[Old School]

I cannot recall the government specifically requiring the filling voids left by underground mining but used to be part of an acceptable, if economically unfeasible, part of "mitigation" plans that regulatory agencies would accept in lieu of restrictions on extraction rates. For example, when mining is done beneath natural gas pipelines, coal companies must limit coal extraction, and obtain approval of a mitigation plan by the owner of the pipeline. (Despite the fact that pipelines can and are constructed to withstand subsidence caused by coal mining and despite the fact that in most cases, coal companies reserved the right to subside when they sold gas rights that they once owned.)

My point is that the US Supreme Court has upheld the right of states to force coal companies to limit mining extraction rates while refusing to compensate them for their economic losses. Forcing a coal company to leave 50 percent of coal in place when they might otherwise recover 70 percent of the coal, is no different than taking 29 acres of land away from an owner who has a deed for 100 acres without compensating the owner for his loss.

When the extraction rate is unreasonably limited through regulations the loss to the owner of the coal is greater than the percentage of coal left because most of the cost of developing a mine is incurred regardless of the extraction rate. In other words, mining an incremental 20 percent of coal reserves is normally much more profitable than mining the first 50 percent.

A better analogy would be for the state to destroy a property owner's house and yard, tell him that he may continue to use the rest of the property, and refuse to pay him for the loss of his house and yard.

I misread your post Hoot, I thought you were implying that filling of mine voids were already taking place. I've known of coal companies reducing the number and width of the entries, whenever mining under road right-of-ways.