Contact a local civil engineer. There can be all sorts of issues with reclaimed surface mining lands. From gas emissions to toxic chemicals, to no way to install septic, to under compacting, to even over compacting... Obviously most severe would be a complete house structural failure or sliding.

"...up to 100 feet deep" - again contact your local civil engineer. This subject is no joke.

We built a manufacturing plant on fill from strip mining. The soils engineer provided assurances that it could be done. The foundation sank up to a foot in one area of the plant.

@BT Yes, thanks. I am contacting a local civil/geological engineering firm. I am right now trying to understand the process/technology better. There are other homes that have been built in that area. I am trying to do due diligence as there is one home that reportedly has foundation issues - (close to a "highwall?) and likely did not do much soil testing when it was built over a decade ago.

I think you are likely asking for trouble. We built on fill and we were very careful on the types of soil we used for fill and compacted it in 8" lifts, with nuclear testing at the required intervals. Building on soils moved randomly without any foresight for what would be done after, no compaction at the lower levels, etc - feels like it's asking for instability that you can't control or even mitigate with any compaction activity now.

Keep looking IMO too many variables and no way to get guarantees about anything.