How does the pipe have enough play to move 8'?
They don't.
Typically, a line will get put to the proper depth, then the land developer will decide to change the grade. I have worked on several projects where a gas or water main started out with 4' of cover, and by the time all was said and done it had to be dug up and reinstalled to get within specs.
As in your case olds, it can make for some exciting times!
I worked a bridge replacement project with an 6" intermediate pressure line suspended on the old bridge. I had to dig along the line for 400', under it twice, and then they had a guy come in and live weld a tap on it so we could put in a bypass. They had a gas company rep there the entire time I dug around the pipe.
I went for a long walk while that crazy SOB welded on the live line