As much as anything, the story of the Dan River spill is a tale told in metal. The spill owes much of its threat to the relatively small amount of heavy metals contained in the ash, the major pollutants fused to what remains of pulverized coal used in the production of electricity.

And the spill itself stemmed from metal failure, the collapse of a corrugated-metal pipe, known as CMP in the construction industry. It was a failure that almost any qualified civil engineer could have foretold for the utility, and, in fact, several did when they inspected the drainage ponds as many as 30 years before the spill.

But Duke officials did not act aggressively enough on those warnings, later apparently even forgetting that most of the underground pipe was made of metal rather than the longer-lasting concrete, which they assumed extended its entire length, because concrete was what was visible at the pipe’s end beside the river.

In fact, the pipe had been built in phases going back to the early 1950s, with CMP making up the first 800 feet and sturdier, longer-lasting reinforced concrete the final 300. The pipe’s purpose was simple, taking uncontaminated storm run-off under the pond to the river without exposing it to the pollutants in the coal ash stored above.

The first part of the pipe was built in the days when CMP was the material of choice. By 1968, when the final section was installed under the Eden plant’s primary ash pond, the construction industry had shifted to concrete for such heavy uses as tunneling under ponds and major highways.

But CMP’s crimped metal design goes back a long way in American history and has made a lot of projects possible over the years. Two inventors developed its spiral design during the waning years of the 19th century.

The technology really came into its own in the 1930s, when a Midwestern engineer named Merlin Spangler designed a way of positioning it underground to boost dramatically its strength and durability as a drainage outlet.

Spangler’s technique made possible the explosive growth of the American highway system that would be one of the nation’s unique accomplishments in the last half of the 20th century. It involved laying CMP end to end on a firm, stable bed of gravel and then compacting certain soils or other “structural fill” around its sides and top.

This compaction forged the metal and surrounding matter into a cohesive unit, able to bear the crushing weight of passing traffic on a roadbed overhead.

A common use involved taking small streams and storm water from one side of a street or highway to the other, taking it underneath the pavement to prevent washouts and other forms of wear caused by repeated flooding.

In the case of the plant on the Dan River, the culvert would bear up for 60 years against the weight of untold tons of coal ash and millions of gallons of water, all stored only a few feet above.

Low-cost, sturdy and effective: What could be better? Unfortunately, CMP has an Achilles heel that goes back to the chemistry of its most basic material, an eighth-inch-or-so piece of steel that’s highly prone to corrosion.

Water and dirt seep into the CMP culvert over time through corrosive spots or weakened joints, causing more rust and sometimes even carving a lengthwise incision in the bottom. The grit in the soil eventually can bisect the pipe, almost like a saw blade.

No longer able to retain its circular shape, the pipe eventually folds in upon itself.

That could be what happened to a 4-foot-long section of the Dan River pipe that had been carrying supposedly uncontaminated storm water from the Duke Energy grounds on the Eden side of the pond, beneath the primary ash basin and into the river – a journey just a bit shorter than four football fields end to end.

After the spill, clean-up crews fished what they believed to be the failed section of pipe from the river, downstream from where the drainage pipeline ended at the river’s edge. They theorized the pipe collapsed into a flattened shape that could be thrust like a spitball through the rest of the culvert by the coal ash unleashed from above.

“We believe the object recovered from the Dan River on April 13 is a 4-foot section of the 48-inch, storm water pipe that failed on Feb. 2,” Duke spokesman Jeff Brooks said at the time. “This section of pipe appears to have collapsed in upon itself, allowing the section to pass through the remainder of the pipe and into the river.”

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Contact Taft Wireback at (336) 373-7100, and follow @TaftWirebackNR on Twitter.

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