COAL MINE IMPACTS - New York State Department of ...



SOMETHING OLD, SOMETHING NEW:

COAL MINE TREATMENTS ALONG AN INTERSTATE INTERCHANGE

Donald V. Gaffney and Lucas T. Turko

Michael Baker Jr., Inc. Michael Baker Jr., Inc.

4301 Dutch Ridge Road 4301 Dutch Ridge Road

Beaver, PA 15009 Beaver, PA 15009

(724)495-4254 (724)495-4245

dgaffney@ lturko@

Abstract

The interchange of I-79 with the Penn-Lincoln Parkway, west of Pittsburgh, PA, was constructed in the late 1960’s. Abandoned and flooded underground mine workings in the Pittsburgh Coal were encountered unexpectedly at cut slope locations on two different ramps. At these locations, water drained freely until control measures were implemented.

The existing mine treatment along the ramps consists of seals constructed of random embankment material, notched into the proposed cut slope, with drainage pipe along the face of the coal. Mine water from this pipe discharges into the existing roadway drainage system, which now shows the impact of this acid drainage. The seals have no apparent stability problems. However, there apparently are leaks in the existing mine seals, contributing to wet areas.

Final design for reconstruction and improvement of the interchange has just been completed. Since the existing mine seals are apparently functioning, the basic geometry was maintained for the proposed design. Mine pool volume and recharge estimates were developed. Minor modifications to the old designs were made to improve efficiency, tighten the specifications, and use modern materials.

However, because of current environmental regulations, mine water encountered during new construction must be collected and treated prior to discharge. In addition, rock from project excavation with the potential for producing acid runoff also now requires special handling. Soil, rock and water quality testing was performed, and their quality characterized. A “Coal Impacts Report” was prepared for the project to document this work.

Fitting these new treatments along with the old seals and drains proved to be a challenge within the project constraints. In addition, the level of investigation, imposed requirements, and resulting designs offer an insightful comparison between construction in the 1960’s and today.

1.0 Introduction

Baker was awarded a contract by the Pennsylvania Department of Transportation, District 11-0, to design two new connecting ramps at the interchange of Interstate 79 (a.k.a.: S.R. 0079, L.R. 1016) and the Penn-Lincoln Parkway (a.k.a.: Routes 22&30, Parkway West) west of Pittsburgh, and improve the geometry of the other ramps. The two new connecting ramps will allow motorists traveling eastbound on the Parkway (away from the airport and towards the city) to access Interstate 79 northbound, and motorists traveling southbound on Interstate 79 to access the Parkway westbound to the airport, respectively. This project is commonly referred to as the “Parkway Ramps” project.

Final design for reconstruction and improvement of the interchange has just been completed. The design of the interchange requires excavations into existing hillsides and cuts slopes for the proposed highway. The hillside areas adjacent to the project have been disturbed by past mining activities and highway cut slope construction. This includes surface and deep mines associated with the Pittsburgh coal seam as evidenced by existing strip benches, existing cut slope coal seals and associated mine water discharges observed above the roadway levels. The existing coal seals were constructed as part of the I-79 highway construction project, circa 1968. Since the existing mine seals are apparently functioning, the basic geometry was maintained for the proposed design.

2.0 A Time of Farms and Mines

Before suburban sprawl reached this area, it was primarily farmland and woods. The Pittsburgh Coal seam was mined in the 1800’s and early 1900’s. An aerial photo, circa 1936, shows croplands, orchards, and woodlands, with contour strip mines and evidence of subsidence on underground-mined hillsides. Subsidence is primarily in the form of potholes created from collapse of where rock cover was inadequate to support mine overburden. Strip mining typically followed after underground mining.

Coal mining activities in the area were confined to the Pittsburgh coal seam. Within the project area the seam dips to the southeast at a grade of 2 % to 2.5 %, although there are local structural variations that create small dams and pools. Surface water drainage associated with abandoned mines is located along the hillsides adjacent to both of the proposed ramps. The Pittsburgh Coal Company operated the now abandoned Moon Run Mine in the areas adjacent to proposed construction of both ramps. The extent of the mine is documented on various mine maps, including the WPA Project No. 4483 Map.

3.0 Initial Expansion West of Pittsburgh

In the mid-1950’s, US Routes 22&30 were relocated to follow Campbells Run Road along its southern valley slope. During construction, the strip mined hillside immediately south of the east-west roadway was cut back including excavation of strip mine overburden. However due to the elevation and structural dip of the coal seam, no mine waters were encountered and the mine itself was not a concern.

