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Geotechnical Engineering


Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop

Potential Environmental Impacts Due to Mining Related Activities Corridor O Study Area Clearfield and Centre Counties


Pennsylvania Department of Transportation
Engineering Dl2-0
Skelly and Loy, Inc.


Skelly and Loy, Inc., was assigned the task of reporting the potential impacts from mining activities within the zone of Corridor O, a proposed four-lane limited-access highway. The mining activities within this area of Centre and Clearfield Counties were intense due to past coal and clay extraction from several seams of coal and associated bottom clays. Nearly every form of underground and surface mining technique has been used in the corridor except underground mechanized longwall mining. The purpose of this report is to summarize the potential environmental impacts of abandoned mine lands, subsidence, and acid rock drainage (ARD, this drainage is also termed to acid mine drainage, or AMD) which is found along the alternative routes of the corridor. There are three separate design sections within Corridor O. Sections 2 and 3 are the sections primarily impacted by mining activities; section 1 has seen minimal mining activity and has received less attention than the other two sections.

Through a series of public meetings and design sessions, the Pennsylvania Department of Transportation's (PENNDOT) Corridor O project team reduced the initial large study area to several smaller alternative design zones (ADZs). At present, these ADZs have been reduced further to specific routes which are presently being explored with drill holes to ascertain the geotechnical characteristics along the proposed alternatives.

General Discussion

Coal was being mined in Corridor O in the 1800's. The coal was used for steam generation and metallurgical purposes and evolved into a fuel for electrical power generation which continues to present day. The towns of Philipsburg and Clearfield were heavily industrialized locations at the start of the twentieth century with two major railroads, rail shops, coal mining, ceramics/brick manufacture, and lumber production providing economic opportunity for European immigrants. This industrialization was accomplished without great regard to environmental concerns.

Mining regulations were imposed early in the twentieth century to provide improved worker safety. However, it was not until the middle of the twentieth century that Pennsylvania led the nation in establishing laws and regulation on the mining industry to reduce the environmental impacts caused by mining activities. There were large areas of Pennsylvania that had already been damaged by these activities. Many abandoned mines that were left prior to regulations still present dangerous conditions. Also, coal refuse piles, surface mine spoils, and unsealed deep mines were abandoned and now generate acid drainage.

Since the advent of State and Federal regulations and reclamation fund taxes on coal production, the conditions and impacts from mining have improved. Because of the intensity of mining within Corridor O prior to regulations, there are still large areas within the corridor that suffer the consequence of abandoned mine hazards, subsidence, and acid rock drainage.

The stratigraphy of Clearfield and Centre Counties is well known due to the extensive mining practiced and the studies conducted by Pennsylvania Geologic Survey (PAGS). The stratigraphic column of Sections 2 and 3 of Corridor O consists principally of Pennsylvanian age rock and coals. There are exposures of Mississippian age rock at the south end of Section 2 and in the Bigler/Dale area of Section 3. Although there are a few small clay and shale pits in Section 1, mining has had tremendous impacts in Sections 2 and 3 of Corridor O. There were several mineable seams of coal located within the limits of Sections 2 and 3.

These coals and associated strata display a variable nature. The coals vary in thickness due to splitting of coal benches into smaller units and due to erosional and depositional phenomenon in the peat forming paleo-environment. This means that a seam extensively mined in one area of the corridor may still exist as thinner, unmineable bench in another area. Regardless of mining or not, the problem of disturbing an intact coal seam can produce acid rock drainage just as readily as cutting through an old surface or deep mined area. Of course, where deep mines are encountered, there is the added problem of surface subsidence.

The sedimentary rock of Sections 2 and 3 is fairly flat-lying which is characteristic of this geologic physiographic province known as the Allegheny Plateau. Gentle folding has caused the coal seams to vary in depth and has resulted in the removal of coal through erosion in some locations.

Stratigraphically, the "A" coal and Mercer coal are the lowest seams and outcrop near the Sections 1 and 2 boundary. The strata dip toward the axial trace of the Houtzdale-Snowshoe syncline which is oriented southwest to northeast and passes near the village of Hawk Run. The "B" through "F" coals are successively higher than the "A" coal. The "A" coal is at its deepest point at the trough of the syncline. The full extent of coal measures was available in the vicinity of the village of Hawk Run; the F through the B seams were commercially extracted here. The coal and rock strata rise up the western flank of the syncline to the Laurel Hill Anticline which passes with the southwest to northeast trend near Bigler. The crest of the anticline is evidenced by the older Mississippian age rocks at the surface along Moravian Run and its tributaries. The coals and Pennsylvanian Age rocks have been eroded from these higher locations of upward stratigraphic folding.

