Mine Subsidence

The state of Colorado has a rich legacy of mining. The search for gold and silver in the mid-1800s and the development of mines throughout the Colorado Mineral Belt to extract that wealth drove the exploration and settlement of Colorado, and was an important factor in Colorado becoming a US territory and ultimately achieving statehood in 1876. Extensive coal resources were also discovered and exploited early in the state history to help drive the continued growth and expansion.

While hard-rock mining has diminished in Colorado, its citizens and government agencies continue to wrestle with the physical hazards and environmental impacts caused by mine drainage. However, Colorado has huge coal resources and coal mining remains an important part of the Colorado economy today, click here.

Hard Rock Mine Subsidence

Underground hard-rock mining generally follow ore-bearing veins and mineralized ore bodies. The entrances to underground workings are by adits and shafts. The mine workings tend to be smaller and follow the ore by narrow tunnels, drifts, and stopes. Roof failures of these workings can cause subsidence at the ground surface or leave dangerous surface openings. The Colorado Division of Reclamation Mining and Safety has an Abandoned Mine program that is tasked to identify and safely close, or seal dangerous mine openings in Colorado.

On right is a 2006 subsidence hole that opened on I-70 west of the mining town of Idaho Springs. This photo shows the hole has been filled with flow fill. Note the yellow dashed line that shows the alignment of the tunnel towards a rock opening, partially obscured by the guardrail, in the highway cut slope across the westbound lanes.

Coal Mine Subsidence

Colorado coal has been mined since 1864. Mines that were closed prior to 1977 are considered to be historic coal mines by the Office of Surface Mining (OSM) and pose particular risks to surface development. Mine subsidence is one of these risks.

Mine subsidence is the movement of the earth’s surface caused by the collapse of underground mine voids or entries. Over time, gravity and the weight of the rock overlying the mine cause the layers of rock to shift and fall downward into the mine area. As one layer falls down, the void moves upward toward the ground surface where it can cause holes, cracks, tilting, and sags. Even a few inches of settlement can cause significant damage to structures, roads, and utilities. Mine subsidence can occur abruptly or gradually over many years.

Many factors contribute to the risk of coal mine subsidence. Some of the more common factors include the depth of the mine workings, the geometry of the mine, how much coal was extracted, the overlying geology, and groundwater fluctuations.

Much of the Front Range, including El Paso, western Jefferson and southern Boulder and Weld Counties have problems with subsidence as a result of abandoned mine tunnels that snake under developed areas.

The Graden Coal Mine shaft collapsed in 2007 south of Dacono in Weld County. Note the land development activities in the background. Photo by Celia Greenman.

A short definition

Ground subsidence is the sinking of the land over man-made or natural underground voids. In Colorado, the most common type of subsidence is the settling of the ground over abandoned mine workings.

Characteristics

Subsidence may occur gradually over many years, or abruptly — in an instant. It may occur uniformly over a wide area as local depressions or pits separated by areas which have not visibly subsided. In Colorado, it is most common in the sedimentary rocks over abandoned coal and clay mines. The crystalline rocks in which most metals are mined have greater strength and are less likely to settle or collapse. Subsidence in Colorado also occurs where underground water has dissolved subsurface materials (see karst subsidence). Another form of regional subsidence that can create large ground fissures is the withdrawal of ground water by wells. Although serious in other western states, this type of subsidence is rare in Colorado. Subsidence caused by collapsing soils is discussed under the Collapsing Soils heading.

Aggravating Circumstances

Weight, including surface developments such as roads, reservoirs, and buildings; man-made vibrations from activities such as blasting, heavy truck or train traffic may accelerate the natural processes of subsidence. Fluctuations in the level of underground waters caused by pumping or by injecting fluids into the earth can initiate sinking to fill the empty space previously occupied by water or soluble minerals.

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Legal definition

H.B. 1041, 106-7-103(10): “Ground subsidence” means a process characterized by downward displacement of surface material caused by natural phenomena such as removal of underground fluids, natural consolidation, or dissolution of underground minerals, or by man-made phenomena such as underground mining.

