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Accelerated geosynthetic closure of a mine refuse site

August 1st, 2016 / By: , , / Feature

Figure 1 (Above) Cambria Site 93, coal refuse pile in Somerset County, Pa. Image courtesy of Google Earth.
Figure 1 Cambria Site 93, coal refuse pile in Somerset County, Pa. Image courtesy of Google Earth.

Case history: Cambria Site 93 in Pennsylvania

Introduction

During the coal mining process—underground or surface—other unwanted soil and rock materials are mined along with the coal. A separation technology is used to best segregate the coal from the “refuse material.” After separation, various amounts of coal still exist in the refuse piles of material left behind. These piles are typically steep-sloped and have a dark gray color. Some piles with high commercial value have been re-mined for their coal content, while the historic industry practice for reclamation or closure has been to minimally regrade the piles and cap with a soil layer that will minimize water intrusion and promote the growth of vegetation.

The requirements have been changed in recent years to include a geosynthetic liner within the cap. There were reports that some of the initial cap liner installations were met with issues. In 2014, PBS Coals Inc. (PBS) raised the bar on effective closure of coal refuse piles. PBS is a subsidiary of Corsa Coal and has been a coal producer in Pennsylvania since 1963.

Close Site 93

In 2013, PBS began closure of the Cambria Site 93 refuse pile using a geosynthetic membrane (Figure 1). This site is located approximately seven miles east of Somerset, Pa., and one mile north of Pennsylvania Turnpike Route 76.

An aggressive implementation and construction schedule was put into place to ensure closure of the site in 2014. The first 25% of the cap was required to be completed through the entire cross section (liner, soil, seed) by May, 2014—a challenge in Somerset County considering that May snow is not uncommon in southwestern Pennsylvania.

Site description

The coal refuse pile to be closed encompassed an area of approximately 30 acres. The sideslopes were steep, with average slopes of 2.5H:1V. The pile was designed with these slopes to allow for maximum material storage capacity within the available footprint.

In addition to the steep slopes, a material referred to as “RecMix” was placed over the coal refuse as an alkaline addition and used as the subbase layer for the geomembrane. The RecMix presented additional design challenges because of its variable characteristics along with considerations of frost upheaval during the winter months.

Figure 2 (Left) Sliding of original closure system.
Figure 2 Sliding of original closure system.

Thus, the main design challenges for this system included the steep slopes, accelerated project schedule, and challenging subgrade characteristics. An initial attempt during 2011 to close the facility with a geosynthetic cap system was not successful (Figure 2).

The closure profile consisted of (bottom to top): a 6-ounce per square yard (oz/sy) nonwoven geotextile, 30-mil LLDPE geomembrane, 6-oz/sy nonwoven geotextile, and 12in. of topsoil. The initial closure plan was not an engineered solution and subsequently did not provide adequate shear strengths on the sideslopes. The geomembrane and geotextile interface was not of sufficient strength for the application in addition to a lack of drainage for the cover soil material above the geotextile layer.

Figure 3 View of intact nonwoven geotextile base layer with sliding geomembrane and nonwoven geotextile layer.
Figure 3 View of intact nonwoven geotextile base layer with sliding geomembrane and nonwoven geotextile layer.

The cover soil, top layer of geotextile and geomembrane experienced shear failure and the project was put on hold (Figure 3).

Second attempt deadlines

In October 2013, Pennsylvania Department of Environmental Protection (PaDEP) and PBS agreed on the following construction milestones, all on a finished-area basis, including topsoil placement and seeding: 25% completion by mid-May 2014, 50% completion by mid-June 2014, and 100% completion by mid-August 2014.

To meet the aggressive deadlines, PBS solicited design/build/certify proposals from teams of geosynthetic installers and engineering firms to develop cap system alternatives that would deliver acceptable short- and long-term sliding (veneer) stability while providing PBS with options to balance performance, cost, and regulatory approval.

The notice-to-proceed (NTP) was issued in late January 2014. Immediately upon NTP, a laboratory testing program commenced to evaluate the candidate geosynthetics, stockpiled topsoil resources, and RecMix for a variety of index and engineering design properties. Concurrent with the laboratory testing program, design analyses were prepared using presumptive engineering properties that were adjusted as testing was completed.

Figure 4 (Top) Option 1—Integrated drainage system closure. Figure 5 (Bottom) Option 2— Geomembrane with drainage composite.
Figure 4 (Top) Option 1—Integrated drainage system closure.
Figure 5 (Bottom) Option 2— Geomembrane with drainage composite.

Options

Due to the steep bench-to-bench pile slopes, a textured geomembrane was selected for use in the cap system. Two cap system cross sections were developed and evaluated during design.

Option 1 included a 50mil-thick integrated drainage system (IDS) high-density polyethylene (HDPE) geomembrane with one side of the material having conical spikes and the other side having semi-cylindrical studs, overlain by an 8-oz/sy nonwoven, needle-punched (NWNP) geotextile (Figure 4).

Option 2 consisted of a 40mil-thick embossed textured HDPE geomembrane overlain by a double-sided geocomposite drainage net (GDN) comprised of a 250mil-thick HDPE geonet core with 8-oz/sy NWNP geotextiles laminated to its upper and lower surfaces (Figure 5).

Both cap system options would have 1-ft soil covers, per the existing facility permit.

Testing

To support the design work, a laboratory testing program was developed to determine key engineering index and performance properties required to model what were expected to be “worst case” conditions for the cap system during both installation and while in-service.

The testing program included grainsize distribution, Atterberg limits, natural moisture content, standard Proctor moisture/density, remolded flexible wall permeability, remolded direct shear strength, and interface shear strength.

Samples of the various geosynthetic materials were obtained directly from the manufacturer, while samples of the in-place RecMix and cover soils were obtained from the pile surface and on-site stockpiles, respectively. A summary of the cover soil and RecMix test results is provided in Table 1.

 
Sample I.D. USCS Class W nat (%) Yd (max) W opt (%) K (cm/sec) degrees c (psf)
CvrSoil1 CL 19.3 118.5 13.3 1.3E-07 14.7 1,483
RecMix SC 26.2 112.9 16.1 9.6E-05 40.8 331

Table 1 Summary of Cover Soil and RecMix Laboratory Test Results. Note: Permeability and shear strength properties determined at 90% Standard Proctor maximum dry density and moisture content of optimum +5%.

The protocol developed for the interface shear strength tests included applied normal stresses of 100, 500, and 1,000 pounds per square foot (psf); testing all interfaces under wet (submerged) conditions; using a shear rate of 0.04 inches per minute (ipm); and running the tests to a minimum displacement of 2.5in.

Each of the interfaces were tested individually except for the cover soil/NWNP geotextile/geomembrane stud-side interface in cap system Option 1, which was tested in a “floating” setup due to limitations in being able to isolate and run the NWNP/geomembrane stud-side interface.

Figure 6 Summary of interface shear strength laboratory test results.
Figure 6 Summary of interface shear strength laboratory test results.

Results from the interface shear strength design tests are provided in Figure 6.

These test results indicated that the two cap system options had relatively similar performance characteristics, with Option 1 exhibiting better interface strength performance at the highest normal stress evaluated. Given the increased shear strength results, better transmissivity performance, the benefit of a 50mil-thick vs. 40-mil geomembrane, and since Option 1 had the advantage of a shorter installation time (important due to the completion milestone dates), cap system Option 1 was selected for use at the site.

Figure 7a Phase of closure construction.
Figure 7a Phase of closure construction.
Figure 7b Phase of closure construction.
Figure 7b Phase of closure construction.

Cap and closure

The laboratory testing and engineering design activities were completed by late February 2014 and submitted to PaDEP for expedited review and approval. The redesigned cap system included the integrated drainage system (IDS) geomembrane and an 8-oz/sy nonwoven, needle-punched (NWNP) geotextile as presented in Option 1; underlain with RecMix and overlain with 12in. of screened (4in. minus) material.

The PaDEP approval was received in mid-March 2014 and the selected geosynthetic materials were immediately manufactured, tested, and shipped to the site to support a mid-April construction start date. Construction commenced on time and the 25% (mid-May 2014) and 50% (mid-June 2014) completion milestones were both achieved one week ahead of schedule. The 100% completion milestone (mid-August 2014) was achieved four weeks ahead of schedule (Figure 7).

Figure 8a Placement of cover soil above geosynthetics.
Figure 8a Placement of cover soil above geosynthetics.
Figure 8b Placement of cover soil above geosynthetics.
Figure 8b Placement of cover soil above geosynthetics.

The installed cap system met the unique and challenging design, permitting, and construction requirements presented by the site and project schedule. High-interface shear strengths were provided by the geosynthetic components, thereby allowing the owner to pursue a geosynthetic capping option on the existing finish grade without needing to evaluate expensive material excavation, disposal, and regrading that would have added cost and workdays to the project (Figure 8).

The RecMix material was incorporated into the design, utilizing the results of site-specific laboratory testing with the proposed geosynthetic materials. During construction, design properties were verified by a CQA sampling and testing program. Ultimately, the project design and selected geosynthetic products produced acceptable shear strength during construction activities and beyond.

Figure 9a Completed cap, closure, and revegetation at Cambria Site 93.
Figure 9a Completed cap, closure, and revegetation at Cambria Site 93.

Conclusion

Figure 9b Completed cap, closure, and revegetation at Cambria Site 93.
Figure 9b Completed cap, closure, and revegetation at Cambria Site 93.

The Cambria Site 93 project was completed four weeks ahead of the final completion date, even while facing a late snowmelt and wet start to the spring season (Figure 9).

The success of this project can be attributed to the owner’s forward thinking to request proposals that included the design, construction, and certification under one contract, because this decision streamlined efforts, respectively, and positioned the project team for completing the project ahead of schedule. The expectations of the project were properly communicated and the contractor, installer, engineer, and manufacturer applied the correct amount of resources to ensure their respective participation in the project did not create delays.The success in 2014 prompted an additional closure project at a nearby PBS facility using the same design during the summer months of 2015. The successful performance of closures that used geosynthetic materials, specified through proper testing and design analysis, illustrate that the performance of closures of steep slopes often encountered with coal refuse piles are enhanced with geosynthetic materials. Specific materials such as integrated drainage geomembranes are available that can provide increased shear strength, increased transmissivity performance and decrease overall project costs. The geosynthetic industry continues to provide innovative solutions that enhance the containment capabilities of engineered solutions.

Bob Baker, P.E., is senior geoenvironmental engineering manager at Tetra Tech.Chris Eichelberger is vice president/technical marketing with Agru America.Matt Furniss, P.E., is assistant surface operations manager at Corsa Coal Corp.

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