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, in the August/September 2020 issue of Geosynthetics magazine, presented construction of the test pad 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 evaluation of the geosynthetics in 2016. Part 2 of this article presents 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.
Test pad subareas
As presented in Part 1, AGRU Super Gripnet 50-mil (1.27-mm) high-density polyethylene (HDPE) liner and AgruTex 081 nonwoven geotextile were incorporated into a landfill final cover system. To evaluate various conditions, a test pad divided into several subareas was constructed. The subgrade protrusion height, cover soil lift thickness, cover soil maximum rock size and subareas with cushion geotextile are listed in Table 1 on page 20. Test conditions for adjacent Subareas C and E were identical, and test conditions for adjacent Subareas D and F were identical.
To evaluate the effect of subgrade protrusions, the American Association of State Highway and Transportation Officials (AASHTO) No. 57 angular, crushed stone aggregate and one larger rock were placed within each subarea to create approximate 1-inch (2.5-cm) protrusions. The geomembrane and geotextile were installed, and 18 inches (45.7 cm) of soil was placed over each subarea using an LGP dozer.
Presuming construction equipment may need to utilize the benches of the completed final cover for access, a temporary access road investigation was performed. The investigation consisted of the placement of an additional 18 inches (45.7 cm) of cover soil (3 feet [0.9 m] total) on the lower test pad bench, then trafficking the bench with a fully loaded 45-ton (40.8-tonne) articulated truck. Half of the bench was constructed with cushion geotextile beneath the IDS geomembrane and the first 18 inches (45.7 cm) of cover soil, with a maximum particle size of 4 inches (10.2 cm); the other half had no cushion geotextile and cover soil with a maximum particle size of 9 inches (22.9 cm). The resulting combinations of materials tested in the various test pad subareas are presented in Table 1.
2016 temporary access road construction and investigation
After the initial 18 inches (45.7 cm) of soil was placed using an LGP dozer over the lower bench of the test pad, an additional 18 inches (45.7 cm) of nonprocessed (maximum 9-inch [22.9-cm] particles) soil was placed on lower bench Subareas C, D, E and F, for a total cover thickness on the bench of 3 feet (0.9 m). A fully loaded Cat 745C articulated truck with Michelin 29.5R25 tires at 65 psi (448 kPa) tire pressure then made 100 passes over the bench with the 3-foot (0.9-m) thickness of cover (Figure 1).
Afterward, portions of the trafficked bench were exhumed. GAI Consultants Inc. (GAI) observed the geotextile was not torn at any of the exhumed locations. The exposed geotextile was then manually cut and pulled back to expose the IDS geomembrane. Protrusions were visible in the IDS geomembrane, but the IDS geomembrane was not punctured at any of the exhumed locations. The exhuming activities are shown in Figure 2. After observation, the geotextile was patched and the cover soil replaced.
After construction, soil samples were collected from processed and unprocessed soil on the test pad. Processed and unprocessed soils were classified as Gravelly Lean Clay with Sand (USCS CL).
Based on the temporary access road test pad construction and 2016 evaluation, it was concluded that where 1-inch (2.5-cm) angular protrusions were placed on the subgrade, and 3 feet (0.9 m) of unprocessed final cover soil containing rocks up to 9 inches (22.9 cm) were placed over the IDS geotextile and geomembrane and trafficked with 100 passes of a loaded 45-ton (40.8-tonne) truck, the 8-ounce/square-yard (271-g/m2) nonwoven geotextile was not noticeably damaged and the 50-mil (1.27-mm) HDPE IDS geomembrane was not punctured; therefore, this test met project requirements for final cover placement on trafficked benches. This procedure was included in the project specifications and approved by the West Virginia Department of Environmental Protection (WVDEP) as part of the landfill closure permit.
2018 test pad evaluation
Although it was originally expected that the test pad geomembrane would be included in the final cap, it was later decided that it would be more cost effective to remove the test pad rather than to attempt seaming it to new surrounding geomembrane. This provided an opportunity to exhume and view larger areas of geosynthetics, and to view the underside of the geomembrane nearly two years after test pad construction. Since carefully exhuming the soil and geosynthetics was an added expense to the project, the client asked that only limited areas be carefully exhumed; the remainder of the test pad would be removed less carefully.
The carefully exhumed areas included locations where rocks had been previously placed above and below the geosynthetics, and where the geotextile was patched from the partial exhumation in 2016. Survey equipment was used to locate these areas.
Subarea B excavation
The soil cover for Subarea B was approximately 18-inches (45.7-cm) thick; therefore, soil was excavated using an excavator to a depth of 1.2 feet (36.6 cm), then hand shoveled to locate the geotextile. Once the geotextile became visible, the excavator carefully excavated nearly all the way to the geotextile. The cover soil had occasional large rocks but was predominantly soil, and large rocks were generally not in contact with the geotextile. The soil just above the geotextile was removed by hand with shovels, and the geotextile was swept. The 2018 Subarea B excavation was 7 feet (2.1 m) × 6.5 feet (2.0 m), which was significantly larger than the 2016 geotextile patch.
