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CCR landfill final cover test pad—Part 1

August 1st, 2020 / By: / Feature

By Nina J. Balsamo, John Massey-Norton, John R. Klamut, Terry Queen,
Charles F. Straley and Mark R. Lehner

An integrated drainage system (IDS) geomembrane, which provides both a low-permeability membrane and drainage space above the geomembrane, is being used in the final cover at a coal combustion residuals (CCR) landfill in West Virginia. Part 1 of this article presents construction in 2016 of the test pad (Figure 1) to evaluate 1-inch (2.5-cm) maximum subgrade protrusions beneath the geomembrane, placement of cover soil with a 9-inch (22.9-cm) maximum particle size using a low ground pressure (LGP) dozer, and the condition of the geosynthetics. Part 2 of this article will present the application of heavy traffic loads over the benched portion of the test pad to evaluate its use as a temporary haul road (in 2016), and additional exhumation of the test pad in 2018.

FIGURE 1 Test pad for a landfill final cover and surrounding area

Background and purpose

GAI Consultants Inc. (GAI) is the engineer of record for an active landfill located in West Virginia and owned by a North American electric utility company. The landfill is permitted for disposal of fly ash, bottom ash and synthetic gypsum by the West Virginia Department of Environmental Protection (WVDEP). The approximate surface area of the completed landfill will be 192 acres (77.7 ha). 

A final cover system design was submitted to the WVDEP to meet the requirements of 40 Code of Federal Regulations Part 257.102, “Criteria for conducting the closure or retrofit of coal combustion residual (CCR) units” (CFR 2015). 

The Geosynthetic Institute’s GRI Guide GS11, “Standard Guide for Constructing Test Pads to Assess Protection Materials Intended to Avoid Geomembrane Puncture” (GRI 2012) provides guidance on methodology and construction of field test pads to assess puncture performance of geomembranes. Based on the GRI Guide (modified), a 46-foot (14.0-m) wide by 103-foot (31.4-m) long test pad, consisting of a lower and upper bench, and separated by a 3H:1V slope, was constructed at the landfill site to establish appropriate specifications for the landfill final cover design and construction. 

The purpose of this article is to describe the materials, procedures and equipment used to construct the landfill final cover system test pad and the evaluation of the completed test pad.

Test pad description

The test pad layers followed the requirements of the WVDEP permit and U.S. Environmental Protection Agency (EPA) rule for a landfill final cover, and included the following (from the bottom up):

  • Earthen subgrade
  • 50-mil (1.27-mm) high-density polyethylene (HDPE) integrated drainage system geomembrane
  • Minimum 8-ounce/square-yard (271-g/m2) nonwoven IDS geotextile 
  • A 6-inch (15.2-cm) diameter perforated bench collection pipe 
  • Minimum 18-inch (45.7-cm) thickness of final cover soil over the IDS geomembrane and geotextile

The IDS geomembrane is manufactured to provide a studded drainage surface on the top side and a spiked friction surface on the bottom side. When covered with a geotextile, a drainage layer is created between the geotextile and IDS geomembrane sheet, as shown in Figure 2

FIGURE 2 Final cover system cross section. Note drainage space between geotextile and IDS geomembrane. (Not to scale, figure courtesy of AGRU)

AGRU Super Gripnet 50-mil (1.25-mm) HDPE liner was selected as the IDS geomembrane and Agrutex 081, a nonwoven IDS geotextile, was selected for the test pad. AGRU specifications require the geotextile overlying the IDS geomembrane to be nonwoven.

Subgrade protrusions of 1 inch (2.5 cm) and final cover soil rocks up to 9 inches (22.9 cm) in dimension were used in the test pad to determine whether these features would damage the IDS geotextile or puncture the IDS geomembrane. One half of the test pad was constructed in two lifts: 12-inch (30.5-cm) lift overlain by a 6-inch (15.2-cm) lift (Figure 3), and one half of the test pad was constructed in one 18-inch (45.7-cm) lift (Figure 4). The additional 18 inches placed over the lower bench (Figures 3 and 4) will be discussed in Part 2 of this article. The test pad was two geomembrane roll widths wide, minus overlaps to seam. As the roll width of the IDS geomembrane is 23 feet (7.0 m), the test pad is 46-feet (14.0-m) wide. It is recommended by the GRI Guide GS11 that the test pad width be at least 100% greater than the equipment width for each tested condition. The crawler tractor (dozer) used on the test pad was 8.2-feet (2.5-m) wide; therefore, the GS11 guidance on width was met for the use of the dozer. 