Subsidence due to underground mining is a surface depression resulting from the collapse of roof rock into the mine void. The size of the surface area effected is a function of many variables including overburden thickness, lithology, type and intensity of mining, and surface drainage. There are two basic types of mine subsidence: sinkhole and areal.

Sinkhole subsidence occurs when surface drainage combines with the failure of the roof rock to form an eroded vertical opening from the surface into the mine and roof rock voids. Sinkhole subsidence is common with shallow mine depths and room and pillar mining. Room and pillar mining was the type of mining used in the deep mines throughout Corridor O.

Areal subsidence is a large area of surface depression that occurs when a large panel width is fully extracted. Although pillar removal with second mining (retreat mining) can produce areal subsidence similar to long wall mining, it is not as predictable as longwall mining subsidence with regard to time and dimensions. Retreat mining is not as complete in the removal of pillars compared with longwall mining. Remnant pillars and stumps left by pillar retreat mining can take years to fail, thus causing predictive problems. The prediction problem is further complicated by the mining of several seams in the same area (such as the synclinal area around Hawk Run) where distorted lithostatic stress fields cause pillar line control and hazardous conditions underground during panel advance and retreat. Acid rock drainage (or acid mine drainage) is caused by the oxidation of pyrite (iron sulfide) in water to form iron hydroxide and sulfuric acid. Iron hydroxide precipitates forming a yellow-red sludge known as "yellow-boy". The formation of sulfuric acid causes a significant drop in pH of the water.

Pyrite is found in many rocks and coal. Drainage through the pyrite-bearing strata becomes acid due to this reaction driven by bacterial action on the pyrite crystal. Finely disseminated pyrite (allowing a large surface area) leached by water of high oxygen content (fostering bacterial action) causes the lowest pH conditions of acid rock drainage. Complete descriptions of factors contributing to ARD are found in the 1998 Pennsylvania Department of Environmental Protection (PA DEP) publication entitled "Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania."

The coal and interburden between the coal seams within Corridor O have been studied and researched for many years because they contain the elements to produce severe acid rock drainage. The A and B seams due to their intensely mined areas and highly conducive pyrite mineralogic conditions have produced severe acid drainage problems throughout Corridor O. All of the seams have the potential to produce acid conditions which is why acid/base accounting and standardized testing are needed to determine the neutralization potential of excavated rock.

The prevalence of conditions that produce acid drainage in Corridor O causes particular concern where road cuts are required in the vicinity of old deep mine workings and surface mine spoils. Not only could the cuts exacerbate acid conditions into the right-of-way drainage, but the surface drainage away from the right-of-way could contribute to deteriorated acid drainage conditions in the vicinity.

Mine Openings and Potential Habitat Opening Related to Mining

Mine openings into deep mines along the various ADZs were investigated through research, map studies, and field examination. There were several types of openings to access the coal within the corridor that need to be considered. The terms used for mine openings need further definition. The most common opening is called a "drift" whereby the opening is developed horizontally from the coal outcrop or a highwall. "Slopes" are driven down to the coal seam at an angle from above the coal when horizontal access from the surface is not possible. Slopes are generally equipped with a hoisting mechanism at the surface to lower personnel and supplies and to raise coal by rail car or belt. A "shaft" has a vertical orientation to provide access to the coal for the same purposes as a slope. All openings to the mine are also part of the mine ventilation system either providing for intake air or exhausting the mine atmosphere by fans.

The original mine openings are not now readily visible due to the collapse of the overlying and surrounding rock and overgrowth of vegetation. Also, many of these openings have been mined out with later surface mining. The sources for the locations of the past openings included Pennsylvania Geological Survey documents, PA DEP, and U.S. Office of Surface (OSM) mining records and Pennsylvania Abandoned Mine Lands (AML) inventories. Site investigations and examination of aerial photos were also conducted for those opening locations.

There are some noteworthy items to consider in regard to mine openings beneath or near a selected route. Although these openings may have been filled or deliberately collapsed, renewed surface activities can cause a reappearance of the opening feature. The reappearance of the feature may also be accompanied with extensive subsidence. Any mine working that is intercepted can also be a source of acid drainage and may require hydraulic sealing of the opening to stop the flow of acid water.

Potential habitat locations within Corridor O represent three different mine-related phenomena. Where strip cuts cut through the barrier coal into the entries of old mine workings, the site may have been no longer economically viable due to overburden depths; therefore, the old workings were backfilled or the highwall collapsed over the openings. Erosion and animal burrowing have reopened smaller holes along these unreclaimed highwalls. A second related feature is where auger mining along a strip cut was conducted to produce an opening similar to cuts into underground workings, the backfilled materials have eroded, and animal burrowing has caused small entries into the auger holes extending into the coal. The third phenomenon is that of sinkhole collapse into shallow mine workings. The overburden is so thin that it runs into the old workings and, with animal activity and erosion, the access through the sinkhole becomes larger.

