4.3.7 Mine Subsidence
Total Page:16
File Type:pdf, Size:1020Kb
Schuylkill County Hazard Mitigation Plan Month 2019 4.3.7 MINE SUBSIDENCE Two common causes of subsidence in Pennsylvania are (1) dissolution of carbonate rock, such as limestone or dolomite; and (2) mining activity. Schuylkill County is not underlain by limestone or dolomite; subsidence incidents in the County are related to mining activity. Therefore, for this plan update, this section provides a profile and vulnerability assessment of subsidence related to mining activities. PROFILE Mine subsidence is defined as the movement of the Figure 4.3.7-1. Sinkhole in Porter Township, October 2017 ground surface as a result of readjustments of the overburden due to collapse or failure of underground mine workings. Areas underlain by coal, or other minerals may become susceptible to subsidence. Mine subsidence features usually take the form of either sinkholes or troughs (PADEP 2017). Sinkhole subsidence occurs in areas generally where there is less than 50 feet of overburden, or vertical distance between ground surface and the coal seam. This is commonly associated with abandoned mining activities Source: Andruscavage 2017 because current mining regulations ensure that there is sufficient overburden to reduce the likelihood of subsidence (PA DEP 2018). 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 (i.e. when the roof of an underground mine collapses, it causes the ground above to sink or subside). This type of subsidence is typically localized, affecting a small area on the overlying surface. However, structures and surface features affected by sinkhole subsidence can experience extensive and costly damages. Subsidence troughs can occur over active or abandoned mines. Trough subsidence induced by room-and-pillar mining and occur the overburden sags downward due to the failure of the remnant pillars or by punching of the pillars into a soft mine floor or roof (PADEP 2017). Location and Extent Schuylkill County is a significant part of the coal region that is located in eastern Pennsylvania. The hard coal, or anthracite, beds are located in several narrow bands that run northeasterly between the Blue Mountain and the Susquehanna River. The region is divided into three fields – southern, middle, and northern. The southern field was the first to be developed and its central part is known as the Pottsville District. In the fields, the coal lies in dozens of seams, or “veins,” some only a few inches thick and unworkable and some as much as 40 feet thick. In the County, anthracite occurs in a large number of veins, one above the other, separated by thicker or thinner beds of slate and shale (Schuylkill County HMP 2013). Areas above subsurface mines are prone to sinkholes and/or subsidence incidents. Figure 4.3.7-2 illustrates the areas of anthracite coal fields throughout Schuylkill County. In Schuylkill County, there are two predominant bands of anthracite coal. The first is located in the north central part of the County and runs west to east from Butler Township 4.3.7-1 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 to Kline Township. The other area is located in the central portion of the County and runs from the western county border with Dauphin County to Coaldale and Tamaqua at the border with Carbon County. A smaller third area occurs in the northern part of the County in East Union and North Union Townships. The predominance for subsidence will be in those areas of the County located on one of the three fields described above. Figure 4.3.7-2. Anthracite Coal Field Extent in Schuylkill County Source: PADEP 2018 Note: The green shaded area indicates coal field areas in Schuylkill County; however, this may not provide a delineation of mined activities. Room and pillar mining was the method most commonly used to mine the coal throughout the region, which leaves sections of the coal seam intact to serve as support for the mine (EMFI Coal Mining Methods). The advantage to room and pillar mining was that it could be used to follow the curvature of the anthracite coal seams. Historically, a common practice at the end of a mine’s lifespan was to retreat mine or “rob the pillars” (OSU eHIstory). This process consisted of extracting additional pillars of coal to increase that mine’s yield, but that also decreased the structural stability of 4.3.7-2 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 the mine. Retreat mining was not always documented or reported, which means the extent of extracted coal is not always accurate. Figure 4.3.7-3. Drawing of a Room and Pillar Mine Source: Pennsylvania Mine Map Atlas 2018 Note: The white shaded area indicates the mined-out areas or “rooms.” The blue shaded areas represent the existing coal or “pillars.” The pink shaded areas with a bold outline show areas that have been retreat mined. Figure 4.3.7- depicts Mineral Parcels for the Tax Year 2013. These “mineral parcels” represent mine operations which are taxed by the Schuylkill County Tax Assessment Office based on the permit issued by the PADEP (Schuylkill County HMP 2013). 