Descriptive definition

There are several distinct types of natural processes and man’s activities that may produce ground subsidence. The descriptive definition of the other types of ground subsidence, and how they are affected or produced by geologic conditions are explained on the Ground Subsidence page.

Removal of support by underground mining is a common cause of ground subsidence in many areas of Colorado. Extensive removal of minerals, mineral fuels, rock aggregate, and other materials results in large underground void spaces. Subsequent natural processes including fracturing, chemical changes, caving, flowage, and other related adjustments often produce surface subsidence, fissures, and tilting of the land surface above and/or adjacent to the surface projection of underground workings. Man-induced changes in the hydrology of the underground workings and/or overlying rock and soil materials can affect subsidence. In addition to actual undermined areas, special hazards are posed by certain appurtenant structures such as air shafts and various other mine workings. Additional problems in identifying and delineating areas of potential subsidence include the presence of faults and other geologic complications, and the fact that “final mine maps” may not show the actual extent of mining. Also, discrepancies in survey ties between the mine maps and surface reference points may be sizable. Many undermined areas have incomplete or nonexistent records. Potential subsidence hazards from underground mine working and shafts exist in many parts of Colorado. These include areas of past and present coal mining, “hard rock” mining areas, and undoubtedly others.

Severity of the problem

Geologic conditions conducive to mine subsidence exist in extensive areas of Colorado, and serious instances of damage due to ground subsidence occur frequently. Click the case histories tab on this website. With increased demand for mineral fuels, other mining activities and pressures for intensive urban and recreational development throughout much of the state, these problems will intensify unless recognized and dealt with wisely. These guidelines are intended to help local governments to identify problem areas and prevent needless economic losses in the future development of the state.

Criteria for Recognition

The criteria for recognition of actual or potential mine subsidence conditions include a careful evaluation of all pertinent historic mining activity, surface land geomorphology, and geologic and hydrologic factors of an area. Most historic mine areas have been mapped and those maps are available from the CGS Mine Subsidence Information Center. Click on the tabs on this website. Historic underground mine map boundaries can be inaccurate. Historic evidence includes common knowledge of long term area residents concerning characteristics of land under present and past usages. This kind of information is important but must be carefully evaluated for accuracy and objectivity. Additional sources of information include official records of state, local, and federal agencies (especially with respect to past mining activity). Unofficial sources of information include unofficial mine maps, newspaper accounts, and published books of a historical nature. Accordingly, land use in areas not only in, but near abandoned mines, should be considered having the potential for subsidence, and should be carefully investigated in subsidence investigations by trained engineering geologists and geotechnical engineers.

Consequences of Improper Utilization

The consequences of improper utilization of land subject to ground subsidence will generally consist of excessive economic losses. This includes high repair and maintenance costs for buildings, irrigation works, highways, utilities and other structures. This results in direct economic losses to citizens, and indirect losses through increased taxes and decreased property values.

Damages

Subsidence can result in serious structural damage to buildings, roads, irrigation ditches, underground utilities and pipelines. It can disrupt and alter the flow of surface or underground water. Surface depressions created by subsidence may be filled in, only to sink further because the underground void has not been completely closed. Areas may appear to be free of subsidence for many years and then undergo renewed gradual or even drastic subsidence.

Generalized Map of Areas in Colorado Underlain by Coal Mines

Statewide Historic Underground Coal Mines (ArcGIS.com Map Viewer)

Statewide Historic Underground Coal Mine Extents and Reported Coal Mine-Related Subsidence Events Map (zip file containing Google Earth kmz file)

Extent of Coal Mining in Front Range Counties

Adams County
Arapahoe County
Boulder County
Douglas County
El Paso County
Elbert County
Fremont County
Huerfano County
Jefferson County
Larimer County
Las Animas County
Weld County

Select Front Range Areas Showing Undermined Regions

Boulder-Weld County Area
Colorado Springs Central
Colorado Springs-Rockrimmon
Erie Area
Golden-Lakewood-Morrison Area
Lafayette Area
Superior Area
Tri-Towns Area

Case Histories

Case History 1: Jay Rd. Erie, Colorado, 2009

A large subsidence hole was reported in January 2009 at a residence near the corner of a horse barn. The property owners reported the hole opened up overnight and a fence and gate had been destroyed by the event. The hole measured roughly 25 feet by 25 feet by 15 feet deep and was filled with water. Because of the nature of the opening and the proximity to livestock and human activities, the event was considered a subsidence emergency and was backfilled by the Abandoned Mine Lands program.

