Subsidence is the sinking or settling of the ground surface. It can occur by a number of processes. Ground subsidence may result from the settlement of native low density soils, or the caving in of natural or man-made underground voids. Subsidence may occur gradually over many years as sags or depressions form on the ground surface. More infrequent, subsidence may occur abruptly as dangerous ground openings that could swallow any part of a structure that happen to lie at that location, or leave a dangerous steep-sided hole. In Colorado, the types of subsidence of greatest concern are settlement related to collapsing soils, sinkholes in karst areas, and the ground subsidence over abandoned mine workings.
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 human activities that may produce ground subsidence. In general, the type and severity of surface subsidence is governed by the amount ground surface and the location of removal or compression, and the geologic conditions of a particular site. Some examples of the types of ground subsidence, and how they are affected or produced by geologic conditions are explained below.
Withdrawal of pore fluids, usually groundwater, is a common cause of ground subsidence. Massive lowering of the groundwater table by “mining” of groundwater in a poorly consolidated aquifer results in subsidence of the ground surface. There are no documented cases of serious subsidence from groundwater withdrawal in Colorado. However, several areas of extremely thick and extensive alluvial aquifers may have that potential if intensive future ground water development occurs. This is especially true of such large intermontane basins as the San Luis Valley, Wet Mountain Valley, North and Middle Park, and parts of the Upper Arkansas Valley. A second kind of ground subsidence results from desiccation (drying up) of very wet clay deposits following lowering of the water table. Hydrocompaction produces ground surface collapse from excessive wetting of certain low-density weak soils. This can occur in two general types of soil that are common in Colorado a) wind deposited silts (loess), and b) predominantly fine-grained colluvial soils. In either case, collapse occurs from excessive wetting of previously dry, collapsible soils. Wetting of these materials weakens the already weak or unstable soil structure, which undergoes internal collapse and densification (reduction of air voids). Densification of the weak soil column produces ground surface collapse and subsidence in the vicinity of excessive wetting. Removal of fine material by piping* is probably an additional factor in some cases of subsidence by wetting. Such excessive wetting can occur from irrigation, broken water lines, surface ponding, or drainage diversions.
Dissolution of soluble rock or soil materials also results in ground subsidence. This occurs in areas underlain by highly soluble rock formations—especially gypsum (CaSo4-2H2O), or halite (NaC1); and to lesser extent in limestone (CaCO3) materials. Removal of earth materials by water solution leads to surface collapse. Hydrologic factors that may cause the solution and removal of material may be natural or man-induced. Natural solution is the result of the normal hydrologic processes of downward percolation of surface water and/or lateral movement of ground water within the water table (either the main groundwater table or a perched water table). Man-induced hydrologic changes or activities can have much the same effect on soluble earth materials. Such activities include temporary or permanent stream channel changes, irrigation ditches, land irrigation leaking or broken pipes, temporary or permanent ponding of surface waters, the mining of soluble minerals by means of forced circulation of water within the earth. Soluble rock materials that are subject to possible ground subsidence underlie large areas of western Colorado.
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 all of the basic types of subsidence described above exist in extensive areas of Colorado. Known serious problems of mining related subsidence, hydrocompaction, and dissolution subsidence are known to occur in the state. The various subsidence web pages have case histories. 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 ground subsidence conditions include a careful evaluation of all pertinent historic, geologic, and hydrologic factors or the area, and/or actual periodic measurements. Onset of actual or observed subsidence is in many cases related to changes in land use; accordingly land use changes in areas identified as having potential for subsidence should be carefully scrutinized.
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.
Engineering geologic factors should include a complete survey of existing geologic and engineering data that are available by way of a background study. These data will identify areas in a general way known to be underlain by geologic formations containing evaporite minerals, limestone, and potentially retrievable mineral deposits. More detailed information such as local geologic and engineering studies for highways or dam sites may reveal specific pertinent data and how similar geologic problems were (or were not) solved in areas of actual construction.
Knowledge of hydrologic factors is critical for evaluating most types of ground subsidence. Because of this, it is necessary to define hydrologic conditions to identify potential subsidence areas. The hydrologic analysis should include evaluation of all available geologic data as described above, but in a hydrologic context. Additional hydro-geologic data including published information, well logs, and field information from the site of the investigation should be compiled and evaluated. Finally the impacts of possible land uses should be evaluated as they apply to lands susceptible to ground subsidence.
Consequences of Improper Utilization
The consequences of improper utilization of land subject to ground subsidence will generally consist of excessive economic losses. These may include high repair and maintenance costs for buildings, irrigation works, highways, utilities, and other structures. At times, structures are condemned because of the damage. This causes direct economic losses to residents, and indirect losses through increased taxes and decreased property values. Spontaneous ground openings can be extremely dangerous if one were to open below an occupied structure.