The consensus from the neighborhood contractors, who did the heavy lifting on this, is that because the pipe is laying on ledge-rock, it still hasn't thawed. Think gigantic ice cube. Just talked to a neighbor who can't get grass to grow above that same bit of rock. When the rock heats in summer, after the grass has shown some promise, it kills the grass roots from beneath. Ledge rock is a stone m'f*cker, y'all.
After just shy of 12K, I no longer care what caused the failure. I only care that the insurance adjuster finds it in his heart to cover some/any/a lil bit of all this. If not, I'm gonna dig up a 7 foot section of the trench and lay down in there for a while, after someone covers me up.
On the plus side, our mud-room, a ridiculous extravagance we dove into last fall, thinking we had $2 in our pockets, is just about done. The heated floor is a HUGE hit with the cats. And us. So there's that