A combination of factors, including a previously unknown layer of "slime," led to the Dec. 22, 2008, layer of coal ash sludge that overflowed its aging storage facility at a Tennessee Valley Authority power plant near Knoxville and contaminated hundreds of acres, according to a 1,400-page, three-foot-thick "root cause" analysis released June 25.
The analysis, conducted by Los Angeles-based AECOM Technology Corp., found that the angled geometry of the site, along with increased loads due to higher fill and the wet-placed loose ash, along with the weak slime foundation, all led to the failure, AECOM Senior Principal Engineer Bill Walton said. While the failure itself occurred within an hour, the dike had been on the verge of collapse for some time, he noted. The failure released 5.4 million cu yd of heavy metal-contaminated wet coal ash from a settling pond and flooded 300 acres of nearby land. Cleanup is expected to cost up to $825 million.