Stantec has received the Grand Award from the American Council of Engineering Companies of Kentucky. The award recognizes Stantec for its design and technical solution for the Tennessee Valley Authority’s recovery from the Kingston coal ash spill. The project also received a National Recognition Award at the ACEC 2014 Engineering Excellence Awards competition.
In December 2008, the dikes at TVA’s power station in Kingston, Tennessee containing wet coal combustion ash failed, and 5.4 million cubic yards of coal ash were released. The ash slurry flowed out from the site, impacting the adjacent Emory River In the aftermath, TVA committed more than $1 billion for the response, clean up, and site restoration.
Stantec’s team was at Kingston within hours after the spill to assist TVA with emergency actions. As the recovery progressed, Stantec developed a site closure scheme, completed detailed engineering designs for a containment structure, and remained on-site to help with recovery efforts.
Today, TVA has achieved several key milestones at Kingston. During the first two years after the spill, about three million cubic yards of ash dredged from the river was shipped by rail to a permitted, off-site landfill. Since then, the remainder of the released material has been excavated and stacked back inside the footprint of the failed impoundment facility. A two-mile long perimeter containment structure has been built around the new 240-acre ash landfill, which is being capped with a flexible membrane cover system. In 2014, Stantec’s design was recognized by ACEC for engineering excellence.
The idea of placing large volumes of ash back within the failed area was met with initial skepticism, said officials. To prove it could be done without causing another failure, a test embankment was built across six acres of the failed dredge cell. Geotechnical instrumentation was installed to measure water pressures and movements within the underlying ash. Stantec established criteria and monitored conditions. In the end, more than 250,000 cubic yards of ash were safely stacked to a height of 45 feet above the failed surface. In addition to restoring confidence, the test program established protocols that would be followed throughout the project.
By far, the most difficult design challenge was how to contain the ash in the event of a large earthquake, said officials. Tennessee regulations require landfill facilities to withstand a 2500-year earthquake. Engineering analyses showed that these events would liquefy the sandy soils underlying the ash landfill.
Borrowing an approach used elsewhere to stabilize large embankment dams, Stantec designed a grid of buried walls that enclose the ash landfill. The engineering design was complex and required advanced numerical modeling of the seismic behavior, using both dynamic 2D and static 3D structural analyses. The design was subject to extensive independent review and regulatory oversight. The completed retaining structure, built to depths of 70 feet, is one of the largest walls of its type ever constructed in the U.S.
Craig Zeller, EPA’s project manager at the site, sees how far the cleanup project has come. “It was a daunting project at the start. But five years later, after dredging, digging, and stacking ash day after day, week after week, the end is in sight. The project is on schedule. Ash is out of the river. The perimeter walls are done. The environmental restoration is underway. Kingston is a fantastic case study for a large-scale cleanup,” Mr. Zeller says.
TVA’s Kingston, Tennessee, Fossil Plant supplies power that is critical to the regional economy. On-site storage of the recovered ash, made possible with the perimeter containment structure, represents TVA’s best option for meeting both the economic and environmental goals of the project. The result has restored public confidence that a failure like the 2008 event would not happen again at Kingston, not even during a major natural disaster like an earthquake, said officials.