What if you already have a pipeline on your property? Do they go under or try to squeeze it in just under the topsoil?
How come they are burying it? The Alaskan Pipeline looks like it is all above ground. Isn't that cheaper? And when crossing land that is not suitable for agriculture isn't that the best way?
I am thinking they normally bury the line several feet down at a minimum. It is deep enough a farmer can plow and such without worrying about the pipe. They normally set a base material around the pipe and it is heavily wrapped with insulation. If the galvanic protection is properly maintained, I think it is better to put it underground than below.
My employer bought an older pipeline from someone years ago and had to replace a section of it after a farmer practically pulled up a section with his plow. I don't know if the pipe got shallower due to settling or if it was erosion. There were a lot of other problems with that line that cost money.
Remember that there are near millions of miles of pipelines crossing back and forth across the US. If you don't have pipeline on your land, there is likely one not far away. There are also a number of regulations governing installation and maintenance.