4.0 Interstate 79 Construction

The hillside areas west of the I-79 mainline and north and south of the Parkway were disturbed during construction of interchange ramps as part of the I-79 construction activities, circa 1968. Mine voids were encountered and mine water was discharged during construction for the proposed rock cuts. This water ran freely into nearby streams as emergency remedial designs were developed and implemented. The final grading of the I-79 cut slopes included mine water collection drains and coal seals at the coal elevation within the cut slopes. These existing coal seals and mine drains will be impacted during proposed excavation activities and will have to be relocated and/or reconstructed to accommodate the design. The current conditions of these existing coal seals are summarized below.

4.1 Existing Conditions at Ramp H

The elevation of the bottom of the Pittsburgh coal seam at the Ramp H cut slope varies from approximately 1040 ft-msl at the northern extent to 1035 ft-msl at the southern extent. Mine seepage discharges from two 8-inch diameter vitrified clay pipes (VCP) into a 18-inch diameter bituminous coated corrugated metal pipe (BCCMP) located at the southern extent of the existing coal seal at Ramp H. Original BCCMP slope pipe was completely corroded after 30 years and now allows the water to flow over the cut slope into an existing bench inlet. Then the water is carried through an18-inch diameter BCCMP beneath Campbells Run Road where it is discharged into Campbells Run. One of the VCP outlets is believed to be a drain at the base of the existing coal seal, while the second appears to be directly tied into an existing buried mine opening.

Nine test borings were drilled recently on the hillside above the coal elevation at the proposed Ramp H cut slope area to determine mine void locations and water levels in the mine. The Moon Run Mine and Pittsburgh coal seam north of S.R. 0022 and west of S.R. 0079 dips in a southeasterly direction and outcrops above the highways. A significant area of the Moon Run Mine is the apparent source of the continuous seepage discharge in this area.

4.2 Existing Conditions at Ramp C

An existing contour strip bench exists above the proposed Ramp C alignment and an existing coal seal is located in the cut slope above Ramp C. Surface water seepage from the existing coal seal is evident near the low point of the mine. The surface water seepage is believed to be associated with the existing coal seal drainage. The seepage flows over the cut slope face into the roadside swale where it ponds before flowing into the existing storm water facility. The existing storm water system then outlets the flow into an existing concrete lined groin channel. Acid mine water is discharged down the groin channel to a tributary stream, which then conveys the flow beneath I-79 through a 72-inch diameter corrugated metal pipe into Campbells Run. In addition, the seepage flows over the backside of the cut slope over original ground into an existing groin channel. Wetlands have been created by these overland flows.

Seventeen test borings were drilled recently specifically to investigate the coal and mine conditions on the hillsides above the proposed Ramp C cut slope areas. Similar to mine conditions along Ramp H, the Ramp C mine area exhibited no discernable pattern of pooled mine water. Numerous voids and fractured rock encountered above the mine level, as noted in the test borings, give strong evidence of a significant caving of rock strata in the mined area. Mine water is ultimately making its way to the low point and discharging through observed pipe outlets and seeps, but the mine void does not appear to contain a large continuous impoundment of AMD. The mine water impounded near Ramp C appears to be located within isolated pockets, based on the available data. In addition, free drainage to the lowest parts of the mine is likely impeded by the presence of significant amounts of mine gob and roof fall material.

5.0 Planned Interchange Improvements

The mine workings present in the adjacent hillside areas will be exposed by proposed excavation activities. The excavations will generate additional temporary flows of Acid Mine Drainage (AMD) and rock materials that are potentially acid producing. Where the existing mine seals are disturbed, they will be reconstructed.

5.1 Mine Seal Embankments

Since both existing mine seals were constructed as emergency responses at different times and by different individuals, they have slight variations in design. One has a bench at the top; the other has a bench at the bottom. They both used VCP, placed along the face of the coal. One allowed surface discharge into a natural stream, the other completely tied to the roadway drainage system.

The new design incorporates benches at both the top and bottom of the seal. It will use PVC pipe, with a manifold system extending back into mine openings. All drainage will be positively discharged into the roadway drainage system.