4.3.7-3 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 Figure 4.3.7-4. Mining Parcels in Schuylkill County Source: Schuylkill County HMP 2013 According to the 2018 State HMP, there are 655 active/inactive/abandoned mines in Schuylkill County. Refer to Figure 4.3.7-5 which illustrates these locations which align with the location of the anthracite coal fields. 4.3.7-4 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 Figure 4.3.7-5. Pennsylvania Coal Mining Operations Source: Commonwealth of Pennsylvania HMP 2018 The red circle illustrates the location of Schuylkill County. Range of Magnitude Damages from mine subsidence are generally classified as cosmetic, functional, or structural. Cosmetic damage refers to slight problems where only the physical appearance of a structure is affected. Functional damage refers to when the structure’s use has been impacted. More significant damages that impact structural integrity is classified as structural damage. This includes situation where entire foundations need replacement due to severe cracking of supporting walls and footings (PADEP 2017). No two subsidence areas or sinkholes are exactly alike. Variations in size and shape, time period under which they occur (i.e. gradually or abruptly), and their proximity to development ultimately determines the magnitude of damage incurred. Events could result in minor elevation changes or deep, gaping holes in the ground surface. Subsidence and sinkhole events can cause severe damage in urban environments, although gradual events can be addressed before significant damage occurs. Primarily, problems related to subsidence include the disruption of utility services and damages to private and public property including buildings, roads, and underground infrastructure (PEMA 2013). 4.3.7-5 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 Past Occurrence The Pennsylvania Department of Conservation and Natural Resources (PA DCNR) Interactive Map (Figure 4.3.7-) does not show any sinkholes or surface depressions in Schuylkill County (PADCNR 2018). Additionally, the 2013 State HMP did not identify any sinkholes or subsidence incidents for Schuylkill County (PEMA 2013). Figure 4.3.7-6. Sinkholes and Surface Depressions in Schuylkill County Source: PADCNR 2018 Note: The red outline indicates the location of Schuylkill County. While the figure above does not show historic occurrences of sinkholes or subsidence documented by PADCNR, there have been occurrences throughout the County. The 2013 HMP discussed specific subsidence events that occurred in Schuylkill County through 2011. For the 2019 HMP update, mine subsidence events were summarized between January 2012 and April 2018. For events prior to 2012, please refer to Appendix X. The impact of mine subsidence could be easily illustrated and quantified for this plan update through the number of claims and total claims issued through the Mine Subsidence Insurance Program at PADEP. However, that information is not attainable by the County from PADEP due to privacy issues (Schuylkill County HMP 2013). 4.3.7-6 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 With numerous sources reviewed for the purpose of this HMP update, loss and impact information for many events could vary. The accuracy of event information, impacts and monetary figures discussed is based only on the available information in cited sources. 4.3.7-7 SECTION 4.3.7. MINE SUBSIDENCE Schuylkill County Hazard Mitigation Plan Month 2019 Table 4.3.7-1. Mine Subsidence Events in Schuylkill County, 2012 to 2018 FEMA Declaration Number County Date of Event Event Type Location (if applicable) Designated? Losses/Impacts November 12, Sinkhole Mahanoy N/A N/A A home on Park Place Road shifted on its foundation during the night of 2016 Township November 12th. The family evacuated their house to find their home leaning as the backyard gave way. A hole approximately 40 feet wide and 30 feet deep opened up. County officials believed it was caused by mining done beneath the hole and an unfilled mine path that caved. October 6, Sinkhole Porter Township N/A N/A PennDOT shut down a section of Route 125 after a portion of the highway 2017 collapsed. It occurred approximately 2.25 miles outside of Donaldson in Porter Township. The hole was approximately 25 feet deep and 25 feet wide and began filling up with water. It was reported that the sinkhole stretched across both lanes of the roadway and was about four feet deep.