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Interstate Technical Group on Abandoned Underground Mines
|Glenshaw||Upper Freeport Coal|
The Pittsburgh Coal was the most widely mined coal seam that affected the Expressway, although both deep and strip mines were encountered in the Redstone Coal, Waynesburg Coal, and Washington Coal. For the most part, overburden rock of the Pittsburgh Coal consists of laminated shales and sandstones which are subject to raveling and collapsing upon loss of support due to coal removal.
The same year the Expressway received environmental clearance, the Pennsylvania legislature passed Act 54, which changed the rules by which underground coal mine operators could mine and impact surface features. No longer were they required to protect structures and other features from damage. Instead, they could compensate for or repair damages after mining. This allowed operators to convert from old, room-and-pillar techniques to long-wall mining. It has been reported that sections of Interstate 70, near the southern terminus of the Expressway's first construction sections, have been subsided as much as 5 feet by mining since the passage of this act. Allowance for subsidence in the Act and the desire not to have subsidence under the Expressway led to a disagreement with an active coal mine operator as to financial and operational damages from construction of the roadway. Legal issues are not the subject of this paper. However, the Expressway also crossed through the surface facilities of the mine, creating technical problems discussed below.
The PTC recently completed two other major expansion projects, the James E. Ross (Beaver Valley) Expressway (Route 60) and the Amos K. Hutchinson (Greensburg) Expressway (Route 66). Both of these projects also traversed Pennsylvania's bituminous coal field, and required treatments addressing underground coal mine impacts. The PTC had also been performing major renovations along its mainline (Interstate 76) in this area, dealing with underground mines. During its original construction through this area over 50 years ago, the PTC took precautions against impacts from coal mines. All of this practical experience came to bear on the approach to designs related to underground coal mines on this project.
The construction of the Mon-Fayette Expressway in Washington and Allegheny Counties, PA, required addressing the issues related to underground coal mining. A basic design philosophy was adopted which differentiated first between active and abandoned mines underground, and second between structures and roadways at the surface. Active coal mines required negotiations not only for purchase of support coal and changes to MSHA-permitted surface activities, but also coordination of construction activities above ground with mining activities underground. Abandoned mines had the potential for subsidence, exposure, acid mine drainage, and mine fire problems.
Initially, design guidelines for treatment of abandoned mines were developed and issued to the prime consultants. These design guidelines were developed considering the PTC's experience with underground mines, understanding of the technical aspects of the issues, and benefit-risk cost comparisons. The guidelines evolved into a series of generic special provisions for underground mine-related roadway construction. Underground mine-related special provisions for structures were developed individually, considering the unique conditions presented in each case.
Original Guidelines for Structures - Recommendations to treat mines were defined in terms of depth to the coal seam and area of treatment. Wherever coal was found from 0 to 100 feet below the bottom of foundations, consultants were directed to recommend treatment to achieve 100 percent support. Depths greater than 100 feet were to be evaluated on a case-by-case basis. A second depth of 400 feet was provided, beyond which treatment would only be considered if the consultant "strongly believes treatment is warranted."
The area of treatment should be based on the entire structure, not just individual substructures. Projecting angles for treatment outside the structure footprint were established based on depth to the seam:
|Depth to Coal||Angle|
|0 - 100 feet||35 degrees|
|100 - 300 feet||25 degrees|
|300 - 400 feet||15 degrees|
Other widths would be considered where the consultant "strongly believes treatment is required."
While these guidelines did not directly reflect coal seam subsidence characteristics, they did represent an improvement from Pennsylvania's historical use of treatment to 50-percent support within a projected angle of 15 degrees starting 15 feet from the structure footprint.
Final Guidelines for Structures - The depth to the underlying mine still determined the method of Stabilization for the bridge foundations. However, the areal extent of treatment was modified from the entire structure to individual substructure units. Also, the types of treatment became more standardized.
If the depth to the underlying mine was less than 50 feet below the bottom of footing elevation for the substructure, a deep-mine undercut was performed where the overburden between the substructure and the deep mine was removed. Then, depending on the specific conditions, either spread footings were placed below the mine level and the backfill placed, or the excavation was backfilled with specially selected material and then end-bearing piles were then driven into the stratum below the mine. In locations that the depth was greater than 50 feet below the bottom of footing elevation for the substructure, a detailed subsurface Stabilization (grouting) plan was developed to fill the mine openings, overburden voids and subsidence fractures. Footings or piles were either placed to bear on the grouted rock, or piles were placed in holes drilled through the grouted rock to bear on the stratum below the mine.
Original Guidelines for Roadways - Design considerations for the construction of roadways considered mine conditions both above and below grade. Above-grade abandoned mines would be exposed by excavations for the roadway. Along with the subsidence condition, local dip of the mine and the presence of water which could produce acid-mine drainage were critical to evaluation. Coal exposures had to be drained if water was present, and large openings had to be closed to prevent unauthorized and unsafe entry into the workings. Above-grade workings had been successfully addressed in several previous PTC projects, so special provisions for construction and treatment details were provided.
