I'm just kind of spitballing this but: Depending on when the pipe is laid (and sometimes you don't have a choice) if the clay is dry and then gets wet and expa

I'm just kind of spitballing this but: Depending on when the pipe is laid (and sometimes you don't have a choice) if the clay is dry and then gets wet and expands it could crack the pipe. A cracked interior will always spread. Clay, when dry, is rigid and I would think if the earth moves it's probably going to shove the pipe rather than move around it. That'd probably cause the pipe to either fracture or dislodge. Even if none of the above happens that means that if they pipe does develop a defect for something more common (eg: a tree root intruding) then the running water will be split. It'd cause the clay on the bottom part of the pipe to begin to wash away either back into the pipe or it'll find porous material to seep through eventually eroding the structural support of the pipe. I've seen situations like this that basically create mini cave systems and the surface of the earth will appear in-tact until something heavy drives over it. That said, I never worked in Atlanta so I'm not familiar with what defects or the infrastructure look like there.