Pipes and Liners

Stormwater pipes. They’re in the countryside, at beaches, in forests. They’re in cities in the desert. They’re made of concrete, corrugated steel, and plastic. In time, they may be retrofitted, because they’re incomplete, inadequate, or failing. In areas where they can’t be dug up and replaced, they might be reinforced with sophisticated plastic liners that often improve the flow.

The flow of water in pipes and channels is still calculated by the Manning formula, which was created by a man who began his career as an accountant in Ireland 200 years ago.

Robert Manning had no formal training in fluid mechanics or engineering, and if it hadn’t been for the great potato famine of 1846, he might have remained an accountant. Instead, he went to work in the drainage division of the Irish Office of Public Works, where he later became chief engineer. He supervised the construction of the Dundrum Bay Harbor, designed a water supply system for Belfast, and was responsible for a number of other harbor navigation and arterial drainage projects.

At the time, none of the seven best-known formulas for estimating the flow of water in channels and pipes were accurate enough for Manning. He studied them all and eventually redeveloped one that had been presented by the French engineer Philippe Gauckler. It’s sometimes called the Gauckler-Manning formula, and it determines the Gauckler-Manning (or simply Manning) coefficient, which is still used by hydraulic engineers.

This coefficient depends on a number of factors, including the roughness or smoothness of the pipe or liner wall. The smoother it is, the lower the coefficient, and the greater the pipe’s or liner’s hydraulic capacity. For example, according to the Engineering Toolbox Web site: engineeringtoolbox.com, Manning’s coefficient for plastic and PVC is 0.009; for finished concrete, 0.012; and for corrugated metal, 0.022. The coefficient for a clean, straight, natural stream is 0.030.

The following projects show a variety of ways that stormwater pipes and systems can be successfully retrofitted.

Oakey Boulevard Storm Drain
The city of Las Vegas, NV, doesn’t take chances with its stormwater management systems.

“We get approximately four inches of rain per year,” says Robert Welch, P.E., engineering project manager for the flood Control Section within the city of Las Vegas’ Department of Public Works. “We design for a 100-year storm event, which is 2.77 inches in six hours, and we assume full development upstream.”

Welch handled the planning for a stormwater project that is part of the Clark County Regional Flood Control District’s five-year master plan update. The city uses its major roadway corridors to convey water through city limits, and this project took place along a section of Oakey Boulevard, an east-west street in an older residential neighborhood that had experienced flooding in the past, especially the busy intersection of Oakey and Rancho Drive.

Often residential neighborhoods are developed and the infrastructure is cobbled together as funds become available, which was the case in this 1960s-era neighborhood, he says. The only existing pipes were at the intersection of Oakey and Rancho. They were built as a bubbler system in the 1960s and are just 18 inches in diameter. They were undersized for the flow, which ruled out the option to reline.

Rosa Cortez, P.E., who also works for the city’s Department of Public Works, was the engineering project manager for the design and construction of this project. Cortez is also managing the design of the last phase of the master plan within the corridor, which continues from Cahlan Drive along Oakey Boulevard west to Valley View Boulevard.

“We’re using two types of storm drain facilities based on how much floodwater we’re trying to convey,” says Cortez. The city installed reinforced concrete boxes (RCBs) on Oakey and reinforced concrete pipe (RCP) on most of the north-south streets to collect the flows at the intersections. Both come from various suppliers. The majority of the city’s storm drains are made of RCP. Some have been installed for close to 40 years and are still functioning well, probably because the area receives so little rainfall, Welch says.

The central Las Vegas area is relatively flat, although the street slopes can vary. Much of the surface is impervious, both because the area is fully developed and because the caliche soil, which is common in the desert, restricts drainage because its particles are cemented together by calcium carbonate. Stormwater comes from the west and ultimately ends up in the Las Vegas Wash.

This project covers a little more than half a mile of Oakey Boulevard, between Birch Street on the east and Cahlan Drive on the west. It eliminated the bubbler drains and completed the master planned storm drain infrastructure, which included the installation of a new sewer line and the relocation of an existing water line.

