Our home is on a slab and the cast iron pipes had the bottoms rotted out. The found the problem after lifting the toilet and sending down a camera on a snake end. The monitor showed that the bottoms of the pipes had actually eroded away and this was the reason our water, waste, garbage and all ran out sooo slow.
This was covered by our insurance, but the mess was something else. Not one time did the crew of 15 workers ever come in the house, they did all the work via tunnels under the slab and re conecting to the base pertruding from the slab and using pvc and actually running all new lines. then connecting up to the main and sewer.
works great, they left the old pipe in place and just ran all new lines.
Then packed the holes back with sand and a chemical that I forgot the name, that expands when wet and acts like support.