When there is a hard freeze, ice forms on the surface of mud or soil. The expansion of water as it freezes causes a pressure drop that pulls water from below the surface. As the photo above illustrates, this pressure can draw out a considerable amount of water, which freezes into crystals or ribbons. The rupture in the ash pile started in the northeastern corner of the pile where the curvature maximized the surface area, providing more exposure to the cold air. The wicking pressure would be strongest in the corner, and the flow created as ice formed on the surface of the pile is just the sort of trigger engineers were looking for. Since they failed to consider the cold weather, they were unable to find a cause.
Why does it matter? Just a couple years prior to the spill, an outside consultant warned TVA of this danger and recommended that they lower the water content in the pile during winter months to prevent a freeze-triggered collapse. They did this the previous year, but TVA cycled managers to other plants during a reorganization in 2008. Apparently institutional memory was lost and the pile was not dewatered last winter.
Monday, December 14, 2009
Sour Persimmon has a theory on the probable cause of the Kingston ash spill last Christmas that also likely explains why it's not TVA's theory: