Chris wrote:
....

I'm pretty tempted to just drop a piece of 4" ABS into the hole (ie a
butt joint to the clay) and back fill with concrete patch.

Ideal would be able to slip the ABS into the clay as danville suggests
rather than having a butt joint that really isn't sealed.

If forced to the latter, I'd put a heavy bead of silicone caulk to make
a seal between the two ends and try to put a wrap around the clay
section and the joint before the backfilling so that down the road if a
repair were needed the clay wouldn't be directly in contact w/ the concrete.

--