These features are usually very subtle and difficult to find. The locations shown should not be considered all inclusive, and it should be understood that these openings can open and collapse shut with weathering collapse and mass wasting.

The major consideration here is that threatened and endangered species such as the Indiana bat (Myotis sodalis) may be found to be using the mine opening as habitat. Studies are presently being performed to determine the presence of the Indiana bat at these locations. If needed, structures such as "bat gates" have to be installed to allow access for these species while prohibiting human access. Because rock and soil erode away from these structures and because they are subject to vandalism, maintenance has to be provided.

Subsidence Potential

Due to the extensive underground mined areas, there is potential for subsidence of the surface to occur. There are large areas that have already collapsed due to mining of the pillars. There are areas of sinkhole subsidence where the roof in shallow workings has collapsed to the surface. The phenomenon of subsidence usually occurs within a few months of mining but not always. As the support system in the abandoned mine deteriorates, the stress above and below the coal changes, and further overburden movement ensues. Activities on the surface also change the equilibrium conditions in the remnant pillars and can lead to renewed overburden movement appearing as surface subsidence. Most all of the deep mines crossed by potential routes of Corridor O have been abandoned for decades and most likely have collapsed to some extent. However, surface excavation can cause a renewal and further consolidation of the fractured overburden above the coal. It should be noted that there are areas where surface mining in the last 50 years has removed large areas of previously deep mined coal. It should also be noted that there were several areas of patterned ground observed. Patterned ground is a hummocky surface area that reflects collapse over mine entries with shallow mine workings.

Those areas where underground mining was conducted should be avoided due to the unstable nature of coal overburden with many decades of weathering of the overburden and coal pillars. The potential to predict subsidence is extremely difficult due to the long term of pillar loading and many variables.

Realistically, there is no routing through the corridor that can avoid areas of underground mining. Therefore, some measures in dealing with underground workings should be anticipated. The method used will be dependent on the amount of cover above the deep-mined coal seam, the seam thickness, and whether the mine void is extant or if the overburden has collapsed into the void.

If the workings are shallow and the cut grade is within 20 to 40 feet of the top of the mined coal seam, it would be most expeditious to cut down and remove the coal pillars and stumps. Any voids related to subsidence are eliminated with this removal of coal and adequate subgrade compaction can be accomplished. This coal would be "incidental to construction" and can be donated to a nearby coal-using institution to maintain the noncommercial aspects of extraction.

Where the overburden exceeds 20 to 40 feet above a mine void, the use of backflushed fill into mine voids is the best way to prevent later subsidence. The backflushed materials should be controlled to restrict the flow of materials to the immediate area of the roadway. This can be accomplished with remotely placed barriers located in entries where backflush materials could flow away from the area to be supported. Detailed studies of mine maps, if available, and on-site borehole camera monitoring will be needed to insure that backflush materials are accurately placed.

The overburden may need to be consolidated with low pressure grouting techniques. Where drilling has shown the RQD (rock quality designation) to be low in the overburden above a deep-mined seam, low pressure grouting may be used to consolidate the overburden to prevent renewed failure and collapse into the old workings resulting in surface subsidence damage to the road.

The technique used to stabilize the surface could employ crushed and screened waste materials such as power generation ash rather than aggregate. There are sources of high calcium fly ash (a pozzolanic material) that could be used for backflushing and grouting.

Mine-Related Hydrogeologic Conditions

Due the great extent of mining in Sections 2 and 3 and because the mined seams and disturbed overburden contained pyritic minerals conducive to the generation of acidic waters, the water quality in these two sections tends to be extremely poor as evidenced by low pH values and high sulfate concentrations. As previously stated, finely disseminated (framboidal) pyrite and marcasite (FeS2) crystals in the coal and overburden reacts with oxygenated recharge water percolating through the mine overburden to form sulfuric acid and iron hydroxide. The chemical reaction is as follows.

Fe S2 + H(OH) + O2 ----> Fe (OH)3 + H2 (SO4)

The reaction is enhanced by bacterial action and high levels of dissolved oxygen in the leaching waters. The acid drainage is often recognized by the yellow-red staining of the channel with the iron hydroxide precipitate.

The severity of the acid drainage conditions has been a concern for decades in Pennsylvania. Operation Scarlift was a mine remediation program instituted by the Pennsylvania Department of Environmental Resources in the 1960's. Skelly and Loy participated in some of these Operation Scarlift studies of watersheds polluted due to acid rock drainage in Centre and Clearfield Counties.