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Case History 2: Erie, Colorado, 2008

In December of 2008, a report of a large subsidence hole in a field west of Erie was reported. The hole was about 50 feet in diameter and 35 feet deep before being filled with water. The field where the hole appeared was under consideration for annexation by the town for future residential development. A geophysical investigation conducted 3 months prior did not show any evidence of voids in the area. The hole was located outside of the mined area shown on the mine map indicating that the mine map was inaccurate. During the mitigation process, a secondary subsidence pit of smaller dimensions was found directly west of the original hole. Both holes were backfilled by the Abandoned Mine Lands program.

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Case History 3: Marshall, Colorado, 2007

In May 2007, a consultant reported mine-related subsidence features in a property near Marshall being considered for residential development. Upon investigation, a number of subsidence holes and mine-related features were found on the property. An abandoned building showed significant damage possibly related to the old mine. No records of the mine were found, but several old foundations around the property indicate a mine entry exists west of the site. Due to the unknown mine, lack of records, and the presence of subsidence features, the property owner elected not to pursue development.

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Case History 4: Country Club Circle, Colorado Springs, Colorado

The Country Club Circle area of Colorado Springs has a long history of mine subsidence from multiple mines that were active during the early 1900’s. The mining in the area is very shallow, and there is little bedrock between the mine roof and the ground surface. The area is developed with residential neighborhoods, and residents in the area have been living with subsidence for decades. Starting in 2005, the Abandoned Mine Lands program has been working to try to grout the homes in the area to prevent further damage. The money for this effort was made available by the Office of Surface Mining based on the history of ongoing damage and the age of the residential structures involved. Many of the residents are enrolled in the Mine Subsidence Protection Program. Structures built after 1989 would generally not qualify for assistance and would have to rely on the developer to adequately address potential subsidence hazards.

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1986

2005

Case History 5: Colorado School of Mines, Golden, Colorado

The Colorado School of Mines (CSM) has had ongoing subsidence issues related to the old clay mines west of the main campus for decades. At one point, one of the married student housing units was so badly damaged that it was uninhabitable. In recent years, the school has made the subsidence-prone area into an athletic field; however, ongoing subsidence-related issues are still being reported.

During the construction of the fields in 2004, depressions started occurring. In the spring of 2005 the area reactivated from the snow melt runoff. Several open holes in the field were visible, and the street near the sorority houses was damaged. The structures themselves were not, as they sit on deep piers on the sandstone that was not mined.

A grouting program was implemented to try to stabilize the area in 2005; however, in 2006 additional street damage occurred and several new depressions were found in the field. In 2008, DRMS conducted a geophysical survey to try to establish areas where voids still existed so that a mitigation strategy could be developed to avoid future damage.

Subsidence damage to the CSM married student housing unit circa mid 1990s. Note displacement in wall blocks.These units were removed and replaced with an athletic field in the early 2000s. Photo by Dave Noe.

subsidence road damage near the sorority houses above the CSM athletic field

Spring 2005 damage to the road near the sorority houses above the CSM athletic field. This damage was likely initiated by subsidence, then exacerbated by the rupture of a storm water drain. Photo by TC Wait.

Continued road settlement in 2008

Continued road settlement in 2008 prompted geophysical survey efforts to discern remaining subsurface voids. Note subsidence holes visible in the field below. Photo by TC Wait.

Subsidence Bibliography

Open the coal mine subsidence bibliography (pdf).