Combination of conditions caused TVA ash spill

Combination of conditions caused TVA ash spill

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KNOXVILLE (WATE) -- A combination of conditions that evolved over a long period of time caused the ash spill at TVA's Kingston Fossil Plant, officials announced Thursday.


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Those conditions included:

  • The ash storage pond was built on top of an unusually weak foundation.
  • TVA had increased the amount of ash poured into the pond.
  • The loose, wet ash was having trouble draining.
  • There were design problems related to the slope of the wall.

A geo-technical engineering firm called AECOM was retained by TVA in January to do the independent analysis on the cause of the spill.

AECOM Vice President Bill Walton said his firm found what TVA engineers never noticed in the years leading up to the devastating collapse. "It is an unstable material and of course the ash above it was loose also."

That material is referred to as a layer of slimes, or a mix of ash, water and sediments. That's what the retaining wall was essentially built on top of.

The ash spill began shortly before 1:00 a.m. on December 22, 2008 when a series of dikes that contained the ash failed.

In less than an hour, eight acres of private property were inundated, including three homes with severe damage and nearby railroad tracks were buried under 10 to 20 feet of sludge.

The ash and debris also went into the Emory River channel, otherwise known as the Watts Bar Reservoir.

A flood wave went before the ash flow when the dikes failed. The wave was at its highest west of the Swan Pond Circle intersection when it reached 784 feet. That's almost 47 feet above the Watts Bar Reservoir pool level.

TVA surveyors estimated 5.4 million cubic yards of ash were released. However, engineers said there was no sign of seepage from the pond leading up to the collapse.

AECOM's report says the firm believes "the loose wet fly ash liquefied and flowed north into the sloughs and waterways at lower elevations" where it was stored.

The failure of the ash containment likely "started as a wedge block or translational mass that slid over the weak foundation slime layer at the north end of site."

After the first sudden failure, there was apparently a series of slides as the ash grew weaker.

AECOM determined that excessive rain for two days before the spill was only a minor factor in the collapse. The report also dismissed natural disasters such as earthquakes.

AECOM arrived on site to begin its analysis on January 8.

There were previous surface slides at the ash pond on November 6, 2003 and November 1, 2006. 

One of the residents who lives nearby, James Settles, is also the foreman of dredge cells at the Kingston Fossil Plant. He said his dog was barking incessantly the night of the ash spill and he got his first call at 1:00 a.m.

When Settles left his house, he "told his wife what time it was and that he would not get any sleep since he expected the 2003 or 2006 seep had reoccurred."

In its report, AECOM says it took hundreds of soil borings and numerous core samples, and did lab testing on the samples.

AECOM also did exploratory excavations; installed instrumentation; studied maps, photographs and surveys; analyzed relics from the coal ash release; reviewed design records and drawings; and interviewed TVA engineers and site operations personnel.

The EPA, Tennessee Department of Environment and Conservation and TVA are all involved in the recovery effort at the site.

The cost of that effort is estimated to be between $675 million and $975 million.

The Southern Alliance for Clean Energy (SACE) responded to the report saying, it, "...clearly lays to rest the misinformation pointing to the event as a natural disaster. There is no evidence that this is anything other than a failure of human engineering to contain a toxic material."

According to SACE Executive Director Stephen Smith, "The report also leads one to the conclusion that wet storage of ash causes significant engineering risk. This should push Congress and EPA to act quickly to regulate coal ash and phase out wet storage."

Smith testified on ash storage before the Senate Committee on Environment and Public Works in January, calling on TVA to do a "cleanup, not a cover up."



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