Back Breaker

A contractor successfully cleans 300 feet of stormwater retention tunnels in a remote mountainous region of southwest Virginia
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Two 66-inch reinforced concrete stormwater retention tunnels in the mountains near Bristol, Va., were beginning to flood nearby roads. The tunnels, installed in 1990 by the state Department of Transportation, were never cleaned.

Atlantic Plumbing and Utilities in Virginia Beach won the per-hour bid with a maximum of 120 man-hours. “We didn’t know how much material to expect,” says foreman Clint Carpenter. “In fact, nobody knew.” He considered using a mini-excavator, but the track was too wide to fit down the manhole vaults, and the bucket was too wide to scour the last two feet of sediment off the pipe floor.

Other obstacles set much of the work back to a time before the Industrial Revolution. Besides grueling labor, the men battled clogged hoses, tight constraints, frustration, and the risk of heat exhaustion and dehydration. Rising above everything, they removed 176 tons of material from 300 feet of pipe in 110 working hours, including dump times.

Steep slopes

The tunnels, one 50 feet long and another 250 feet, were buried in the side of a mountain 15 feet above and parallel to a state access road with limited traffic. The 8- to 10-foot-deep manhole vaults were 10 feet beyond the edge of the road and up the side of steep grades.

To reach them, the crew parked a Vactor 2100 sewer cleaner on the side of the road. The machine had a 15-cubic-yard debris body; 1,500-gallon water tank; 4,500 cfm/18 inches Hg Dresser Roots positive displacement blower; and 80 gpm/2,000 psi waterjetting system.

Work began in the shorter tunnel’s downstream manhole vault. The men extended the boom 25 feet, adapted the 8-inch tube to accept a 6-inch flex hose, then ran it the rest of the distance to the vault. They climbed up the steep grade using their hands to maintain balance. While in the tunnels, the crew followed every OSHA confined-space standard, including gas meters and radio contact with the attendant/entry supervisor.

A 5-foot-tall baffle at the back of the downstream vaults filtered stormwater through a 6-inch hole near the bottom. If water drained too slowly, it overflowed the baffle and entered the next storm drain.

“Our winters can be wet, and we wanted any water entering the tunnels to drain so we wouldn’t have to pump it,” says Carpenter. “Rain made the clay and sediment much heavier to handle, but not wet enough for dewatering.” Because the landfill could not accept water-laden debris, the team cleaned using filter bags.

“They were running so much water and there was so much debris that they went through four bags the first day, which was not cost-effective,” says Carpenter. The only other solution was to remove the material by hand, eight to 10 hours per day.

Pick and shovel

Dense clay, sand, rocks and litter filled one-quarter of the pipe behind the baffle. The men, moving upstream and hunched over, attacked the solid mass with pickaxes and spades, shoveling the material into the hose. To relieve their backs, they took turns working seated on an overturned pail and rotated with the entry supervisor.

They averaged a full load before lunch and one after. Besides shoveling 9,000 pounds of sediment per day, they carried out numerous rocks larger than six inches. “Our biggest stroke of luck was having the landfill 10 minutes away,” says Carpenter. “The quick turnaround enabled the guys to move 18 tons of material in four days.”

As the men cleaned, however, the recesses in the hose trapped material, constricting the diameter enough to affect vacuum and making the hose more difficult to handle. “We use flex hoses around Virginia Beach because we’re not pulling from great depths and it’s not muddy or full of clay,” says Carpenter. “But Wise County has lots of clay from the mountains, and that caused the problem. We had to quickly come up with something different that was cost-effective.”

Must come out

The men settled on 6-inch black corrugated bell-and-spigot PVC pipe. The 20-foot sticks had smooth interiors. “They also were much easier to handle than 100 feet of hose,” says Carpenter. The crew made a special adapter, then used it to attach a 45-degree elbow to the end of the aluminum tube once it almost reached the bottom of the vault.

They attached a 5-foot section of corrugated pipe, then another 45-degree elbow, enabling a second 5-foot length to lie flat on the tunnel floor. Cleaning advanced five feet at a time. Reaching that distance, they added another 5-foot section. Moving ahead, they replaced the two 5-foot pipes with a 10-foot section.

Advancing another 10 feet using 5-foot sections, they replaced the entire length with a 20-foot section, then repeated the process. The vacuum drew the extensions together, creating airtight joints.

Even with the trash going up the pipe much more easily, the blower on the truck ran wide open. Occasionally, the men used a little water to loosen the clay and clean the vacuum tube.

At the worst point, 80 percent of the tunnel was blocked. “It was like going through a wall,” says Carpenter. “Although it was cold, I worried about overworking the guys and their suffering heat exhaustion and dehydration. I let them set their own pace and take breaks as needed.”

The crew laid 150 feet of pipe before switching operations to the upstream drop inlet. In three weeks, they removed 158 tons of material and wore the tips flat on four spades. They jetted both stormwater tunnels to remove any remaining debris and vacuumed the slurry. Atlantic Plumbing and Utilities returned in spring to clean two sections of 66-inch retention tunnels totaling 1,000 feet.



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