Types of Mine Subsidence

Typically in Illinois, mine subsidence will appear in one of two forms: pit or sag. Pit subsidence forms a bell-shaped hole, usually 6-8 feet deep and from 2-40 feet across. The mine will most likely be shallow, less than 100 feet deep, and the bedrock over the mine less than 50 feet thick and consisting of weak rock materials such as shale. Ground movement will be swift and sudden. Sag subsidence, the most common type of mine subsidence, appears as a gentle depression in the ground and can spread over an area as large as several acres. The first signs may appear suddenly within a few hours or days, with gradual movement continuing anywhere from a couple of years to more than a decade. Frequently, damage is subtle, and perhaps dismissed as normal wear and tear until multiple signs appear. Sag subsidence can develop over mines of any depth and are usually caused when coal pillars, left intact by miners to support the mine roof, disintegrate or collapse.