Mine Subsidence Illustration
View this color illustration showing the typical effects of mine subsidence, including captions and photographs further describing the types of mine subsidence that can occur.
"Mine Subsidence ? An Overview
In order to consider potential impacts of underground mining on overlying structures, water resources, and surface land, it is first necessary to have some understanding of the mechanics of mine subsidence.
Mine subsidence can be defined as movement of the ground surface as a result of readjustments of the overburden due to collapse or failure of underground mine workings. Surface subsidence features usually take the form of either sinkholes or troughs.
Sinkhole subsidence is common in areas overlying shallow room-and-pillar mines. Sinkholes occur from the collapse of the mine roof into a mine opening, resulting in caving of the overlying strata and an abrupt depression in the ground surface. The majority of sinkholes usually develop where the amount of cover (vertical distance between the coal seam and the surface) is less than 50 feet. This type of subsidence is generally localized in extent, affecting a relatively small area on the overlying surface. However, structures and surface features affected by sinkhole subsidence tend to experience extensive and costly damages, sometimes in a dramatic fashion. Sinkhole subsidence has been responsible for extensive damage to numerous homes and property throughout the years.
Sinkholes are typically associated with abandoned mine workings, since most active underground mines operate at depths sufficient to preclude the development of sinkhole subsidence. In accordance with the current regulations, the Department will not authorize underground mining beneath structures where the depth of overburden is less than 100 feet (30.5 m), unless the subsidence control plan demonstrates that proposed mine workings will be stable and that overlying structures will not suffer irreparable damage.
Subsidence troughs induced by room-and-pillar mining can occur over active or abandoned mines. The resultant surface impacts and damages can be similar, however the mechanisms that trigger the subsidence are dramatically different. In abandoned mines, troughs usually occur when the overburden sags downward due to the failure of remnant mine pillars, or by punching of the pillars into a soft mine floor or roof. It is difficult, if not impossible, to predict if or when failure in an abandoned mine might occur, since abandoned mines may collapse many decades after the mining is completed, if the mine workings were not designed to provide long-term support.
Potential Impacts of Underground Mining on Structures
Damages to structures are generally classified as cosmetic, functional, or structural. Cosmetic damage refers to slight problems where only the physical appearance of the structure is affected, such as cracking in plaster or drywall. Functional damage refers to situations where the structure?s use has been impacted, such as jammed doors or windows. More significant damages that impact structural integrity are classified as structural damage. This would include situations where entire foundations require replacement due to severe cracking of supporting walls and footings.
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Potential Hydrologic Impacts of Underground Mining
Underground mine openings can intercept and convey surface water and groundwater. When excavated below the water table, mine voids serve as low-pressure sinks inducing groundwater to move to the openings from the surrounding saturated rock. The result is the dewatering of nearby rock units via drainage of fractures and water-bearing strata in contact with the mine workings. There is also the potential for impacts to more remote water-bearing units and surface water bodies depending on the degree of hydrologic communication. The extent and severity of the impact on the local surface water and groundwater systems depends on the depth of the mine, the topographic and hydrogeologic setting, and the hydrologic characteristics of the adjacent strata. Additionally, the amount and extent of mine subsidence-related changes to the rock mass govern the impacts of underground coal mining on surface water and groundwater.
In the flat-lying sedimentary rocks of southwestern Pennsylvania, underground mining is routinely accompanied by rock fracturing, dilation of joints, and separation along bedding planes. Rock movements occur vertically above the mine workings and at an angle projected away from the mined-out area. Mining-induced fracturing within this angle can result in hydrologic impacts beyond the margins of the mine workings. The zone along the perimeter of the mine that experiences hydrologic impacts is said to lie within the "angle of dewatering" or "angle of influence" of the mine. Angle of influence values of 27 to 42 degrees have been reported for the coalfields of northern West Virginia and southwestern Pennsylvania (Carver and Rauch, 1994; Tieman and Rauch, 1991).
These changes to the rock mass can change the water transmitting capabilities of the rock by creating new fractures and enlarging existing fractures. This typically results, at least temporarily, in detectable changes in permeability, storage capacity, groundwater flow direction, groundwater chemistry, surface-water/groundwater interactions, and groundwater levels. Depending on the ratio of overburden to seam thickness and the type of mining, measurable surface subsidence may occur. As previously discussed, this surface movement ranges in type from broad troughs approximating the area of coal extraction (typical of longwall mining) to complete collapse of overburden from the mine to the surface, e.g., sinkhole subsidence (generally associated with shallow room-and-pillar mining).
The various underground mining techniques have distinctly dissimilar impacts on local water resources. In short, the impacts of room-and-pillar subsidence tend to be localized, irregular, and often long delayed; whereas those of longwall subsidence are immediate, pervasive, systematic, and ultimately predictable (Booth, 1997).
