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.
Subsidence may occur gradually over many years, or abruptly—virtually instantly. 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 (karst subsidence). Another form of regional subsidence that can create large ground fissures is the withdrawl 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 heading Collapsing Soils.
Weight, including surface developments such as roads, reservoirs, and buildings, and man-made vibrations from such activities as blasting, heavy truck or train traffic can 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.
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.
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 in the General Subsidence web page here.
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 sizeable. 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 subidence exist in extensive areas of Colorado, and serious instances of damage due to ground subsidences 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 wisely dealt with. 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 avaiable 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.