I live in what I consider the worst house on a bad street for foundation shifting, in Texas northwest of Houston . It's a slab home, 1977, and extensive foundation work with concrete piers was done about 15 years ago. I am renovating the master bath (1977 folks, so that's a 6x10 bathroom!) and when I pulled the commode I saw that the drain pipe had broken and separated, and about 25% of every flush was likely going under the house and not in the drain. I also see that this is at least the second time it has broken, since there is a broken flexible joint hose clamped around the broken 4" PVC pipe. I am comfortable and experienced with running new supply pipes (through the attic space), but under-the-foundation stuff is new to me. I called in a pro - the quote was $600-900 to bust out the foundation around the drain, join and bring a new pipe up through the floor that I will then cut off after the floor has been cleaned, leveled, and tiled, fill the void left by the foundation work with sand and then concrete.
This is a plumber I've worked with before, and he knows I like to do as much as I can and respects that. So I busted out the foundation (15 minutes with a demo hammer) but discovered a HUGE void under that section of the house. And I've read that after the foundation repair work, the void should not be filled because the clay underneath will still swell rain water and push that section of the house up again past the support of the piers.
After all that background - do you think the new 4" drain pipe that is coupled to the old one under the foundation, needs to be encased in new concrete to hold it together? Or do you think the void should be filled?