The following sections review some general aspects of mining-induced impacts to water resources. However, the impacts of mine subsidence on surface and groundwater flow quantity and quality are not easily generalized.
"?The enhancement of the overburden hydraulic conductivity due to mining is neither uniform nor well-defined. Predicting impacts is difficult and there is no such thing as a ?typical? hydrogeologic setting or mine site." (Parizek and Ramani, 1996)
Potential Impacts on Streams and Surface Waters
The impacts of underground mining on surface waters can range from no noticeable impact to appreciable diminution, ponding, and/or diversion. The formation of subsidence-induced cracks, surface depressions, and/or sinkholes at the bottom of, or adjacent to, surface water bodies, such as streams, ponds, and lakes can lead to complete or partial loss of water due to leakage to the underlying strata. The resultant changes in surface slope can adversely impact drainage along irrigated fields, canals, sewers, and natural streams (Bhattacharya and Singh, 1985).
Room-and-pillar mining is generally less disruptive to nearby surface waters than high-extraction methods. Individual openings have only minimal localized draining impacts due to self-supporting roof members which span the opening to form a compression arch, with the support pillars serving as abutments. This "pressure arch" limits not only the deformational, but also the hydraulic influence of the opening (Booth, 1986). As additional entries are driven, the resultant network of intersecting drains act as a planar underdrain inducing downward leakage from overlying units. However, due to its built-in system of support pillars and limited mining-induced fracturing, significant drainage is typically limited to near-mine units.
Many detrimental impacts of room-and-pillar mining take years or even decades to occur as weak coal pillars deteriorate over time (Sgambat, 1980). Deteriorating or under-sized pillars that fail over time result in vertical extension of mine-induced fracturing. Dewatering impacts under these conditions can reach to a few hundred feet above the mine collapse areas (Rauch, 1985).
Rauch (1985) provides the following description of the dewatering impacts of room-and-pillar mining in the north central Appalachians.
"?Typically the greatest groundwater inflow rates occur near the working face of the mine where groundwater is being drained from storage, especially from fractures in mine roof rocks. In older mine sections, long term groundwater recharge to the mine is under more or less steady state conditions, originating ultimately from infiltration of precipitation or surface water. ? This water typically enters the mine along rock fractures that intersect the mine ceiling, especially along vertical fracture zones . ?Groundwater inflow is especially great in areas of mine ceiling collapse due to the leaving of too little roof rock support or to weak ceiling rock where fracture zones intersect the mine.
This drainage to room-and-pillar mines dewaters some overlying aquifers. The extent of this drainage is best determined from studies of water wells and springs overlying the mines. In general, significant dewatering extends to 20 to 100 feet vertically above drained room-and-pillar mines, but is usually restricted to within about 40 feet vertically of these mines."
"Localized, significant hydraulic impacts of deep headings and uncollapsed room-and-pillar mines will be seen in shallow aquifers only in areas (such as fracture zones) where vertical hydraulic connections are naturally high or where the mine itself is very shallow" (Booth, 1986). Shallow room-and-pillar mining (within 200 feet (61 m) and particularly within 100 feet (30.5 m) of the surface) drastically increases the likelihood of significant impacts to surface waters. This results from the mining?s proximity to shallow, open fractures and unconsolidated surface material. Hobba?s (1993) report on room-and-pillar mining in northern West Virginia found that,
"? mining and subsidence cracks increase hydraulic conductivity and interconnection of water-bearing rock units, which in turn cause increased infiltration of precipitation and surface water, decreased evapotranspiration, and higher base flows in some small streams?. Both gaining and losing streams were found in mined areas."
In deep settings, the impacts tend to be minimal. Bruhn and Speck (1986) reported the following impacts from room-and-pillar mining conducted beneath 600 feet of overburden in northern West Virginia.
"? Structurally, the overburden strata were little affected by the introduction of entries and cross cuts into the panel. Subsidence was virtually nil. Piezometric levels remained at their pre-mining elevations except (presumably) near mine level. Measured piezometric level variations were minor and no more than might be expected from seasonal variations?"
Potential Impacts on Wells and Springs
Wells and springs in proximity to room-and-pillar mining have the potential of being adversely impacted. Commonly the mechanism is direct draining of groundwater to the mine. Generally, where the support pillars are stable, these impacts are localized. Dewatering typically extends to 20 to 100 feet (6 to 30.5 m) above the mine workings. Wells that terminate at depths greater than 100 feet (30.5 m) above the mine roof are generally safe. In cases where support pillars fail, additional subsidence may result in more extensive fracturing. In these instances impacts may be up to 200 (61 m) or even 300 feet (91.5 m) above the mine roof. Subsidence impacts may be extended where mining is close to vertical fracture zones.
Other Causes of Impacts on Structures and Water Supplies
There are many factors that cause damages to structures other than mine subsidence. Most structures have some degree of damage even prior to mining. Most property owners are not fully aware of the condition of their home and property since they do not generally conduct routine and thorough inspections.
