The likely cause of the Dec. 22, 2008, collapse of a coal-ash pond at a Knoxville, Tenn., powerplant that contaminated a stretch of river differs from what the plant’s owner and hired engineer disclosed last month, according to independent engineering analyses and reviews. The new studies point to other factors, including mismanagement and water pressure, as more likely catalysts than the reason cited by the Tennessee Valley Authority.

Photo: TVA

Failure of Tennesee coal-ash impoundment may cost $1 billion to clean up.

In a July 28 report, Marshall Miller & Associates, a Raleigh, N.C. engineering firm, says the “root-cause” analysis of the accident released last month by AECOM Technology Corp., Los Angeles, for TVA, focused “disproportionately” on a low-strength “slime” layer. The firm was hired by TVA’s inspector general to peer review the AECOM report.

William Almes, a senior Miller engineer, says AECOM and TVA’s review should have focused as much, or more, on the pond’s fill geometry and on loose wet ash inside, factors that could have been mitigated by proper management of the structure. His firm’s finding that slime was not at fault has “significant implications for TVA and the power industry,” says Almes. “With or without the slime layer,” there could be a risk of ash-pond failure, he told a House of Representatives subcommittee on July 28. There are some 400 U.S. coal-ash impoundments.

Richard Moore, TVA’s IG, also testified that conditions leading to the failure existed for years and were exacerbated by poor communication and pond maintenance and that the structures are considered landfills rather than dams. In at least three instances, he said TVA management ignored internal and outside engineering reports urging pond stability improvements. The report says the spill would likely have been avoided if corrective action were taken when recommended.

Management and miscommunication were also blamed in a separate report released last month by an Atlanta law firm that was hired by TVA’s board. The board on July 21 adopted a resolution to reevaluate agency practices.

In another review, Barry Thacker, president of Geo/Environmental Associates, Knoxville, concluded the spill was preventable after analyzing it for clients with similar ponds. Thacker cited water pressure that could have been alleviated by drains (ENR 7/6 p. 76). TVA and AECOM dismissed Thacker’s analysis.