Thu11232017

     Subscribe  |  Contact  |  Advertise  |  RSS Feed Other Publications

Wednesday, 15 July 2015 15:12

Not waffle fries, just a sinkhole: Towering mound of dirt go back in once construction finished

Written by 

fr inglesdirtWork continues on repairs to a massive sinkhole that collapsed suddenly near Ingles grocery store in Waynesville several weeks ago.

Despite thousands of cars per day traveling the main commercial drag of Russ Avenue, the gaping cave-in on a lower corner of the Ingles property went largely unnoticed at first. Instead, it was the big yellow bulldozers running around on the surface that hogged all the attention as they mounded up an ever-taller pile of dirt.

The dirt is simply being staged, waiting to go back in the ground once the culprit behind the cave-in is fixed. But passersby were quick to postulate that the earth moving was the beginning construction of a much-anticipated Chick-Fil-A coming to the vacant lot. Alas, the real story starts with an old, rusted culvert.

A giant five-foot pipe had ferried a stream underground for decades, but it corroded over the years, letting the stream seep out, eating away at the soil and creating a cavity that would eventually collapse. 

“The water itself hollowed out a big enough cavern that gravity took over,” explained Ed Williams, a water quality specialist with the N.C. Department of Environment and Natural Resources out of Asheville.

The collapse was triggered by a heavy rain the same day (May 28), but the tipping point had been months or even years in the making.

“The steel starts to rust a little more and the hole gets bigger and it starts to wash away the soil even more. And then you create a big void. Eventually that water is flushing the soil away as fast as it can fall in,” explained David Brown with the U.S. Army Corps of Engineers in Asheville.

Once it started to go, it went, leaving a gaping hole about 20 feet deep, 20 feet across and 40 feet wide.

“It was a huge hole,” Williams said.

All that dirt was sucked down into the culvert, sending a mud slurry whooshing downstream. It blasted out the mouth of the culvert, coating a half-mile of the creek downstream with mud.

 

Unlucky timing

Ingles was on the verge of replacing the old culvert with a new one. A few weeks before the cave-in, Ingles had applied for state and federal permits to replace the underground pipe that carried the stream.

“We were within a week of issuing a permit to replace it,” Brown said.

Signs of trouble had surfaced over the winter. Shifting soil prompted Ingles to hire an engineer, who concluded the underground culvert was failing. Ingles had just developed a plan to replace it, “And then this happened,” Williams said.

It was once common to route streams through giant underground culverts, allowing for development on top of where they once flowed.

“There are piped streams all over the place. They used to go in more readily and now they are much more scrutinized,” Williams said, citing the environmental impacts of confining streams to underground pipes.

And there’s a delayed side effect: the galvanized metal used for the culverts eventually corrodes.

“They seem to deteriorate after about 30 years,” Williams said.

There have been at least three similar sinkhole mishaps tied to underground stream culverts in the region over the past five years — in Asheville, Brevard and Black Mountain. The underground culverts can be a ticking time bomb but are often forgotten about.

Usually, no one around today remembers exactly when they were put in. With the Ingles sink hole, Williams looked at old aerial photos of the site until he found one old enough that it showed the original stream.

 

The cleanup

Ingles faced a costly and a multi-pronged cleanup and stabilization following the cave-in, but has been commended for its response.

“They mobilized pretty quickly,” Williams said.

Workers couldn’t get near the sinkhole at first because it was still unstable, with the walls continuing to crumble in.

The first job was to reroute the creek still flowing through the bottom of the sinkhole, using giant pumps and temporary piping.

“You can’t have water going through there while you are trying to repair it or it will just keep making the sinkhole bigger,” Williams said.

Then, the major job of digging out the area around the sinkhole began — and that’s what the towering pile of dirt visible from Russ Avenue is all about. Once a new culvert is put in, that dirt will go back in the hole. 

The rest of the culvert will be replaced as well — including a section going under the entrance road to Ingles.

Meanwhile, crews had to clean up all the mud that blew out downstream.

Crews used sediment vacuums to suck out mud coating the stream, but much of the work had to be done by hand with buckets and shovels, Williams said. Sediment traps were put in to catch any mud that was still lining the culvert and could keep washing through. The stream eventually flows into Richland Creek, but the mud stopped just before it got there.

“Richland Creek fared well,” Williams said.

Ingles plans to build a gas station in the spot where the sinkhole happened. The gas station is part of a major makeover in store for the entire Ingles site. (see related article, page 4.)

blog comments powered by Disqus

Media

blog comments powered by Disqus