Some of the more common reasons for structural damages other than mine subsidence are:
Settlement due to drying of soils or the weight of surface loads
Landslides and soil creep
Shrinking and swelling of soils
Freezing and thawing of soils
Surface and subsurface erosion
Poor construction methods
Structural deterioration due to age, lack of maintenance, or misuse
Likewise, there are numerous causes, both natural and man-induced, that can mimic the impacts seen from underground mining near water supplies. Naturally occurring impacts to well yield include:
drought (affects springs also)
plugging of the well screen via incrustation
plugging of the well screen by iron bacteria
clogging of the well due to improperly sized well screen and migration of fines into the well.
clogging of the well resulting from corrosion of well screen
Man-induced causes include:
increased groundwater withdrawals (new wells in area)
nearby surface mining activities
Additionally, water quality complaints must be investigated with an appreciation of ambient groundwater quality. In large areas of the bituminous coalfields indicator parameters for mine drainage, such as iron and manganese, occur naturally at concentrations above drinking water limits.
Bhattacharya, S., and M.M. Singh (1985). Development of Subsidence Damage Criteria. Engineers International, Inc., prepared for U.S. Dept. of the Interior, Office of Surface Mining, Contract J51120129.
Booth, C.J. (1990). Hydrogeological Significance of Subsurface Coal Mining, Water Resources in Pennsylvania: Availability, Quality, and Management. Edited by S.K. Majumdar, R.R. Parizek, and E.W. Miller, The Pennsylvania Academy of Science.
Booth, C.J. (1986). Strata-Movement Concepts and the Hydrogeological Impact of Underground Coal Mining. Ground Water, Vol. 24, No. 4, July-August 1996.
Bruhn, R.W, and R.C. Speck, (1986). Characteristics of Subsidence Over Pillar Extraction Panels. U.S. Bureau of Mines, Contract Report J0233920, GAI Consultants, Inc., July 1986.
Cifelli, R.C. and H.W. Rauch (1986). Dewatering Effects from Selected Underground Coal Mines in North-central West Virginia. In: Proceedings, 2nd Workshop on surface Subsidence Due to Underground Mining, West Virginia University, Morgantown, W.Va., p. 249-263.
Hill, J.G., and D.R. Price (1983). The Impact of Deep Mining on an Overlying Aquifer in Western Pennsylvania. Ground Water Monitoring Review, Vol. 3, No.1, pp. 138-143.
Hobba, W.A. (1993). Effects of Underground Mining and Mine Collapse on the Hydrology of Selected Basins in West Virginia. U.S. Geological Survey Water Supply Paper 2384, U.S. Department of the Interior.
Kendorski, F.S. (1993). Effect of High-Extraction Coal Mining on Surface and Ground Waters. 12th Conference on Ground Control in Mining, West Virginia University.
O?Steen, W.N. (1982). Evaluation of Aquifer Dewatering by Underground Coal Mines. M.S. Thesis Report, Department of Geology and Geography, West Virginia University, Morgantown, W.Va., 107 p.
Parizek, R.R., and R.V. Ramani (1996). Longwall Coal Mines: Pre-Mine Monitoring
and Water Supply Replacement Alternatives. Final Report on Legislative Initiative
Program 181-90-2658, Environmental Resources Research Institute, Pennsylvania
State University, University Park, Pa.
Peng, S.S. (1992). Surface Subsidence Engineering. Society for Mining, Metallurgy, and Exploration, Inc., 161 p.
Rauch, H.W. (1985). A Summary of the Effects of Underground Coal Mines on Quantity of Ground Water and Streamflow in the North-Central Appalachians. Eastern Mineral Law Foundation, Sheraton Hotel at Station Square, Pittsburgh, Pa.
Rauch, H.W., W.N. O?Steen, G. Ahnell, and D.F. Giannatos (1984). Predictions of Aquifer Dewatering over Underground Mines in the Pittsburgh, Sewickley, and Upper Freeport Coals of Northern West Virginia. In: Proceedings, West Virginia Surface Mine Drainage Task Force Symposium, 11 p.
Rauch, H.W. (1989). Ground Water Impacts from Surface and Underground Coal Mining. Chapter 25 in Proceedings of Conference on West Virginia Ground Water, 1987 ? Status and Future Directions, Water Research Institute, West Virginia University, Morgantown, WV, 21 p.
Sgambat, J.P., E.A. Labella, and S. Roebuck (1980). Effects of Underground Coal Mining on Groundwater in the Eastern United States. EPA Interagency Energy/Environmental Research and Development Program Report, EPA-600/7-80-120, 182 p.
Stoner, J.D. (1983). Probable hydrologic effects of subsurface mining. Ground-Water Monitoring Review, 3(1):128-137.