East of Philipsburg and north of Route 322 is a drainage basin designated by the PA DEP as being unsuitable for mining (UFM Petition 14829902). This basin is drained by Black Bear Run, which has been classified as an exceptional value (EV) waterway due to its biologic diversity and chemical characteristics. In addition, Black Bear Run feeds the surface water reservoir maintained by the Cooper Township Municipal Authority. The reservoir provides drinking water requiring minimal pre-treatment for thousands of residents. The water in Black Bear Run was monitored by Skelly and Loy during and after drilling and is of exceptional quality for this region. As noted earlier, changes in the UFM regulations which would allow extraction of coal for projects of public safety (i.e., highways) are being processed by PA DEP. Presently, even incidental removal of coal for public safety is prohibited under the regulations.

The consideration of ways to avoid mine-related drainage issues is a significant effort for the design team, given the prevalence of existing and potential problem areas throughout the corridor. During highway construction efforts, there will be certain situations that should be avoided, including the following.

  1. Road cuts within old strip mine operations, particularly those strip mines with extant permits and ongoing water treatment

  2. Road cuts into and adjacent to old deep mine workings where acidic mine pools are likely

  3. Placing fill into wetlands which have been created by mine water discharges

  4. Placing fill consisting of coal overburden without complete acidity/alkalinity testing in accordance with PA DEP standards

  5. Failure to protect surface water and groundwater sources by placing acidic fill or cutting into old workings

  6. Failure to fully evaluate the local groundwater table elevations and chemistry in areas receiving bridges or other structures, since structural foundations may be severely affected by low pH and high sulfate concentrations.

There are some areas that pose exceptionally difficult problems related to acid drainage conditions. Such an area is Moshannon Creek, which is severely polluted due to drainage from its banks as well as from below the stream. There are acidic discharges easily observed near the PA DEP Regional Office in Hawk Run. It is interesting to note that this office building was originally built as part of a water treatment complex to treat the discharge from this location. Another discharge occurs on One Mile Run near Loch Lomond Road. Since Moshannon Creek was undermined in several locations, the flooded underground workings most likely contribute to the stream's base flow. It should be assumed that low pH and high sulfate waters will be encountered near Moshannon Creek.

Another example is found west of McDowell Road and east of Route 970 to the north of Route 322 and to the south of I-80. This is an area of extensive surface mining with discharges from deep mines occurring at several locations around the perimeter of this large area. There are several measures that can be employed to reduce and minimize the acid drainage problem. These measures include the following.

  • Acid/base testing in cut areas and for fill materials
  • Addition of sufficient alkalinity to fill materials to diminish the acid drainage problem
  • Removal from the site of any coal that is encountered

The risk of encountering unrecorded underground mine openings can be reduced by conducting a carefully planned exploratory boring program. Large mines were usually adequately surveyed because of lease requirements. Smaller mines, particularly before imposition of safety regulations, sometimes operated without survey records. Detailed logging of the borings should be include noting water-bearing zones, groundwater quality, drill string advancement, and RQD, in addition to lithologic descriptions. Hole abandonment details should also be recorded. With appropriate forethought, planning, and attention to detail, underground mine workings and potential mine pools can be anticipated and avoided.

The grade lines along the chosen route will undoubtedly intercept coal seams that have been mined as well as some thinner in-place seams. There are some techniques available to deal with AMD if the gradient results in coal interception. Passive treatment techniques can be employed to deal with minor flows. These techniques include wetland development with alkalinity additions via Anoxic Limestone Drains. Grouting and sealing of mine openings can also be used to cut off or redirect the flow of acid drainage.

Where acid flows already exist due to past mining, the improvement of existing wetlands to better function as passive treatment cells is a potential solution to the problem within the right-of way. Where it is unavoidably necessary to use deleterious fill materials during highway construction, the addition of alkaline materials may preclude the perpetual treatment of drainage from the fill. They should be compacted to the greatest extent possible to retard water infiltration and atmospheric exposure, thus inhibiting bacterial activity on the pyritic minerals.

Summary of Conclusions

The Corridor O study area has a long history of deep mining of mostly shallow coal mining. It is not possible to avoid this deep mining due to the intensity and number of commercially extracted seams. This mining has caused some extreme environmental problems for the region. The mining-related problems discussed in this report include mine-related habitat openings, subsidence potential, and mine hydrogeology.

Openings to old mine workings have been compromised thereby allowing access by animals. Should it be verified that these openings are being used by threatened and endangered species such as the Indiana bat, bat-gate structures may be needed.

Subsidence of the surface is a potential regardless of the route chosen. Solutions include removal of the coal stumps and pillars, backflushing with low cost materials, and low pressure grouting in weakened overburden areas.

Acid rock drainage and mine hydrogeology have already developed and contributed to wetlands and waterways in the Corridor O study area. Careful grade design can eliminate culpability of worsening the acid rock conditions; crossings of unavoidable existing acid drainage can be incorporated into passive treatment and mitigation procedures.

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Updated: 04/07/2011

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