Mine Subsidence Hazard Overview

What is Mine Subsidence?

Land subsidence is a gradual settling or sudden sinking of the earth’s surface owing to subsurface movement of earth materials.  Subsidence is a global problem and, in the United States, more than 17,000 square miles in 45 states, an area roughly the size of New Hampshire and Vermont combined, have been directly affected by subsidence.

The most common type of land subsidence in Kentucky is karst subsidence, which is a human-induced hazard.

Mine subsidence can be described as settlement of the ground surface as a result of readjustments of mine overburden overlying voids created during or after the mining process.  These readjustments can be caused by roof falls, pillar failure, pillars sinking into weak floor, coal fires, and other factors.

Where underground mines are overlain by a considerable thickness of consolidated rock, subvertical fractures can propagate upward toward the surface, resulting in downward settling of the strata.  Alternatively, shallow mines overlain by a thinner rock overburden may collapse, causing overlying soil and unconsolidated sediment to sink into the resulting void.  Both processes result in a surface depression that worsens over time. 

Propagation of fractures and stresses from underground mine collapse leads to vertical displacement (collapse), tilting, horizontal displacement, and strain at the surface.  Subsidence does not occur above all mines.


What is the Mine Subsidence Risk in Kentucky?

Mine subsidence in Kentucky is most often associated with coal mined in underground mines, but can also be associated with other minerals such as limestone, lead, and zinc mined in the subsurface.  Coal-mine subsidence is the dominant type, because significantly more coal has been mined than limestone and vein minerals, and the thin-bedded strata above many coal beds is more susceptible to fracturing and is weaker than thick limestone sequences or limestones containing vein minerals.

The U.S. Bureau of Mines estimated that 2 million acres of land has been influenced by coal-mine subsidence, mostly in the eastern United States; in Kentucky, 37,200 acres in urban areas has been estimated to be threatened from coal mine subsidence. 

The hazard in Kentucky is limited to the eastern and western coal fields.  The mined areas cover about 800,000 acres of the land surface of Kentucky, or about 3 percent of the land surface.

Subsidence can damage manmade surface structures, modify surface drainage (and result in ponding), and modify groundwater and aquifers. The most documented hazards related to mine subsidence are surface cracks and building damage. Subsidence typically causes cracks in foundations, walls, and ceilings, and separation of chimneys, porches, and steps from a structure. In some cases, water, sewer, and gas lines have been broken. Telephone lines and power lines can also be damaged by subsidence.


 


Link to the State Hazard Mitigation PlanMine Subsidence Risk Assessment