The pipes on most of the north-south streets range from 18 inches to 48 inches in diameter to collect the flows at the intersections. The boxes on Oakey Boulevard range in size from 10 feet by 8 feet to 5 feet by 5 feet.

The contractor, Las Vegas Paving, encountered a number of challenges. “It’s not just digging a hole and putting in a box,” says Cortez.

Although they were working in a busy residential neighborhood, crews had to keep all the roadways open so vehicles could continue to travel on them. They also had to limit the noise disturbance to nearby residents. Instead of excavating the entire project at once, workers began at one end of Oakey and worked in sections, almost like a conveyor system, Cortez says. They also worked through the night during two weekends. On the first one, they installed all the utilities at the Rancho-Oakey intersection so they’d be out of the way and the RCB installation would go smoothly, Cortez says.Another challenge was that excavating caliche soil causes an unusual amount of vibration in the ground. The contractor had to control and monitor the vibration, which could have damaged the swimming pools and the foundations of nearby homes in the old, established neighborhood.

Las Vegas Paving excavated approximately 15 feet below the surface in order to install all the storm drain facilities, but the water table is only 7 to 12 feet down. Workers had to pump the water out of the trenches and pipe it into a bypass system, but the pumping became the third constraint, she says.

They had to be careful to not draw down too much water too quickly, or the change in hydrostatic pressure in the soil could have damaged the pools and the house foundations–especially as the groundwater would return to its original level once the project was completed.

This return of the groundwater to its original level could also put pressure on the newly installed RCPs and RCPs, causing “float.” To counteract the buoyancy, the contractor compacted the soil on the bottom before installing them and backfilled with CLSM (controlled low-strength material). CLSM is a self-compacting material composed of water, portland cement, aggregate, and fly ash. According to the Portland Cement Association, it’s a cost-effective alternative to compacted backfill, is easy to use in limited spaces, makes excellent bedding for pipes, doesn’t settle under heavy loads, and allows a rapid return to traffic.

The Oakey storm drain system between Birch and Cahlan now can convey approximately 1,000 cubic feet per second of stormwater. The project took about three years from the design to the finished construction. The construction itself took 10 months.

“It went very smoothly,” says Cortez. “The contractor is experienced and has done that kind of work before.”

Nags Head, North Carolina
Working on a stormwater pipe project on a beach in North Carolina sounds like a plum job, until you find out more about it.

“We had to do it during the winter because of tourism in the summer,” says John Abel, division bridge manager with the North Carolina Department of Transportation (NCDOT). The temperature was 40 degrees, the winds blew at 40 miles per hour, and it snowed. The installers had wear wetsuits inside the existing pipe to clear the sand out–and the tides kept allowing the sand to seep in around the joints of the existing 1980s-era
concrete pipe.

The pipe is 36 inches in diameter and extends 670 feet, from NC 12 to an outfall at the beach. It was sagging, and sand was leaking in so badly that sinkholes were forming in the sand dunes. An additional problem was that the upstream area flooded occasionally.

The NCDOT has easements that allow excavation and installation or replacement of pipes, but the state’s Coastal Area Management Act (CAMA) requires the approval of all adjacent landowners before any work is done, Abel says, and one landowner wouldn’t sign. Because the DOT didn’t have a CAMA permit, the crew had to work within a 15-foot easement that was designated as a public access area. A liner was the best solution because it could be installed in the limited space. Because of the poor condition of the pipe, a liner with high structural integrity was needed.

The DOT decided on a fiberglass reinforced polymer mortar liner from Hobas Pipe USA, which is under license from Hobas Engineering AG of Switzerland. Although the liner has a 28-inch inside diameter and a 30-inch outside diameter, its thin, smooth walls provide 6% more flow than the original pipe did, which eased the upstream flooding problem. In addition, the liner is very corrosion- and abrasion-resistant, so it will perform well in the harsh environment.

“They ran the numbers and gave us the Manning coefficient,” he says. “It allowed greater velocity than before.”

The project took place between January and March 2010.

“What we were hoping would take two weeks ended up taking six weeks,” says Abel. The installation was very simple and easy. Figuring out how do deal with the sand was the hard part.