FIGURE 3 Test pad cross section, subareas C, E and A

The test pad length consisted of one 20-foot (6.1-m) wide upper bench, a 63-foot (19.2-m) slope length, plus one 20-foot (6.1-m) wide lower bench, for a total length of 103 feet (31.4 m). The slope length of 63 feet (19.2 m) exceeded the GS11 length requirement of 300% of the placement and compaction equipment length of 12 feet (3.7 m). The test pad was constructed in September and October of 2016.

FIGURE 4 Test pad cross section, subareas D, F and B

Processed and unprocessed soil

To obtain final cover soil with a maximum particle size of 9 inches (22.9 cm), roots and rocks larger than 9 inches (22.9 cm) were removed. This sub 9-inch (22.9-cm) particle soil was referred to as “unprocessed soil.” 

To prepare “processed” soil, a root rake was used to remove rocks, roots and woody material greater than 4 inches (10.2 cm) (Figure 5). Both processed and unprocessed soils classified as gravelly lean clay with sand (USCS CL).

FIGURE 5 Root rake used to remove particles larger than 4 inches (10 cm) from processed soil

Test pad subareas

The test pad layout is shown in Figure 6. The test pad was divided into subareas to allow testing of various combinations of conditions, such as the subgrade protrusion height, cover soil lift thickness, cover soil maximum rock size and subareas with cushion geotextile. The resulting combinations of materials tested in the various test pad subareas are presented in Table 1. Test conditions for adjacent Subareas C and E were identical, and test conditions for adjacent Subareas D and F were identical. 

FIGURE 6 Test pad schematic—2016

Construction of subgrade, geosynthetics and cover soil

Subgrade preparation and subgrade protrusion placement
The subgrade was proof-rolled prior to installation of the geomembrane. Since proof-rolling resulted in a smooth subgrade, and one purpose of the test pad was to evaluate the effect of subgrade protrusions, American Association of State Highway and Transportation Officials (AASHTO) No. 57 angular, crushed aggregate was scattered over a small area within each subarea of the prepared subgrade. One larger rock was also placed in each subarea near the No. 57 aggregate to create approximately 1-inch (2.5-cm) protrusions. The center of each area of placed aggregate was surveyed so that it could be located during subsequent exhuming of the test pad. 

TABLE 1 Combinations of materials tested

IDS geomembrane and IDS geotextile installation and testing
The IDS geomembrane was installed over the prepared subgrade with the embossed spikes facing down against the subgrade soil and the drainage stud side facing up. The two roll widths of liner were welded along the centerline of the test pad. The IDS geotextile was installed on top of the IDS geomembrane to create a drainage layer between the IDS geotextile and the IDS geomembrane. The geotextile panels were installed parallel to the slope and sewn together. 

Cover soil preparation
Large rocks were present in the cover soil, but it was unknown if or where they would contact the geosynthetics, and there were no plans to exhume the entire test pad. Therefore, a hard sandstone rock up to 9 inches (22.9 cm) in dimension was placed directly on the IDS geotextile within each subarea. These large rocks were placed in the same vicinity as the subgrade protrusions, but slightly offset. This would allow the effects of rocks both above and below the geosynthetics to be examined at the same exhumed location. 

Placement of cover soil
Cover soil was placed in two lifts of processed soil over Subareas A, C and E and in one lift of unprocessed soil of Subareas B, D and F (Figure 7). Each lift of cover soil received four passes from a crawler tractor (LGP dozer). This dozer had a ground pressure of 4.8 psi (33.1 kPa). After compaction, GAI performed nuclear moisture-density tests on the cover soil. 