Coal mines below grade were again classified by depth. Those from 0 to 50 feet below grade were to be treated to ensure 100 percent support. Below 50 feet, treatment was to be provided on a case-by-case basis. At depths below 100 feet, only cases where the consultant "strongly believes treatment to be necessary" would be considered.
The area to receive treatment was more difficult to define, given consideration for both roadways on embankment and roadways in cut. For embankment areas, the treatment was to be within an area defined by the greater of the original right-of-way width or 100 feet horizontally beyond the outside shoulders. In cut areas, the treatment area was defined by projection of a 15-degree angle from the bottom of cut to the coal seam. Other widths would be considered where the consultant "strongly believes treatment is necessary." In all cases, treatment was to be limited to over-excavation and backfill.
Final Guidelines for Roadways - As with the structure guidelines, the roadway guidelines were modified during the design process. For roadways, the type of treatment became more flexible.
If the depth to the mine was less than 50 feet below road grade, a deep-mine undercut was performed, similar to that for structures. This involved the excavation of overburden materials and inspection of the exposed mine. The removal of the coal mine was not required if all voids were removed or fully collapsed during excavation. Intact coal typically was only removed if it was within five feet of the final subgrade elevation. A blanket of rock was placed at the bottom for drainage, and the excavation was then backfilled with normal embankment material.
In areas where the depth to the mine was greater than 50 feet below the road grade, nothing was done during the earthwork stage of construction to stabilize the mine. However, in some areas where the depth to mine was greater than 50 feet below grade, flexible pavements have been used on the Mon-Fayette Expressway.
Original Guidelines for Active Workings - Originally, guidelines for active workings only addressed support issues. The guidelines just applied the same criteria presented for roadways and structures for defining the minimum extent where coal removal should be limited.
Final Guidelines for Active Workings - Much more design coordination with mine operators was required than was originally envisioned. In addition to limiting coal extraction locally, surface activities were affected. These activities, performed under State and Federal permits, included coal haulage and disposal. Impacts to these operations required not just developing new designs, but also filing modifications to the permits.
One particular activity addressed during was the construction of the roadway through an existing and active fine-coal refuse slurry pond. The slurry pond consisted of clay-sized particles of coal, shale and sandstone that were the byproduct of a coal preparation plant. The original roadway design required the construction of a temporary dike through the pond and the removal of the fine coal refuse to construct the proposed roadway embankment and a new slurry pond dam. In addition to the analyses to satisfy the PTC's stability concerns, analyses were needed to satisfy the concerns of the mining operator, Pennsylvania Department of Environmental Protection, and MSHA. The design had to address the potential loss of waste storage capacity, by increasing the total slurry pond height to offset the areal loss.
Original Guidelines for Secondary Impacts - Secondary impacts of underground mining of concern during design were mine fires and acid mine drainage (AMD). The primary principle was to avoid or minimize contact with either of these. No mine fires were anticipated to be encountered. Where AMD was to be encountered, the guideline was "don't make it any worse than existing conditions." If necessary, the designer was directed to provide special drainage pipes for acid environments. Provisions were made to monitor and document water flow and quality before, during and after construction to verify the guideline was met. Short-term mine water discharges during construction were handled through a special provision which called for treatment (if necessary) and controlled release of this water.
Final Guidelines for Secondary Impacts - Handling of AMD was not modified significantly during design. Coal seams in roadway excavations above grade were to be treated by excavating benches and backfilling to the original slope line with durable rock. The benches were graded to drain any water into ditches or roadway drainage. Mine water anticipated to be encountered at or below grade was assumed to be handled by drainage systems installed for groundwater control.
Modifications Through Construction
Application of the Guidelines for Structures - The use of deep mine undercuts provided the most sound and conservative means of Stabilization for bridge foundation construction. When the coal and overburden materials were removed, conditions could be visually inspected to ensure proper removal of all unstable foundation material prior to backfill. The major concern with the method was the potential for settlement of backfill materials, especially where additional embankment was to be placed such as in the abutment areas of the bridges. This was handled through proper construction control.
The use of subsurface Stabilization grouting plans was considerably more difficult and costly. One reason is that the mine conditions could never be visually inspected during construction. In preparation for the grouting plan, the inferred location of the deep mine from local mine maps typically was plotted on the foundation plans and then a grouting plan was designed to stabilize the foundation footprint for the substructure units. Although the mine maps were fairly accurate, they were not always plotted accurately on the foundation plans. This was typically due to the lack of reliable surface references on the mine maps. The designs that used these maps to produce grids of grout holes for the location of mine voids and subsurface subsidence fractures upon actual drilling sometimes found predominantly intact coal instead of open-mine conditions. This ultimately made it more costly to locate subsurface voids and fractures for Stabilization, by the addition of grout holes. It was determined in the field that use of primary grid patterns in the influence area of the foundation footprint should be completed without reference to the provided mine maps and then conditions analyzed and treated by performing secondary or offset holes in areas of intact coal.