The pipe at the roadway end was under roughly 5 feet of sand. As it extends toward the beach, it’s buried nearly 20 feet under the dunes. At the outfall to the ocean, it rests on a pile support system that holds it in place. “The wood takes a beating,” says Abel. “The sand rubs it away. It’s like sandpaper out there.”

The amount of development nearby added to the challenge. “When the pipes were put in, there wasn’t a lot of development. Now almost every piece of property is developed. It’s a whole lot harder to do maintenance on the pipes now.”

The DOT installed the liner from NC 12 toward the ocean. The work crew used two excavators to remove the 4 or 5 feet of sand above a 30-foot section of pipe and stored it in an area approximately 60 feet long, 15 feet wide, and 2 to 3 feet high. They cut the top half from the section of pipe and began installing the liner, which is centrifugally cast fiberglass reinforced polymer mortar (CCFRPM). It is manufactured in sections that are ready to install and its joints are leak-free. Because the existing pipe had separated so badly, another challenge was keeping the sand from filtering back into it while the work was underway. Workers in wetsuits would take a fire hose, get down inside the pipe, and jet the sand out the end into the ocean. As soon as they got it cleaned out, more sand would seep in.

“We tried to work when the tide was outgoing, so the water would help push the sand from NC 12 to the ocean,” says Abel. But the tide was a challenge in its own way because when it rose, it allowed more sand to seep into the pipe. The crew had to stop work when the tides were extremely high due to lunar tides or strong winds from the easterly direction.

The crew slid one section of Hobas pipe at a time through the opening in the top of the existing pipe and pushed it in, then slid the next section in and forced the two together until they were fully joined. Once all the sections were in and connected, workers grouted around the liner and the existing pipe and then replaced the cut top sections onto the existing pipe.

“We didn’t put in as much of the liner as we’d hoped,” he says. “The sections didn’t quite align properly to allow full installation to the ocean, but we were able to get the pipe past the problem area.”

The DOT is very happy with the end product. “Due to the harsh environment of the Outer Banks, with the northeasters and hurricanes, if NCDOT can achieve another 50 years of life from this structure, we would be extremely pleased.”

Clay County, FL
When Tropical Storm Faye blew across Florida in 2008, stormwater rushed through one of the corrugated steel culvert pipes under State Road 16 in Clay County and began to suck the surrounding soil into the pipe.

“It created a void along the side of the road about 10 feet deep and 20 feet across. You could have put a couple of minivans in this hole,” says Spencer Townsend, project manager for the Clay and Nassau counties local sites for Transfield Services North America.

Transfield Services, a multinational Australian company, provides operations, maintenance, and asset and project management services around the world. In Clay County, it performs routine and preventive maintenance along 358 lane miles of roadway for the Florida DOT, but this project was anything but routine.

SR 16 is a two-lane, two-way highway in a rural area and an important truck route that links I-95 in the east to US 301 in the center of the state, Townsend says. There’s very little cover between the 1950s-era pipe and the road, and the pipe–96 feet long and 13 feet in diameter–was much larger than most. Crews couldn’t rip up the road and put in a box culvert because they had to keep the road intact while they were working on the pipe, and the product had to be high-strength because there was so little cover.

Their solution–to spray on high-build, abrasion- and corrosion-resistant mortars from AP/M Permaform of Des Moines, IA–was unusual, too.

“It’s a newer application for this type of product,” says Townsend. “They’ve used it for years in sanitary sewers and manholes, but I believe this is one of the first times it’s been used on such a big pipe and not in a vertical application. It’s very high strength, and it has an additive that makes it very sticky, so it’s perfect for shooting on a pipe.”

Transfield Services partnered with the Florida DOT; Coastal Construction Products, which is based in Florida and supplied the products; and T.V. Diversified Inc. in Lake Worth, FL, the contractor. Diversified built a cofferdam to stop the water, which flows from Black Creek to St. Johns River, from entering the pipe during the project.

“One of the biggest challenges was containing the water,” says Townsend. “The contractor made it look easy.”

Workers pressure-washed the wall of the pipe, which had been treated with bitumen to prevent rust. Because of its age, though, it was corroded where the surface of the water met the air. They built an approach slab and wing walls at one end of the pipe and a departure slab with wing walls at the other.

