The earth’s surface can subside because of underground mining when rock is removed at depth. Although subsidence can occur due to hard rock mining, this article only considers the effects of coal mining.
When coal is extracted underground, gravity and the weight of the overlying rock may cause the layers of rock to shift and sink downward into the void left by the removal of the coal. Ultimately, this process can affect the surface, causing the ground to sag and crack and holes to form. Merely an inch of differential subsidence beneath a residential structure can cause several thousand dollars worth of damage.
Subsidence can happen suddenly and without warning. Detailed investigations of an undermined area are needed before development occurs to resolve the magnitude of the subsidence hazard and to determine if safe construction is possible. While investigations after development can determine the extent of undermining and potential subsidence, often, existing buildings cannot be protected against subsidence hazards. The cost of remedial measures is often extremely high.
Inactive Coal Mines and Population Growth
Coal mining in Colorado started in the 1860s and is a continuing activity in many areas of the state. As of August 1977, Federal and State laws require that potential surface subsidence be taken into account in mining plans. Prior to that time, the effect of mining on the surface was not fully considered.
Many old mines are located near present urban areas. With Colorado’s population growth in the last 25 years, not only have many homes been built over abandoned mines, but many homeowners are unaware of previous mining or the extent of mining in an area. A name such as Coal Mine Avenue may seem fanciful rather than significant. Subsidence over abandoned coal mines is a potential hazard for thousands of homes along the Front Range Urban Corridor, and these numbers will continue to grow as more people move into the state. The map above indicates general locations of inactive coal mines throughout the state.
Room and pillar mining was the mining technique used almost exclusively in early Colorado mining and is still in use today. Approximately 50 to 80 percent of the coal was removed by this method. “Rooms” of coal were mined, and pillars of coal were left in place to support the roof of the mine.
Longwall mining is a newer mining technique where subsidence is expected and planned. In this high coal-extraction method, a panel of coal is removed in the form of a large continuous room, leaving the roof unsupported except along the face of coal being mined by machinery. As the working face advances, the roof sags into the mined void, as shown in cross section below. This technique is more likely than room and pillar mining to produce an immediate effect at the surface.
When and How Much Subsidence Can Occur
Where longwall mining is active and subsidence is a well-documented and predictable action, surface response to ongoing mining can be accurately estimated. However, in the case of room and pillar mines, especially where they are inaccessible and record-keeping may be inaccurate, predictions of when subsidence will happen are not possible. Several factors contribute to the timing of caving at the mine level and subsidence appearing at the surface. Pillars and timber left in place can hold the roof of the mine up for long periods of time.
How much subsidence will occur and the features that will appear at the surface depend not only on the type of mining but on geology and several physical features of the voids left by mining.
Some general rules of thumb are:
— the larger the mine opening height and width, the larger the subsidence feature at the surface;
— the shallower the mine below ground, the more noticeable the surface subsidence evidence; however, in Colorado pits have been found over mines as deep as 350 feet;
— the strength of the rock above the coal seam influences whether subsidence will reach the surface and the kind of features that can appear.
Subsidence Hazard Area Identification
A residence or other structure may be subject to subsidence if it is located over or close to an undermined area.
Therefore, the first step in determining the subsidence potential at a specific location is to discover if the area is undermined. Several published sources of information are available for the locations of inactive mines. Maps showing the extent of inactive coal mines and actual maps of coal mines are available for viewing from the Colorado Geological Survey.
Some recent housing developments in the Front Range Urban Corridor, in response to Senate Bill 35 (1972), have had subsidence hazard investigations completed prior to development. Individual site-specific investigations involve examining the available data and drilling exploratory holes for information on the present condition of the mine. These investigations are completed to determine how the subsidence hazard can affect proposed development, if safe building areas exist, and what areas should be avoided.
These studies, when available, are often on file with the builder, city, or county. They also may be available for inspection from the files of the Colorado Geological Survey. To determine if one of these studies is available for a specific subdivision, the subdivision name (as platted) and location should be known.
Identifying Subsidence Damage
The different physical processes involved with subsidence will produce different types of damage. The vertical drop of pit subsidence can be damaging to buildings. However, a driveway or major structural member of a building can often bridge a hole for a short time.
Damage to structures or ground changes may suddenly appear or may be more gradual. Other ground hazards such as swelling soils, heaving bedrock, collapsible soils, or natural sinkholes can also cause cracking, shearing, other distortion-type damage to structures, and ground changes such as spontaneous openings, sags, and fissures. When such damage occurs, the first course of action is to examine the references that were mentioned above.
For homes built before 1989, homeowners within an undermined area are eligible to participate in the Mine Subsidence Protection Program (MSPP), a federal program operated by the Colorado Division of Reclamation, Mining, and Safety (within the Colorado Department of Natural Resources). The annual premium is $35 per year. Call 1-800-44-MINES for more information.
Homes built after 1989 are not covered by the MSSP, which makes decisions about where to build especially important. Private subsidence insurance is available in Colorado.
What To Do in a Subsidence Emergency
If the ground sinks suddenly on or near your property and you have reason to believe the area is undermined, you should do the following:
1) Determine if your building is served by natural gas. If so, contact the Public Service Company of Colorado or similar utility. If gas lines are cracked or broken, there is a potential for fire or explosion.
2) Contact your city or county safety department/fire department and the Office of Surface Mining Reclamation and Enforcement (OSMRE), Federal Reclamation Projects Branch in Denver. OSMRE controls federal abandoned mine reclamation emergency funds. If the office representative determines the incident qualifies as a subsidence emergency, they will monitor the situation and repair the subsidence feature. They will not repair damage to a structure.
3) Contact your city water and sewer department so that these lines can be checked for damage.
4) Large windows should be taped to help prevent flying glass, should distorted windows shatter.
5) The homeowner should start a written log. This log will help investigators to determine if the damage is caused by subsidence.