5.2 Acid Mine Drainage (AMD) Treatment

In coordination with PennDOT and PADEP, the following design assumptions for the temporary AMD treatment system were established: 1) No permanent treatment of final AMD discharge, 2) Existing discharge points of AMD will be maintained for temporary and final discharge, and 3) Temporary and final discharge of AMD will not degrade the present water quality of Campbells Run. In addition, the temporary treatment will be designed to meet the requirements during active earthmoving and will be removed upon completion of earthmoving activities.

The temporary treatment goal is to discharge treated water with no net acidity and minimal levels of iron, and manganese. Theoretical removal of all iron, and manganese, was used for design calculations, although in reality less than 100% of these dissolved metals will be removed. It is anticipated that temporary treatment effluent concentration levels of these metals will approximate CFR Title 40 – Protection of the Environment, effluent reduction attainable by the application of the best practicable control technology currently available. The following effluent limitations will apply: Iron, total – 7.0 mg/l; Manganese, total – 5.0 mg/l; Total Suspended Solids – 90 mg/l; pH – 6.0 to 9.0 at all times; Alkalinity greater than acidity.

In order to prevent an uncontrolled release of AMD it was necessary to estimate the volume of water anticipated to be released and develop a plan to temporarily collect, treat, and discharge the water during earthmoving activities without additional impacts to Campbells Run water quality.

In order to determine the quality of AMD discharges from abandoned mine workings in areas proposed for the construction of Ramp H and Ramp C, and to determine the impacts of these AMD discharges on the receiving stream (Campbells Run), the flow and chemistry of the surface water sampling locations, as well as of the stream were reviewed. Testing found that discharges from the proposed cut areas of Ramp H and Ramp C, respectively, are acidic discharges with low pH and very high concentrations of Iron, manganese and aluminum. Streams with low pH and high concentrations of aluminum adversely impact the streams. Therefore, temporary treatment of these seepage points are proposed to minimize impacts of AMD discharges on Campbells Run.

Dewatering of each of the mine pools in about one month’s time is desirable. An initial pumping rate of 150 gpm was assumed for design. At that rate, dewatering of the estimated Ramp H mine pool of 17.53 million gallons requires 81 days, while 21 days are required to dewater the estimated 4.61 million gallon mine pool at Ramp C.

Neutralization of the anticipated acidity in the AMD can be met by active temporary treatment of the AMD discharge with Calcium Oxide (Pebble Quicklime) or other approved method by the future Contractor. Water may be pumped from boreholes placed at higher water locations in the mine, or the roof of the mine may be removed, locally, to create a natural or artificial sump for dewatering.

As the water is pumped from the mine it may be treated with Calcium Oxide (Pebble-Quicklime) using a portable mixing unit. The mixing unit is essentially a waterwheel with a small bin holding pebble-quicklime. The rate of chemical feed is dictated by the movement of pumped water through the waterwheel, which causes an auger-feeder to dispense the lime pebbles from the bin. The mixing unit shall be set up at the beginning of a riprap lined channel to be used to convey pumped mine water (AMD) to the settling pond. No power source is necessary for this temporary treatment. A portion of the pumped mine water will be used through a control valve to run the waterwheel for a desired rate of feed of pebble quicklime into the riprap lined channel. The riprap lined channel helps to agitate and aerate the AMD flow (after addition of pebble quick-lime) while being conveyed to the settling pond. The riprap lined channel to convey the AMD flow into the settling pond should be at least 500 feet in length to ensure maximum mixing and reaction of the pebble quicklime with the AMD flow and neutralization of acidity. Limestone riprap is preferred to channel lining since it will impart additional alkalinity to the pebble quicklime treated AMD flow. Since the AMD flow is anticipated to be relatively low, a trapezoidal 2-foot bottom width section with a depth of 1 ½ feet and side slopes of 2H:IV will be adequate to convey the flow.

The settling pond to be provided at the end of the limestone riprap lined channel will have sufficient capacity to provide 24 hours of detention time for the treated AMD. Additional storage volume for sludge produced from oxidized metal precipitates, suspended solids, and unreacted lime is required to maintain adequate detention volume. The effluent can then be safely conveyed by open channel or pipe to a stable outlet from which it will ultimately be discharged back into the Campbells Run.

Due to the extensive subsidence that has occurred over the mined area, the temporary treatment ponds will be constructed at locations beyond the mine limits to prevent potential subsidence impacts or infiltration of water back into the mine.

