Trenchless sewer replacement for belly/sag/dip???

Hello,

I am not a plumber, but I have a 4" cast iron sewer pipe under a slab approx 80 feet long. The commercial building was built in the 1950s, and a drain camera reveals the cast iron pipe is in pretty bad shape, although we didn't see any cracks. The main problem seems to be that there are two "bellies" (sags/dips) in the pipe in which the camera was fully submerged. One belly is maybe 6 feet long, the other is maybe 12 feet long. We have occasional backup of multiple bathroom toilets, and that's what we're trying to solve. This sewer line connects to bathrooms for three small tenants with one bathroom each, and one larger restaurant tenant with five toilets and a urinal. We saw one lateral connection and we don't know if all three of the small tenants' bathrooms come in there or if there's other lateral connections in the long belly that we can't see. There is a 45 degree bend between the two bellies.

What are my options? I'm hoping there's a trenchless solution.

Thank you very much!

stu

If trenchless won't solve it, is there a pumping solution?

I dont really know of a trenchless option for a sag, espicially cast iron. That would tell me the pipe is probably rotton and falling apart in the dirt.

I know cast iron thats somewhat stable can be sleeved. Let us know what you find out.