FIGURE 7 Unprocessed soil placed over Subareas D and F and compaction of 12-inch (30.5-cm) lift of processed soil over Subarea A.

Initial evaluation

The cover soil was exhumed from sloped Subareas A and B where the subgrade protrusions and large cover soil stones had been placed. GAI observed that the geotextile was not torn at any of the exhumed locations. 

After the cover soil was carefully removed and the geotextile examined, the geotextile was manually cut and pulled back to expose the underlying geomembrane. Visual observation of the exposed IDS geomembrane indicated subgrade protrusions, but the geomembrane had not been punctured at any of the exhumed locations.

2016 conclusions and regulatory approval

Based on the test pad construction and 2016 evaluation, the following conclusions were made:

  1. When 12 inches (30.5 cm) or 18 inches (45.7 cm) of final cover soil (processed or unprocessed) was placed and compacted on a 3H:1V slope with four passes of an LGP dozer with ground pressure less than 5 psi (34.5 kPa), subgrade protrusions of 1 inch (2.5 cm) did not puncture the 50-mil (1.25-mm) HDPE IDS geomembrane; therefore, the test met the requirements for final cover placement.
  2. When unprocessed final cover soil containing particles up to 9 inches (22.9 cm) in dimension was spread on a 3H:1V slope and compacted to a minimum 85% Standard Proctor Maximum Dry Density (SPMDD), the 8-ounce/square-yard (271-g/m2) nonwoven IDS geotextile was not noticeably damaged and the 50-mil (1.25-mm) HDPE IDS geomembrane was not punctured; therefore, this test met the project requirements for final cover placement.

Based on the test pad construction observations, project specifications allowed maximum 1-inch (2.5-cm) subgrade protrusions (but not sharp objects), as approved by the quality assurance engineer; final cover placement required a maximum equipment ground pressure of 5 psi (34.5 kPa) during placement of a 12-inch (30.5-cm) or 18-inch (45.7-cm) lift thickness, and the specifications for final cover soil stated, “Final cover soil shall be a soil matrix which shall not have rocks exceeding 9 in (23 cm) in any dimension.” 

The final cover specifications (based on the test pad evaluations), the test pad report, and the design report for the landfill final cover system were submitted to WVDEP in December 2016 and approved by WVDEP. Construction of the IDS geomembrane final cover was initiated in 2018, and more than 16 acres (6.5 ha) have been constructed to date.

References

Balsamo, N. J; Massey-Norton, J.; Klamut, J.; Queen, T.; Straley, C.; and Lehner, M. (2019). “CCR landfill final cover test pad.” Proc., The World of Coal Ash Conference 2019, University of Kentucky, Center for Applied Energy Research, Ash Library. http://www.flyash.info/2019/072-paper.pdf (Accessed 5/29/2020)

Code of Federal Regulations (CFR). (2015). 40 CFR Part 257.102, “Criteria for conducting the closure or retrofit of coal combustion residual (CCR) units,” April 17 (EPA rule). 

Geosynthetic Research Institute (GRI) (2012). GRI Guide GS11, “Standard guide for constructing test pads to assess protection materials intended to avoid geomembrane puncture,” Oct. 19. https://geosynthetic-institute.org/grispecs/gs11.pdf (Accessed 5/29/2020)  

Nina J. Balsamo, P.E., is a senior project engineer at GAI Consultants Inc. based in Pittsburgh, Pa.

John T. Massey-Norton is a retired hydrogeologist at a major electric
utility company.

John R. Klamut, P.E., is a senior project manager with GAI based in Pittsburgh, Pa.

Terry Queen is senior lead construction technician with GAI based in Charleston, W.V.

Charles F. Straley, P.E., P.S., is a senior engineering manager at GAI based in Charleston, W.V. 

Mark R. Lehner, P.E., is a senior engineering manager at GAI based in Pittsburgh, Pa.


Project Highlights

Coal Combustion Residuals Test Pad

Design engineer: GAI Consultants Inc. 

Geosynthetics products: Agru Super Gripnet® 50 mil HDPE liner and AgruTex 081 nonwoven geotextile

Geosynthetics manufacturer: Agru America Inc.