Sinkhole-causing drainpipes to be replaced
If all goes as planned, come September, the
Virginia Department of Transportation (VDOT)
will install a new drainage pipe on Route 29 near the Hollymead Town Center. A section of pipe there failed not once, but twice, eroding the earth underneath the southbound lane, causing two sink holes that opened up there eight months apart in front of the Seminole Commons shopping center.
“This sinkhole was basically the result of a pipe failure,” said Darin Simpson, the Assistant Administrator for VDOT’s Charlottesville residency. The excessive rainfalls filled the existing pipe where it became almost pressurized. That created a situation where because of the breach of the pipe, it would pull the roadbed material down to the outlet end and where the pipe ultimately discharged, and over time, it just pulled enough roadbed material that the road couldn’t bridge itself and it lost stability.”
Water pipes can be built from a kind of plastic, concrete, or corrugated metal. The first pipe that failed was made of the latter. Simpson says it likely deteriorated over time due to the elements, especially salt used to clear roads of ice and slush.
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After the first pipe burst in November 2005, Simpson said VDOT used plastic material as a replacement pipe which did not adequately match the existing connections. Eventually, heavy rainfall doomed it as well and another sinkhole formed in July of 2006.
“It surcharges, which means it runs at full capacity, and now it’s pressurized.” That led to vacuum conditions at the location of the first repair. For the second repair, VDOT left the replacement pipe underneath the ground, but strengthened it with a geotextile filter fabric, as well as a concrete collar.
“We get the heavy rains, the pipe is still going to surcharge, but we feel comfortable at that joint it’s not going to break,” he said.
Many people assumed that the large amount of development near Hollymead Town Center played a role in the pipe failures.
“The Target and all of the new Hollymead development goes into the big stormwater retention pond that you see when you drive in that parking lot,” he said. Developers are now required to build such ponds to handle stormwater.
“When you have a storm event it will only release water slowly into the system so you’re not getting a rush. When you pave and you put structures and concrete and asphalt down, you’ve obviously changed the conditions of the land, when it all used to be vegetation, and the run-off wasn’t as bad. So now, once you pave it and everything, you get larger amounts of runoff. And, the pipe that failed, I don’t think those measures were in place. And it’s just one of those things that they did. Someone wanted to develop [Seminole Commons], and they said, we have a pipe here, you can tie into it, and it’s just, lessons learned.”
Simpson says he is confident the drainage pipe won’t fail again, but VDOT will spend at least $250,000 on a project to replace a 260 foot section. The new concrete carrier pipe will be thirty inches in diameter, half a foot larger than the existing pipe.
“Hydrologically, that six inches gives us a lot more capacity then what it might sound like to folks,” he said. He adds that VDOT has studied the watershed to see if there are any more possibilities of excess water flow in the next major storm.
When the replacement work starts this September, contractors will use a process called “bore and jack” to tunnel underneath the road. Simpson said this will only cause a minimal disruption to traffic on Route 29, shutting down only one lane.
Could it happen again at another spot on Route 29 North?
Simpson says it’s hard to tell how old the pipes are because of the upgrades made to Route 29 over the years. The south bound part of Route 29 is older than north bound, but VDOT records can’t confirm if the pipes along the south side were ever replaced.
“When 29 was two-lane, there were some portions that we could find that were built back in the 1920’s. We found other plans when it was upgraded to four lanes, what they did according to the plans, they basically left the existing pipes in and tied on new pipes, and extended them.”
In the meantime, new developments have been built along the Route 29 corridor, including Forest Lakes and Hollymead Town Center. Simpson says the impact on pipes by any future development projects will be looked at on a case by case basis.
“Our requirement would be that at a minimum, you have to tie on to the existing pipe and bring it out to a point that meets design standards for the outfall,” he said. When construction on roads and new development begins, VDOT will then inspect each pipe to check for its structural integrity.
Simpson said the camera has recently been sent through the existing pipe, and revealed no further deterioration. Bids on the replacement pipe project will be accepted through June 27th.
Sean Tubbs & Brian Wheeler