By Anna M. Saindon
Coal ash pond closures often face unique challenges due to location, available borrow soils and construction constraints. These challenges can lead to alternative methods of closure to comply with Resource Conservation and Recovery Act (RCRA) Subtitle D and coal combustion residuals (CCR) regulations. This article summarizes the construction of the first permitted structured geomembrane/turf component system cap used in Illinois and briefly discusses the constructability benefits and challenges that should be evaluated during the design process of CCR pond closures.
At the beginning of the design stage, a site-specific feasibility analysis for alternative closure methods is suggested. When evaluating the capping-in-place method, the traditional soil/geomembrane cap should be evaluated against other methods such as turf products and include potential reuse options such as solar power generation in the life cycle evaluation.
A typical RCRA Subtitle D cap includes a smooth subgrade, geomembrane (typically 40-to-60-mil [1.0-to-1.5-mm] high-density polyethylene [HDPE]), geonet (if required by slope steepness and stormwater design), 2.5 feet (0.76 m) of cover soil, 0.5 feet (0.15 m) of topsoil and a vegetative cover. Since these are the current industry standard covers, the design and approval process is relatively straightforward. Typical Subtitle D covers have issues with erosion, high total suspended solids (TSS) in stormwater runoff, and difficulties establishing and maintaining vegetation. The availability of borrow soils, lost air space, difficulties in building partial closures, impacts to the surrounding population centers and limited land reuse options affect the cost-effectiveness of the typical Subtitle D cover option.
Turf products have advanced to meet the performance requirements of RCRA Subtitle D regulations and provide flexibility in areas with poor-quality soils or low soil availability, long-term maintenance at decommissioned facilities or economic challenges over the life cycle of the project. Turf products require a high level of communication with regulators in states where they have not yet been used, but may net a significant amount of time and monetary savings over the life cycle of the project. A structured geomembrane/turf component system was used in design and construction quality assurance for ash pond closures and will be the turf product discussed herein.
The structured geomembrane/turf component system involves a smooth subgrade overlain by a linear low-density polyethylene (LLDPE) or HDPE geomembrane (typically 40-to-50-mil [1.0-to-1.3-mm] thick), a geotextile with turf material (Figure 1 on pages 12–13), and a 0.5-inch (1.27-cm) sand or sand/concrete mixture. The structured geomembrane can have either large or small knobs depending on the surface water flow that needs to be controlled.
The subgrade material is typically a clay for solid waste facilities to protect the liner from the municipal waste material. For CCR ponds and landfill caps, fly ash is a suitable subgrade material that has good physical and chemical properties for placing the structured geomembrane directly on top of the fly ash. Bottom ash and clinker cannot be in direct contact with the structured geomembrane, both for abrasion issues and for reducing the usefulness of the spikes on the underside of the structured geomembrane.
The knobs on top of the structured geomembrane (the knobs come in various sizes and structures) control the water flow down the slopes (Figure 2). The geotextile with turf material (turf component) protects the geomembrane from impact and solar radiation that damages exposed geomembrane material, while breaking up wind uplift pressures. The 0.5 inch (1.27 cm) of sand infill provides protection to the geotextile and allows for light vehicles to drive on top of the system. The sand material can be replaced with sand/concrete or sand/epoxy mixes in channels where the maximum stormwater velocity exceeds 4 feet per second (1.22 m/s).
The structured geomembrane/turf component system eliminates issues such as soil erosion, high TSS in stormwater runoff, difficulties establishing and maintaining vegetation and long-term maintenance. In some areas, in-place structured geomembrane/turf component systems have had solar systems installed on top for long-term beneficial reuse. However, if borrow soils are readily available, the up-front cost of installing the structured geomembrane/turf component system can be prohibitive.
Power plant in Illinois
During the design of the CCR pond closures, a variety of alternatives including traditional RCRA caps and alternative caps were analyzed. During this process, consolidating ash within two basins and a structured geomembrane/turf component system cap was chosen. This preserved existing roadways and pipelines constructed on a bottom ash pond berm while minimizing the area of in-place closure. The chosen method saved approximately $1–$2 million in up-front costs versus the other closure methods considered, not including the additional long-term maintenance savings of traditional cap options.
