What Is Mine Subsidence?

What Is Mine Subsidence?

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.

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.

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.

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.

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