UPDATE: Residents denied entry to press conference, media and invited guests only.

ORIGINAL REPORT:

The Root Cause Failure Analysis was presented by Bill Walton, Sr. AECOM engineer, who said it is "voluminous and complex" and comprises ten binders.

The failure initiated in NW corner of cell 2, spread rapidly, mobilized wet ash behind, stacked up on dike c, which breached. The ash flow crested 47 feet above the Watts Bar reservoir pool elevation.

Possible causes eliminated: earthquake. karstic or bedrock instability, artesian impacts (i.e. groundwater), surface seepage or piping (only minor erosion observed). Rainfall was a minor contributing factor. A rapid Watts Bar drawdown was a minor contributing factor.

The failure was "technically unusual," and there were four contributing factors:

1. Increased filling rates into dredge cells

2. Hydraulically placed loose wet ash

3. Fill geometry and setbacks

4. Unusually weak slimes foundation

Rapid loading rates did not allow time for water to drain making the ash more unstable. "Slimes" (high water content, high liquidity, low permeability, low strength ash) destabilized the bottom of the cells. Basically, the older (lower elevation) dikes on which the new ones were built "slid off" the "slime" (if I understand it correctly).

The increased filling rates exceeded design estimates, mainly due to different coal sources that produced more ash.

UPDATE: The full TVA failure analysis report. Summary:

The north end of Dredge Cell 2 was on the verge of failure due to the high stresses and creep in the loose wet layer of weak slimes. The deformation of the slimes in turn caused the overlying collapsible wet ash to liquefy. Failure of the Kingston dredge cells was sudden and complex in nature due its geographic setting and being built within the Watts Bar Reservoir after the lake was formed. It took a forensic type study to determine the propensity of the ash to liquefy at low strain levels when the material cannot drain and thus becomes undrained, and to locate the slide plane in the unusual, creep susceptible, low undrained shear strength slime layer that underlies Cell 2. In AECOM’s opinion, subsurface conditions at the dredge cells were unusual and rarely found. The consequence of failure in the slimes led to the collapse of the dredge cell and loss of the saturated contents of the ash landfill due to the breach of perimeter Dike C.