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COAL MINE IMPACTS

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COAL MINE IMPACTS Powered By Docstoc
					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@mbakercorp.com                       lturko@mbakercorp.com

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.




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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.




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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.




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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.




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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.



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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.




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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.




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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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