2010-05-07_PERMIT FILE - C2009087 (52)

Home Browse Search

• d fracturing. Dewatering impacts under these conditions can reach to a few hundred feet the mine collapse areas (Rauch, 1985). (1985) provides the following description of the dewatering impacts of room - and - pillar mining forth central Appalachians. indwater 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... u Top 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. a Top 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