serves only to retain fill under tank bottom, support weight of sidewall and anchorage for tank in wind (when empty). Small anchor bolts may be supplied for wind anchors.
If that is case, you could cast concrete under nozzle for continuity of ring wall.