One theory offered was that the "acid" from the waste lays in the bottom of the cast iron pipe and erodes the pipe and causes it to crack. Maybe that's true, but my hunch is that with the house vacant, the liquid in the pipe will dry up and no longer be acidic rather than just lay there and eat through the bottom of the pipe. I think that maybe if the bottom is cracked, it is due to having been weakened over the years by the prior constant flow of acidic waste running along the bottom of the pipe.
Another theory is that without liquid waste flowing in the pipes while the property is vacant, the already-corroded pipes tend to dry out, and then they crack at weak points due to the drying process and changes in temperature in the vacant property. That seems maybe a little more feasible to me, but I have no way of knowing if that theory is correct.
I have noticed that in two out of two properties that I bought that were either vacant before I bought them, or vacant afterward, the vertical cast iron pipes in both had longitudinal cracks in them and had to be replaced. One horizontal cast iron sewer pipe also had longitudinal cracks, but those cracks were on the top of the pipe, not along the bottom where acid allegedly would have eaten through.
So, I am leaning toward the theory of old, corroded, and now dried out, cast iron pipes in a vacant property tend to crack due to being dried out and the temperature changes while vacant.
Does anyone know if any of these theories are correct, or what the true cause of cracked cast iron sewer pipes is in vacant properties?