One of the advantages of the material is that it cost about a quarter less than slip-lining, he says. Another is that it can be custom engineered. Because there was so little cover between the pipe and the road, the engineer on the project specified a maximum of 2 inches and a minimum of 1 inch of cover to be sprayed evenly inside pipe. Fortuitously, the pipe was bolted together with bolts that extended 1 inch from the highest part of the corrugation. “We used that as a gauge,” he says. “The bolts were consistent throughout the pipe.” Workers used PL 8000 as well as a small amount of PL 12000, mixed with water from the stream. “The water quality was tested,” says Townsend. “It turned out to be from good to perfect. All we had to do was put in a rough filter.” One person sprayed it from a hose while a second hand-trowelled behind him. The application took about two weeks and was completed in March 2010.

Transfield Services plans to monitor the pipe, but Townsend isn’t expecting it to need maintenance. There’s a safety feature built into the concrete, and the pipe is so large that it isn’t subject to blocking or sediment buildup.

“The whole project went pretty smoothly,” he says. “I think it was largely due to the cooperation among the supplier, the contractor, the DOT, and ourselves. Everyone worked to ensure that this would be a successful project.”

County of Fairfax, VA
Trees and underbrush had grown up the hill since a concrete storm pipe had been installed in the County of Fairfax, VA, and by 2009, the pipe was failing.

Instead of replacing it, Pleasants Construction, which is based in Clarksburg, MD, and works on site development and water and sewer line construction, used Reline America Inc.’s Blue-Tek, a glass fiber reinforced pipe and a UV cured-in-place pipe (CIPP) lining. Blue-Tek rehabilitated the existing pipe and allowed it to stay in place, says Tim Cook, director of construction for the Rehabilitation Department of Pleasants Construction.

“We do a lot of the drain work for the County of Fairfax,” says Cook. “Around 2007, the county put a ban on all CIPP that’s based on styrene. Blue-Tek liner products don’t release styrene.”

This part of the county is quite mountainous, Cook says. The upstream manhole was buried in a pine grove about 40 to 50 feet from a street, at the end of a row of homes. From there, the pipe, which is 140 feet long and 15 inches in diameter, goes down the densely wooded hill, which drops about 35 feet before it reaches the outfall at a stream.

Once the company located and uncovered the manhole lid, workers measured the drain diameter and length, and calculated the pipe’s depth in the ground to come up with the thickness necessary to support the weight above it.

“Blue-Tek is probably one of the thinnest, if not the thinnest, on the market,” he says. “It’s almost like glass, so it allows the maximum flow.” It’s also strong enough to rehabilitate pipes that are structurally damaged, are leaking, or have reached their expected lifespan. Its strength lies in the seamless, spirally wound glass fiber that is made airtight with polyester or vinylester resins. High-grade plastic films protect the liner both inside and out from damage in the field and eliminate the potential for styrene contamination at the jobsite.

Blue-Tek liners are very cost-competitive, Cook says. They’re also versatile and don’t require a lot of equipment to install, which allowed Pleasants Construction to use the liner for this project. The homeowners didn’t want any of the nearby trees removed, so crews were limited to a 10-foot easement for pedestrian access.

“This backs up to a nice streambed,” says Cook. “They wanted it to look like no one had been there. It was a little tricky to figure out how to do it. We had to modify the equipment for the job so we could do everything from the top.”

The pipe was obstructed by a fencepost, so workers first removed a section of fence, which they replaced when the project was completed. They diverted the stream and cleaned debris, obstructions, and sharp objects from the existing pipe; inserted a slipsheet for ease of pulling and for additional protection; and installed the liner with a constant-tension winch for a steady pull so they wouldn’t risk tearing the liner. Once it was in place, they inserted packers at each end, secured it with straps to prepare for pressurizing, inflated the liner, and inserted Reline America’s UV light train to cure the liner. After the liner was cured, they removed the inner film. The project took about six hours from setup to cleanup.

“Once the liner is cured, you cut the ends off, and you have a brand-new pipe,” says Cook. The liner has a product life of 50 or more years.