I suggest you download Australian Standard As 2566.1 & .2. This gives a very comprehensive design guide and installation procedure for buried flexible pipelines.
If it PE you may well direction drill the pipeline.
The methodology is the same, you need to find the appropriate material qualities for the pipe type you are using. The Plastic Pipe Institute (plasticpipe.org) USED to have their engineering manual on-line, now it looks as if it will cost you $35.
A coal pile usually has a large caterpillar tractor on top pushing the material around. The expected loading from the tractor will be much different than what you would expect from just a material pile.
I'm assuming that the coal pile is more than just a few feet high. unless your pipe is relatively close to the surface of the coal, the live load from the dozer will probably not affect your pipe. it will probably be designed for dead load