The geotextile was examined, then removed. Bumps on the upper surface of the geomembrane clearly indicated where stones had been placed on the subgrade prior to geomembrane installation. No signs of damage to the geotextile or IDS geomembrane were identified.
The geomembrane was then cut along the edges of the excavation, removed and flipped over. A grid of holes from the geomembrane bottom spikes was readily apparent in the subgrade, as was a scattering of No. 57 aggregate and one medium-sized stone (Figure 3), which accounted for the depressions in the geomembrane. The medium-sized subgrade stone had subrounded corners. The No. 57 stones were angular but were pushed about halfway into the subgrade. The underside of the geomembrane had obvious depressions where it had been over the subgrade stones, but there were no scratches or other signs of geomembrane damage (Figure 4).
Subarea F excavation
The exhumation of Subarea F proceeded in a similar manner. The goal at this location was to examine the geosynthetics in the path of the tires where the loaded haul truck had made 100 passes in 2016. The 2018 Subarea F excavation was 6.7 feet (2.0 m) × 11.5 feet (3.5 m). Again, the area was excavated partway down with the excavator; then the general depth of geotextile was located by hand shoveling. Note that the location of the 2016 geotextile patch in Subarea F had previously been exhumed (after trafficking) and re-covered with soil in 2016. Two large rocks were found close to, but not directly on, the geotextile. The cover soil was again predominately soil but had more large rocks than the cover material at the Subarea B excavation. The exposed geotextile appeared to have been previously cut to examine the underlying geomembrane, as would be expected from the 2016 test pad study, but the geotextile in the area over the subgrade rocks also had two small tears (Figure 5) that had not been recorded during the 2016 exhumation. The remaining area of geotextile showed no sign of damage.
After the top surface of the geotextile was examined, the geotextile was removed. The upper surface of the IDS geomembrane was examined, and the IDS geomembrane was removed. The impressions of the underlying No. 57 aggregate and one larger rock were easily visible on the underside of the geomembrane (Figure 6). The two small ruptures completely penetrated the geomembrane, but there was no other detected damage to the geomembrane, not even scratches.
Closer examination of the geomembrane ruptures indicated they may have been cut from the topside, as evidenced by slight scratching and burring along the topside of the geomembrane. Also, the geotextile tears mirrored the ruptures in the underlying geomembrane, indicating that both geosynthetics may have been accidentally cut through during the exhumation. The rock beneath the ruptured geomembrane protruded more than 1 inch (2.5 cm) (Figure 7), exceeding specification allowance.
Subarea D excavation
Exhumation of an additional portion of the test pad bench was performed and observed. The geotextile and geomembrane over a subgrade protrusion were removed and examined. Although a significant subgrade protrusion was found, no geomembrane or geotextile damage was observed.
2018 conclusions and recommendations
The 2018 exhumation of the geosynthetics further confirmed the 2016 findings. The following conclusions were made based on 2018 field activities:
Subgrade protrusions from scattered No. 57 angular, crushed aggregate did not damage the geomembrane on the slope or on the bench with the specified installation procedures.
A subrounded stone protruding approximately 1 inch (2.5 cm) did not damage the geomembrane on the slope with the specified installation procedures.
Geomembrane spikes were well embedded into the subgrade side slope.
It is inconclusive whether the small ruptures in the Subarea F bench geomembrane were caused by the angular subgrade rock or were from damage caused by twice removing the overlying materials; however, the subgrade rock beneath the ruptures protruded more than 1 inch (2.5 cm).
Although there were angular rocks as large as 9 inches (22.9 cm) in dimension in the cover soil, they generally were not in contact with the geotextile because they were suspended in a soil matrix. They did not damage the exhumed geotextile with the specified installation procedures.
2020 update
More than 16 acres (6.5 ha) of the final cover system have been constructed at this site to date using these materials and procedures. No apparent issues have occurred during the installation of the final cover system. In addition, no issues with the installed final cover system have been identified since the end of construction.
Overall conclusions/limitations
The construction of test pads in general accordance with the Geosynthetic Research Institute (GRI) Guide GS11 may be used to demonstrate that typical stone/protrusion size requirements may be modified to allow for less conservative requirements for the construction of final cover systems. For the soils and rock types present at the subject site, subgrade protrusions up to 1 inch (2.5 cm) (but not sharp objects) were acceptable and rocks up to 9 inches (22.9 cm) in the cover soils were also acceptable. The use of test pads to evaluate site-specific soils and liner configuration can provide significant cost savings by reducing screening and handpicking of rocks from subsoils and cover soils. The method should be employed on a case-by-case basis, as results may vary depending on the subgrade material, geosynthetic materials, soil type and borrow sources.
In addition, the potential for long-term stress cracking may not be readily observed during test pad construction/observation but should be considered in final design recommendations based on site-specific conditions. Last, a robust construction quality assurance and construction quality control program is of the utmost importance so that full-scale construction is consistent with the methods and criteria used for the test pad construction.
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.
All figures courtesy of the authors.
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).
SIDEBAR: 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.