In 2019, a coal-fired power plant in Illinois was scheduled to be retired. The decision to close the plant was made to address the recently approved revisions to the Multi-Pollutant Standards rule from the Illinois Pollution Control Board.
As a part of the plant retirement, the owners laid out a plan for the permanent closure of the two ash ponds located on the property.
This project is a terrific example of how turf reinforcement mats (TRMs) can be integrated as part of a vegetated solution. TRMs provide long-term protection and a host of other benefits (see Figure 1).
Background
In 1976, the Duck Creek Power Plant was commissioned to supply power to the surrounding area. The power plant was a bituminous coal facility generating 441 megawatts (MW) of energy. The plant was owned by Ameren, an Illinois power company, until March 2013 when it was purchased by Dynegy, which later merged with Vistra Corp.
Ash ponds are also known as surface impoundments or ash basins. They are engineered containment structures typically located on the sites of fossil fuel power plants. Details of the two ponds at this site are provided in Figure 2. These structures are designed for the disposal of two types of waste byproduct: bottom ash and fly ash (see Figure 3). These waste products are generated during the coal combustion process. The pond structure is utilized as a landfill to prevent the release of contaminants into the environment.
Ash ponds utilize gravity to settle out large particulates and reduce the total suspended solids (TSS) from the power plant’s wastewater before treatment. These containment basins are generally built using a ringed embankment of various heights. The height is determined by the capacity needs of the site. Many embankments are designed using specifications associated with embankment dams and include clay cores to prevent seepage and failure. According to the U.S. Environmental Protection Agency (EPA), as of 2012, there were 735 surface impoundments in the United States.
Project team
Vistra hired the engineering team of Civil & Environmental Consultants Inc. (CEC) from Pittsburgh, Pa., to develop the final closure design solution and create specifications for the best management practices (BMPs) to be used on the project. Central Landscaping of Princeville, Ill., was hired to complete the swale stabilization work on-site.
Design solutions
Upon closure of the two ash ponds, the owner was faced with engineering a solution to stabilize and contain approximately 143 surface acres (57.87 h) of residual ash. A time-tested closure design would be implemented at both pond locations (see Figure 4). As part of the capping design, the plan would require a layer of clay soil to minimize seepage of surface water into the stabilized pond core. The clay layer and crowned surface design would significantly increase rainfall runoff.
This runoff would require a system of swales designed to handle flow volumes and shear stress levels that vegetation alone would not withstand. To prevent damage to the containment structure, the solution needed to provide long-term support to the channels, eliminate erosion, be easy to maintain, re-vegetate quickly and blend in with the environment.
Best management practice specification
After evaluating the performance, specifications, appearance and cost of various BMP options, the CEC team chose to use Recyclex TRM-V® by American Excelsior Co. as part of a vegetative swale solution. The TRM chosen used fibers made from 100% postconsumer recycled polyester fibers (green plastic bottles).
Eighty percent of the fibers were 5 inches (12.7 cm) or greater in length. These fibers differ from many of the synthetic fibers used in other TRMs on the market. They are tightly crimped and curled to allow the fibers to interlock. The fibers retain 95% memory of their original shape after loading by hydraulic events. The fibers have a specific gravity greater than 1.0. This means, unlike some other fibers used in TRMs, they will not float during hydraulic events. Keeping TRMs close to the seedbed means less erosion and seed migration.
The fibers are stitched between two strong layers of UV-resistant polypropylene netting to form a 3D matrix that is designed to provide long-term support for vegetation and root systems.
This TRM can be installed like a standard erosion control blanket by placing it over the prepared soil and seedbed. It can also be installed over the prepared bare soil, soil-filled, seeded and then covered with a temporary erosion control blanket. This soil fill method allows the root system to grow directly through the TRM and into the soil. The environmental benefits of vegetated swales are well documented (see Figure 5).
The plan
Prior to installation of the TRM, the contractor provided finish grading services and applied a seed mixture containing tall fescue, perennial ryegrass, creeping red fescue, redtop, smooth bromegrass, medium red clover and farm rye applied in a hydromulch slurry.
The designers utilized the TRM for the swales with slopes that were less than .5H:1V and had shear stress expectations of less than 8 pounds/square foot (39 kg/m2). Central Landscaping installed the TRM in both 8 foot (2.4 m) and 16 foot (4.9 m) wide rolls with a length of 450 feet (137.2 m). This configuration allowed for quick and efficient installation of the TRM. The product was installed according to the manufacturer’s specifications and followed the recommended staple patterns (see Figure 6).
Observations
According to Central Landscaping, the vegetation establishment exceeded expectations. Furthermore, the TRM has controlled all noticeable erosion and protected the containment structure (see Figure 7). Central Landscaping attributes the excellent performance to the fact that the TRM has a specific gravity of greater than 1 and it does not float. “When TRMs don’t float during hydraulic events, the soil and seed stay in place hastening re-vegetation for improved short-term and long-term protection,” according to the company.
Results
Vistra, CEC and Central Landscaping worked together to design a solution, select the correct BMPs and execute the plan. In total, more than 80,000 square yards (66,890 m2) of TRM was installed on this project. The TRM was able to handle the flow volumes and shear stresses that the vegetation alone would not withstand. The design eliminated erosion and prevented undercutting and soil loss. The completed project will provide the long-term protection that the site and surrounding area requires.
Project Highlights
TRMs With Recycled Fibers Help Contain Fly Ash
Owner: Vistra
Location: Canton, Ill.
Contractor: Envirocon
Construction company: Central Landscaping
Engineers: Civil and Environmental Consultants
Geo product: Recyclex TRM-V
Geo manufacturer: American Excelsior Company