Duke Energy failed that Sunday to successfully convey to North Carolina officials the potentially disastrous nature of what had just happened at the retired steam station, but the company reacted quickly within its own ranks.

Workers lift a section of damaged pipe from the Dan River near the Duke Energy coal ash pond in Eden, in April. - Duke Energy Photo

And with some innovative engineering work, plus another near-catastrophe that had a silver lining, utility employees and contractors stemmed the flow of coal ash into the river in four days.

The security guard who had noticed the sudden drop in the main ash pond’s water level, about 2 p.m., quickly alerted workers next door at Duke Energy’s new power plant, which runs on natural gas. In 2012, the new unit had replaced the 60-year-old coal-fired plant, which once depended on the ash ponds to clean its process water.

Workers from the new plant checked out the pond. They noticed that the part of the dam wall closest to the retired steam plant and farthest from the river appeared to be slumping. They called Duke Energy’s in-house environmental staff, who arrived about 3 p.m.

The initial Duke team called in reinforcements about an hour later and “began assessing what’s going on here and what could they do to address the leakage,” Duke Energy’s director of environmental affairs, George Everett, told state lawmakers in a hearing last year shortly after the spill.

“Now, understand, we all wanted to fix this release immediately. To do so, however, is not easy,” Everett said.

After calling in more outside experts, the Duke Energy team deployed a “boom curtain” in the river — a long, floating device with a mesh screen that hangs down into the water to disrupt the flow of pollutants, slowing them down and raising the likelihood of them settling near the river bank.

Coal ash-contaminated water still gushed out of the broken drainage line into the river, but the curtain deflected at least some of the ash.

Next, the team tried for several hours to plug the pipe at its end near the river with an inflatable stopper, big enough around to fill the pipe’s 48-inch diameter.

But water pressure kept building behind the plug and blowing the contaminents out of the culvert — three times before the Duke crew discarded that tactic as too dangerous.

“There have been people killed by having these plugs blow out the end of the pipe,” Everett said.

So, that first day after the spill, Duke officials and their contractors changed their thinking.

“We realized we had to get rid of some of that back pressure in order to really effect a repair,” Paul Newton, the utility’s president of North Carolina operations, said at a later hearing in Raleigh.

Team members also worked on the other end of the pipe, the one nearest the break, looking for a way to get heavy equipment to the area directly above the broken pipe in the largely drained pond.

Much surface water was gone by then, as roughly 27 million gallons of wastewater continued to leave the pond through the failed culvert. What remained was a layer of discarded coal ash about 30 feet deep, a layer of dirt about a yard thick, and the pipe.

But the saturated ash was too much like quicksand to support machinery. So the team at the front end of the pipe tried to build a road across the ash to the edge of the spill — a road they hoped would be sturdy enough to support equipment needed to stop the leak at its source.

They began working with clay, but the material started sinking into the mushy ash. So, during the next two days, they trucked in 9,000 tons of rock to build a more durable foundation for a road leading to what they envisioned as an “earthen platform” at the edge of the sinkhole that had resulted from the pipe’s failure.

That’s when a blessing in disguise happened, during the early hours of Wednesday, three days after the spill.

“About 2 a.m., the earthen platform collapsed into the crater created by the initial pipe failure,” according to the log kept by N.C. Department of Environment and Natural Resources personnel on the scene. “There were no injuries or equipment loss.”

At first blush, it looked like a big setback for the response team, now working under the watchful eyes of both state and federal regulators.

“But that was actually kind of a good thing,” state DENR official Tom Reeder said later about the collapse, “because all that material went into the pipe, and it basically plugged the pipe.”

What had been a steady flow coming out the end of the broken pipeline subsided to a much smaller jet, about 9 inches wide by just a half-inch deep.

The Duke Energy team quickly built a dam of sand bags to trap that flow around the area where the wastewater emerged and then installed a portable pump to pipe the stuff back to a holding tank for later disposal. That put an end, at least for the time being, to any more coal ash flowing into the river.

Later that day, Duke officials brought in a 200-foot-tall crane capable of lowering a larger pump into the pond near the broken pipe. The second pump pulled 2,000 gallons a minute away from that area, relieving the pressure on the damaged stormwater culvert that two days earlier repeatedly pushed the inflatable plug out the other end.

Meanwhile, members of Duke’s engineering team devised a clever solution to the plug dilemma, designing a stopper made largely of concrete that could be rolled into position on casters from the river side, its diameter only slightly smaller than that of the pipe. Around part of this new-and-improved plug, they draped a tractor tire that they could inflate once the plug was in final position, roughly 44 feet inside the culvert.

That would provide a temporary seal, giving workers time to grout closed any remaining open space between the outside of the plug and the inside of the pipe. The team began positioning the new, stronger plug at 1 a.m. Saturday and had it grouted in place a few hours later. By late the next afternoon, the grout had set permanently, closing off the massive leak that had begun a little less than a week earlier.

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

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