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Sinkholes: Tales from the open road

CORNELIUS – Massive sinkhole.

That not only sounds vulgar – what you might call a hod carrier who accidentally dropped a cinder block on your foot, or an old-home renovation project gone way wrong – it also can be a pain in the pocketbook for a developer.

Under the watchful eyes of an engineer and town officials, work has begun to clean up the 130-foot sinkhole-turned-landslide in the Antiquity subdivision in Cornelius. Photo by Tony Brown

Not to mention dangerous for residents of partially completed subdivisions, a situation that could be even more financially disastrous.

Case in point: the 130-foot-long sinkhole now eating a chunk of pavement, stormwater system, underground electric lines and retaining wall in the Cornelius mixed-use development of Antiquity.

Sinkholes – slow underground erosion that eventually gives way, opening a crater – are not as common in Piedmont Carolinas red-clay country as in sandy Florida, where sinkholes-to-hell swallow whole houses.

But they’re not a rarity here, either.

When they do happen, they tend to happen where raw land has been cleared of soil-stabilizing vegetation, where root-ravaging excavators have graded the landscape and where dirt-disturbing manmade features such as roads and stormwater drains have been installed.

And – this is key – where large swaths of the prepared earth have sat vacant for four or five years.

All of which means – according to a plumber, a geologist, two developers, and three town of Cornelius officials contacted for this story – that developments not fully built-out after a half-decade are prime sinkhole-bait.

 

Mix well, add water

The sinkhole in Antiquity, which has widened into a mudslide, is not the only textbook case of what can happen when you mix the right (or in this case, the wrong) ingredients:

A well-intended municipal code concerning asphalt, a storm drain that’s temporarily higher than the roadway it’s on, a small slip by a subcontractor and about five years of rain.

And, just for kicks, in a quirk peculiar to Antiquity, let’s throw in a colony of beavers practicing feats of engineering at the bottom of the retaining wall.

April 15 was the day a fairly small sinkhole opened up near a storm drain on the edge of Old Canal Street, which was constructed about five years ago, in a section of Antiquity that is fully developed but not yet built-out.

With engineers, town officials, curious residents and TV news cameras peering in, workers from Antiquity LLC, the subdivision’s developer, filled in the hole, then installed new curbs. Paving was scheduled for Monday, April 29.

The sinkhole in Antiquity opened on April 15 as a substantial but relatively manageable gaper, but heavy rains turned it into red-clay gash in the earth on April 28. Photo courtesy of corneliusnews.net

But a Sunday-wrecking rainstorm climaxed with a power outage in Antiquity. When Duke Energy repair crews showed up, they found that the sinkhole had not only reopened but expanded, spilling mud, gravel and pavement where a third of a football field of roadway used to be.

The closest residents were ordered out of their town houses for a couple of hours lest a gas line burst.

Antiquity LLC and town officials discovered the damage now extended to a retaining wall between the road and a deep culvert that is home to the swampy headwaters of the South Prong of Rocky River, which the subdivision’s trademark covered bridge runs over.

Now Antiquity LLC, along with the town and a third-party engineer, are figuring out how to fill the hole for good. To do that, they must determine what happened in the first place.

But there’s more to sinkholes than H2O and man vs. Mother Earth. So The Mecklenburg Times has gathered a panel of experts to get to the bottom of this particular sinkhole and ones like it waiting to happen in a housing development near you.

 

The lineup

John Divine, owner for 30 years of Charlotte’s Divine Plumbing, says it well in his company’s motto:

“Water always wins.”

He was talking about toilets and leaky pipes, gutters and bad roofing jobs and foundations.

But it also applies to land, said Craig Allan, Ph.D., chairman of the department of geography and earth sciences at the University of North Carolina at Charlotte.

Allan patiently explained that the epidemic cratering of Florida is caused by water dissolving limestone bedrock, which is not the case hereabouts.

“Sinkholes here almost always have something to do with something somebody has put in the ground, like infrastructure,” Allan said.

“When significant water seeps in around infrastructure, it saturates the clay subsoils and can liquefy it, and that’s when you get a sinkhole,” the professor concluded.

Allan pointed to the Piedmont’s most infamous sinkhole, a 2002 doozy along the margins and near the drain holes of the new, freshly paved parking lot of a new Buffalo’s Southwest Grill restaurant in Hickory. Three years later, another sinkhole opened in the same parking lot. Then, two years ago this month, yet another sinkhole ate a 12-foot-wide, 18-foot-deep chunk out of U.S. 70, just up the highway from – wait for it – Buffalo’s Southwest Grill.

The likely culprit: Water eating away at subsoils around infrastructure, in this case, a broken pipe connected to the drain hole.

 

Regulatory rascal

Allan’s academic basics make sense to Tom Pearson, president of Pearson Land Corp. and a 28-year veteran of the Charlotte home development industry.

Pearson lived through a sinkhole nightmare a few years ago, in the Telfair subdivision he developed in Mint Hill.

Again, infrastructure and water were involved. But Pearson is specific about both, adding more clues about the Antiquity subdivision: a theory about way ordinances in Mint Hill, Cornelius and other area municipalities make developers build roads.

He even had an aqueous observation almost as pithy as Divine’s: “You can’t teach stormwater to go into drains.”

“Roads in (Mint Hill) subdivisions have 8 inches of stone underneath and 2½ inches of asphalt on top,” Pearson explained. “But on the first go-round, you put in the stone and only 1½  inches of asphalt. You’re supposed to put the other inch on top when the development is 75 or 80 percent built out, which could take four or five years.

“Meanwhile, you’ve installed the curbs and stormwater drains an inch above the first layer of paving. For that four or five years, the water goes to the edge of the asphalt but can’t climb that inch to the drains. So it stands there and migrates through the gap between the asphalt and the curb and into the subgrade, causing the catch basins and pipes underneath to separate. And there goes the farm.”

Considerably smaller than Antiquity’s, the Mint Hill sinkhole still cost Pearson about $8,000 to fix. Better, he said, to have developed in the city of Charlotte, whose ordinances order developers to top-up asphalt on subdivisions’ streets within the first year whether they are built-out or not.

“The time element is important,” Pearson said. “One year of water migration is one thing. But five years?”

The streets in Antiquity were installed four or five years ago. But Pearson is an outspoken free-marketeer who gladly will blame many a developer’s woe on

government regulations.

 

A surprising agreement

Favoring Pearson’s explanation, however, is the fact that Cornelius has development ordinances almost identical to Mint Hill’s. Even more persuasive: Wayne Herron, Cornelius’ planning director, agrees with Pearson.

“That’s exactly right,” said Herron, new to Cornelius but with 18 years in land-use and planning, in Boone and Monroe.

After seeing similar construction-site sinkholes, he said: “If you give water an inch, you might as well give it a mile. Look at the Grand Canyon. Water is a powerful thing, even if it’s just a trickle in a crack in the road. Nature just doesn’t like manmade elements. You see it all the time.”

Herron did defend the town’s ordinances, which he said limit the wear and tear of heavy machinery on the finished streets, which the town then takes over.

Pearson, staying in character, scoffed at the beautiful-paving argument. “Eight inches of stone with 2½ of asphalt is 40 percent stronger than 8 inches of stone with 1½ inches of asphalt,” he said. “You have all those heavy trucks and machinery running over that while it’s not as strong.”

Herron said “the man” to talk to about the Antiquity sinkhole is Tyler Beardsley, project manager for the town’s administration department. When he heard the road-water-infrastructure hypothesis, Beardsley said:

“I think that’s as good a theory as any. I was down here with a third-party engineer hired by Antiquity LLC, and that’s what he thought also.”

Beardsley also said, “This is going to be a very expensive sinkhole and will probably take a while to fix. They’re taking down the retaining wall because it was compromised when the sinkhole widened.”

But Beardsley denied being “the man.”

“Naw, the man you want to talk to is Marc Frye; he’s Antiquity LLC’s man on the ground in Antiquity.”

Frye, development manager for Antiquity LLC, fully endorsed the infrastructure-water calculus and said the pavement-storm drain differential probably softened up the clay that used to be where the sinkhole is now.

But he also thinks an electrical subcontractor – unnamed – might have pulled the little Dutch boy’s last finger out of the dike.

“What happened here was a manmade disaster,” Frye said. “The engineer’s report is not yet in, but all the indicators are pointing in the same direction.”

 

The electrical connection

“It’s partly what you said,” Frye said, but there’s more to it than that. “The electricians came down here in July 2012 and dug behind the stormwater drain box, and they loosened it up. They didn’t do a good job. They left their trenches open. The deposited moisture ate away at the subgrade and loosened up the pipe connection. And that caused the storm drain box to come apart.”

Frye said he’s never seen anything like Antiquity’s crumbling retaining wall in his 30 years in the developing business. “I never go with the low-bidder on something that’s holding up a road,” Frye said. “And I’ve never had a wall failure.”

Then Frye tossed a curveball from way out in left field: beavers.

Even without the sinkhole, the wall might have collapsed at some point because a large and growing colony of industrious rodents (family Castoridae) has taken up residence in the culvert below the wall.

And they’ve been very busy with their own construction project: gnawing down trees and strategically placing them in the trickle of water  called the South Prong of Rocky River, which runs along the boundary between Cornelius and Davidson.

The shores of the resultant beaver ponds now approach the wall’s footers. The dirt below them is eroding.

It’s the water again, this time with help from some furry, bucktoothed critters that neither he nor anyone else can do anything about.

Enter the final expert: Kenny Russell, animal control officer of the Cornelius Police Department.

“Our hands are tied when it comes to a wild animal,” Russell said. “Unless it’s a direct threat to public health or somebody’s life, we have to leave beavers alone.”

Frye sighed.

“We can’t get rid of the beavers,” he said. “And even if we could, they’d just keep coming back.”

Just like sinkholes.

Pending the engineer’s final report, the conclusions:

The plumber says it’s the water, stupid.

The geologist says don’t forget the infrastructure.

The veteran developer says yes, and caution: uneven pavement ahead.

The town planner agrees and compares the situation to a tourist attraction in Arizona.

The town project manager says it will take a lot of cash to eliminate the gash.

And Antiquity’s development manager says all of the above, plus some careless electricians and too many beavers.

The town animal control guy is declaring a “No Hunting” sign.

As for the beavers, they just dam it all.

 

BROWN can be reached (704) 247-2912, tony.brown@mecktimes.com, or on Twitter at @tonymecktimes.

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