VDOT getting ready to grind under Neabsco creek, road

Mike Salmon, VDOT spokesman
Published: July 6, 2008

Editor's Note: Virginia Department of Transportation spokesman Mike Salmon contributed the following for this week's Lane Ranger.

Building a bridge that is three times the size of the original bridge becomes a high tech feat when you tunnel under the creek and adjoining road to minimize disruption. These are part of the Virginia Department of Transporta-tion's plans this week to start the "micro tunneling" portion of the Neabsco Bridge project which entails sending a mole-like machine through the sandy soil, toting a pipeline behind for water and sewer lines.

"VDOT is looking out for the environment here," said Mike Jackson, the site superintendent with E. Ann Jackson Inc., a tunnel contractor from Petersburg.

"This is much safer to the motorists and the workers, and has a much smaller carbon footprint," he said.

The system uses a "cutter head" which chews through the sediment, followed by a vacuum and sections of pipe that are installed as it goes. The cutter head is outfitted with a water system that creates a slurry of rocks, dirt and water and it is piped back out the tunnel to avoid having to cart it out on conveyer belts or railed cars. "As this tunnels through, that casing gets placed," said VDOT senior project inspector Jeff Howard, pointing to a pipe.

"There is a lot less environmental disturbance with this," he added.

The tunnel starts in a square pit about 30 feet deep with a circular entry point that resembles a sewer lid. The cutter head is mounted on tracks for the initial entry, and is steered from an above-ground computer, using mechanical diagrams rather than a camera. There is a laser targeting device in the front of the cutter head. "It's reading with an X and Y axis," said Howard.

This is the same basic concept of the "Chunnel" that went from England to France under the English Channel years ago. With the chunnel, the material was carted out on rail cars and the huge cutter heads were eventu-ally dug into the side of that massive tunnel and left. This micro tunnel device will come out the other side to be used on the next project.

"The Euro tunnel was less technical," said Jackson. "This material here is pretty easy because it's sand," he added.

From the entry pit to the extraction pit is 277 feet, and the cutter head can travel about 40 feet a day. Jackson esti-mates that it will take about five days. "It takes longer to get set up and tear it down than it does to dig the tunnel," he said.

At Neabsco Creek, there are three tunnels being dug—two under the creek for the water line and the sewage line and one under U.S. 1 for a sanitary line. The pipes that are put in do not need to be welded, which saves money and time. The tunneling is expected to be completed in August, and the whole bridge is scheduled to be finished in June 2010.

This same company used a similar device to tunnel under Interstate 66 in the Gainesville area but that micro tunnel-ing device was piloted by a person, and the dirt was taken out by hand. E.A. Jackson, Inc. is well known in this area for this type of work. "There was a clause in the contract for experience with this kind of work, they're one of the best around," said Howard.

After the tunnels are dug, the creek will be shifted and the old bridge torn down. Traffic will be shifted over concrete pipe-like structures, the new bridge will be built, and then traffic will be shifted back on to the existing pattern. The new bridge will be big enough to carry six lanes of traffic when it's completed, but widening U.S. 1 is not part of this project. The new bridge will address the flooding that frequently takes place at Neabsco Creek.

"It flooded four times since we've been here since December," said Howard.