5.3 Acid Rock Handling

Recent pollution incidents were caused by leaching of pyritic materials from rock from highway excavations. These incidents have led the PADEP to require rock anticipated to be removed during highway excavations to be evaluated for acid-generating potential.

In order to assess the acid-generating potential of rock strata to be excavated for the construction of Ramp C and Ramp H, soil and rock core samples were tested for Sulfur Forms, which included pyritic sulfur, sulfate sulfur and organic sulfur. Pyritic sulfur is considered the main source of acid generation.

The test results for the rock core samples along Ramp H indicate that potentially acid-generating rock will be encountered in four different rock layers: a 5.5’ thick sandstone layer, a 0.6’ thick coal layer, a 1’ layer of soft shale, and a 1’ claystone layer. The sandstone is located above the Pittsburgh Coal Formation and the shale and claystone layers are located below the Pittsburgh coal.

The test results for the samples along Ramp C indicate that this cut will encounter potentially acid-generating rock in three different rock layers: a 1’ thick sandstone layer, a 1’ thick carbonaceous shale layer, and a 2’ thick layer of mine gob. The sandstone and shale are located above the Pittsburgh coal in the Pittsburgh Sandstone Formation. The mine gob is located within the Pittsburgh Coal.

The acid-generating rocks identified for Ramp C and Ramp H will require blending with Class 1 excavation materials in order to reduce the likelihood of post construction AMD seeps or discharges from the Contractor’s approved off site waste areas. Currently, the project earthwork balance is in a waste situation with approximately one million cubic yards of waste generation. Minimal embankments are proposed and are not being considered for blending due to their minimal size.

A 0:5H:1V cut slope was recommended for the Ramp H hillside. Excavating the proposed cut slope at a 0.5H: 1V slope will reduce the total excavation volume to approximately 61,000 cubic yards, compared to 139,000 cubic yards based on a 1:1 slope. The total volume of acid-generating materials is approximately 759 cubic yards, which is approximately 1.2% of the total excavation volume. The acid-generating materials will be thoroughly mixed with the surrounding excavated rock during the Class 1 excavation process. The mixed materials will be wasted off-site at the Contractor’s approved waste area.

The total volume of potential acid-generating materials likely to be excavated from the cut for Ramp C is estimated to be 35,455 cubic yards, which consist of the two rock layers identified and the gob materials. The total Class 1 excavation quantity for the Ramp C cut is approximately 644,273 cubic yards, based on a 1:1 cut slope. The total volume of acid-generating materials is approximately 5.5% of the Class 1 excavation volume in this cut area. The overall Class 1 excavation is considered to be net non acid-generating, therefore, blending of the rock material during normal Class 1 excavation process is considered adequate in reducing the potential of acid generation from the off site waste areas. Excavated acid-generating materials will need to be blended thoroughly with available non-acid generating materials during the Class 1 excavation process. These materials will be thoroughly mixed, blended and compacted during placement at the Contractor’s approved off site waste area.

Coal materials excavated are anticipated to be part of the construction coal removal process. This material will be encapsulated or blended in off-site waste areas above the ground water table, for “not removed as marketable coal”. Class 1 excavation blended rock material will be placed on top of a layer of non-acid generating rock. Grading and compacting the blended rock, and covering the blended rock with a layer of compacted clayey soil from Class 1 excavation will reduce the potential for acid generation from the waste area by reducing water infiltration and oxygen diffusion into the materials. The blended rock materials should be placed above the water table at any of the Contractor’s approved waste areas.

6.0 Conclusions

The mine treatment measures have proven to be reasonably successful for the past 40 years. However, they are beginning to show their age. The drainage system in one area was partially plugged, and in the other area the slope pipe has completely deteriorated. Recent mine pool surges associated with a hurricane event have created new seepage areas on the slopes.

Improvements have been made to provide more positive drainage systems behind the seals. The seals themselves will be constructed of the same excavation material as the existing seals. This should improve the overall performance of the mine seals without significantly increasing the cost for the system.

The major differences between construction 40 years ago and today relate to more stringent environmental controls. Had this been a completely new construction, the drainage would not have been directed to the roadway drainage system. A completely separate drainage outlet and discharge point would have been created. In addition, there is heightened concern for environmentally sensitive handling of all materials during construction, including excavated rock and mine gob as well as water.

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