Cleanup of the Kingston Fossil Plant coal ash spill is one step closer to completion. On Mar. 3, the Tennessee Valley Authority (TVA) announced that a retaining wall surrounding a 240-acre ash containment cell was completed nearly a month ahead of schedule.

Kingston was the site of the worst coal ash release in U.S. history when a holding pond ruptured on Dec. 22, 2008. The cell will be used to permanently store recovered ash from the plant.

The new retaining wall is the largest of its kind in the U.S., stretching over 12 miles in total length and containing more than 200,000 tons of cement and other materials. The wall—embedded 70 feet deep into bedrock—is designed to withstand a magnitude 6.0 earthquake.

“I’m extremely proud of the project team for accomplishing this major undertaking,” said Bob Deacy, TVA senior vice president of projects. “They kept safety at the forefront and completed this significant milestone ahead of schedule.”

However, the job is not done. TVA still needs to install the cap and cover over the containment cell. That work is expected to be complete by the end of 2014. The company is also performing a number of restoration projects to improve recreational features and reestablish natural habitat in the area. Those projects will continue into 2015.

TVA is in the course of converting all of its wet ash storage facilities to dry storage systems, a process neither easy nor cheap. The company expects to spend up to $2 billion to complete the work at all of its sites by 2022.

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)