The in-place closure process includes the removal of surface waters and isolation of the CCR unit, CCR grading, construction quality assurance (CQA) services and construction management activities. Water management is variable between sites and between ash ponds on the same site (Figure 3). Retaining experienced contractors and providing comprehensive hydrogeologic information to contractors can mitigate construction issues associated with managing saturated material early in the project that affect the project as a whole.
CQA services involve third-party quality assurance activities on these projects. These included sampling and testing of the subgrade, soils and geosynthetics, both in the field and in laboratories. CQA activities for geomembranes and the geomembrane components of structured geomembrane/turf component system are standardized and include air channel testing and vacuum box testing. A traditional Subtitle D cap requires observation and testing of soil cover and vegetation while the structured geomembrane/turf component system requires observation of the turf seaming, sand installation and sand/epoxy or sand/concrete mix placement (Figure 4). Qualified contractors and geosynthetic installers can save an owner a significant amount of time and money associated with failed CQA testing and repairs during construction.
Construction management services can be performed by the owner, design team or CQA team. On this project, the owner performed some construction management services while the CQA team performed other construction management services. This includes managing the contractors on-site, reviewing submittals, conducting safety programs and addressing security issues. Depending on the size and complexity of the project, this may require a full-time person devoted to construction management activities who reports to the owner.
There are two different ways to seam the geotextile/turf component: stitching (Figure 5) and welding (Figure 6). Most geosynthetic contractors are certified in only one of the two methods. While each method is approved by the manufacturer for the structured geomembrane/turf component system and each makes strong seams, there are differences in timing and weather requirements to consider before choosing a method. The welding method uses the same welding equipment as for the geomembrane liner but with some modifications and at different settings. The welding method does not require flipping the geotextile/turf and allows for multiple consecutive seams to be completed at the same time. Unfortunately, welding is subject to the same weather issues as geomembranes and cannot be used on wet material. Since the turf component can’t be quickly wiped dry, as can be done on the structured geomembrane, there may be longer delays than expected for the turf component to dry out. Stitching requires flipping the turf component, sewing it together, then unfolding the material. This typically allows for fewer seams to be joined at the same time, but this method can be done on wet material. The stitching method also keeps welding operators moving forward in good weather while a small crew works on the turf component behind the main crew or in bad weather cycles.
Geomembranes are affected by weather and will stretch or wrinkle in warm weather and shrink in colder weather. Therefore, another consideration is how to design the ditches so that temperature differences do not affect stormwater flow. This is an issue for any geosynthetic exposed to varying temperature conditions. Though this is typically only an issue for low sloping ditches, the effects of cold weather shrinking or hot weather wrinkling the underlying material is something to consider during the design process in regions where there are distinctly different summer and winter weather conditions. This is a stone-lined drainageway that does not require special mixes for the turf component infill. Lining the drainageway with stone provides weight to keep the expansion and contraction of the liner material from affecting the drainage. Other designs that could work would include “decoupling” the liner material of the main cap and the liner material of the ditch line so that the shrinkage/wrinkling of the main cap does not affect the ditch material. The elongation or shrinkage of the smaller area of geomembrane in the ditch would not be enough to affect the drainage.
A structured geomembrane/turf component system is an effective way to close CCR ponds in place and can be cost-effective in areas where soils are not readily available and long-term maintenance is an issue. Alternative methods require a high level of communication with regulators in states where they have not yet been used, but may net a significant amount of time and monetary savings over the life cycle of the project. At the project in Illinois, the structured geomembrane/turf component system is an effective alternative CCR pond in-place closure design that was selected because it is cost-effective, requires minimal maintenance, can withstand large storm events, can withstand flooding, and addressed the environmental and geotechnical needs at the site.
Anna M. Saindon, P.E., R.G., Ph.D., is project manager with Geotechnology Inc. in St. Louis, Mo.