A method of mining. A continuous miner removes a long column of coal, letting the top of the mine fall behind it. Subsidence is more or less immediate, as opposed to "room and pillar" mining where the roof is supported and subsidence is delayed. This should not be hyphenated as a compound modifier. Hyphenating it makes it a long wall. While it may seem like a long wall, room and pillar mining would also produce long walls and are mined with similar equipment. "Long-wall" mining is not as specific as "longwall mining" as a technique. It's time to drop the hyphens in longwall, coalbed, daycare and other common words.
The employee is experienced in the longwall method of coal mining. We will be mining this site longwall. The drop in the terrain is due to longwall mining.