Additional problems associated with the subsurface Stabilization plans were the subsidence conditions encountered. Generally, the mines encountered during construction were either collapsed (fully subsided) or apparently had been backfilled during mining operations. Most were completely flooded with water. Only the recently abandoned portions of active mines and main haulage ways of the older mines were found to be open voids.
Many of the designs required the use of low strength concrete grout mixes to be injected in areas of voids. The review of drillers' grout hole logs often indicated larger voids (6 to 10 feet) than those encountered during performance drilling (1 to 3 feet). Use of concrete grout in these smaller voids resulted in poor grout takes and thus poor subsurface Stabilization. The use of flowable fill grout without concrete with an 11-inch slump proved to be the best method of ensuring grout take in the smaller voids and fractures, and only use of a stiffer grout with a 5 to 7 inch slump or concrete grout to construct cutoff walls or curtains to prevent flow of less stiff grout from the influence areas.
Application of the Guidelines for Roadways - Most of the time, the guidelines and special provisions for the handling of underground mines worked well in roadway areas. In some locations, such as under small ancillary structures, it was required to remove all the coal in the roadway template. In other locations, it was decided to leave the coal in place and collapse the voids. This was done where an abundant amount of water was discovered at the mine level.
Application of the Guidelines for Active Mines - Contractors had to coordinate with mine operators for sequencing and scheduling of blasting and other activities with impacts on mine operations. Designs related to specific surface impacts had to be adjusted to account for changes in mine operations from the time of design.
For example, during the time from design to construction, the elevation of the fine-coal refuse slurry pond discussed above increased by over 8 feet. This presented a marked change in the schedule of construction as well as the safety of the construction of the temporary dike. The contractor proposed to stabilize the refuse in-place utilizing soil mixing. This involved the mixing of a high-early strength grout with the refuse to harden it to a consistency appropriate for foundation of roadway-embankment and slurry-dam construction. This work was performed using specially adapted tools affixed to an excavator and a crane-supported mixing apparatus. Depths of stabilized slurry ranged from 0 to greater than 50 feet. Design criteria required an unconfined compressive strength of 100 psi. In addition to the revised design, another permit modification was required to be prepared and submitted on behalf of the mine operator.
Application of the Guidelines for Secondary Impacts - During excavation of the largest deep mine undercut on the Mon-Fayette Expressway, a mine fire was encountered. The mine fire was discovered during the completion of blasting boreholes. The PTC and design engineers were notified of the mine fire, and a mine fire expert was consulted. Once the fire was delineated by a series of test holes, the contractor was directed to construct two parallel cut-off trenches along the edges of the undercut to prevent the advancement of the fire under the areas adjacent to the roadway and to excavate the heated and burning coal. The trenches had to be advanced beyond the excavation of the mine fire 'hot' materials. This change in the undercut excavation disrupted the contractor's planned method for constructing the undercut. The mine fire 'hot' materials had to be excavated and hauled to cooling pits outside the roadway limits, where they were water cooled prior to burial. Blasting agents also had to be modified for higher temperature use.
During construction, field personnel had flexibility to determine the optimum locations for out-letting any mine water encountered during excavations. Where significant amounts of mine water were encountered below grade, subsurface drainage wicks were constructed to either outlet water into existing drainage structures or through rock-fill galleries. In addition to the independent water monitoring programs, water was tested for pH and iron content at the location where it would enter the surface waters at the time of initial discharge to verify no special treatment was required.
At first during construction, design specifications were enforced as an attempt to provide the best product possible. However, it was learned early in construction that some of the proposed treatments were not practical to construct. As time progressed and the subsurface conditions were better understood, it also was determined that certain required specifications could be modified to enhance their utility. Specifications were modified through consultation with the PTC, designer engineers, and design and construction managers to allow more discretion during the construction activities. The construction manager was given the flexibility to change construction treatments with the variations in conditions encountered. The modified specifications for grouting provided the ability to adjust mix designs, grouting patterns, and scheduling of production. Modified specifications for over-excavation and backfill also allowed the construction manager to adjust limits to reflect mine conditions encountered.
The authors would like to thank the Pennsylvania Turnpike Commission, Michael Baker Jr., Inc., and TriLine Associates, Inc., for their assistance in preparation and presentation of this paper. We also appreciate the photos and other documents made available to us through the Trumbull Corporation.
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Michael Baker, Jr., Inc.; 1976; A Comprehensive Program for Dealing with Mine Subsidence Emphasizing Local Government Options; Appalachian Regional Commission and PA Department of Environmental Resources
Michael Baker, Jr., Inc.; 1995; Project Manual for Final Design, Mon/Fayette Transportation Projects; PA Turnpike Commission
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