At some point, as stormwater pipes begin to fail, almost every manager will face the decision to replace or repair them. The choice depends on a number of factors: What condition is the pipe in? Is it badly degraded, or does it just have some leaky joints? How much ground will be disturbed if it’s replaced, and is excavation feasible? What’s above the pipe–heavy traffic, light traffic, a building, a field? Are there utilities nearby, either aboveground or below? How quickly does the job need to be done? And, perhaps the most important: How much will the replacement or repair cost?
The decision virtually always comes down to effectiveness and cost efficiency. In some situations, replacing a specific length of pipe with precast concrete pipe is the best way to go. In others, lining it with a material like plastic or fiberglass, either cured in place or pulled into place, is the best solution. In still others, all that’s required is to seal leaky joints.
The following three projects illustrate a wide variety of situations–and solutions–from replacing a section of a 100-year-old cast-in-place pipe in Salt Lake City to relining a culvert under a country road in North Carolina to sealing leaky joints under a busy street in Tampa.
City Creek Aqueduct
A hundred years ago, a construction crew worked overtime to build a stormwater pipe in Salt Lake City, UT. Workers built as much as 140 feet a day, for a total of 3,300 feet of 84-inch-diameter cast-in-place pipe with 7-inch reinforced concrete. Some cave-ins occurred because of the rushed construction.
“They wanted to complete it before snowmelt,” says Randy Wahlen, formerly of the Mountain States Concrete Pipe Association in Sandy, UT, a trade association that works for multiple concrete pipe manufacturers. “They finished far ahead of schedule–and it appears that most of it was done with shovels.”
Wahlen used historical photos and newspaper articles to understand how the pipe was built. “It was cast using an outer frame of round wood staves and an inner frame of four removable quarter sections. It was incredible that they could build the pipe they did, and it was incredible that it lasted 100 years.”
The pipe is part of the City Creek Aqueduct, which was completed in 1910. It contains constant flows from City Creek as well as snowmelt from the mountains to the east and stormwater from the Salt Lake City area. It empties in the Jordan River, which flows into the Great Salt Lake.
A section of the pipe runs under North Temple Street. It was functioning well, but it’s very close to a proposed light rail transit line along North Temple that would connect the city’s airport to its downtown. The load wouldn’t have exceeded the current highway loading on the pipeline, Wahlen says, but repairing the pipe in the future might have been disruptive to the tracks.
Salt Lake County hired Ensign Engineering in Salt Lake City to perform a condition assessment and structural analysis of the pipe. Wahlen assisted with the inspection and analysis.
“The wall thickness was right on,” says Wahlen, “but the pipe had much less reinforcing than we would design today.”
View of the new pipe and the trench with structural backfill
Engineers decided that replacing a section of the pipe with precast reinforced concrete pipe, from Geneva Pipe Company in Orem, UT, was the most cost-efficient option. “Lining in that size of pipe is very expensive,” says Wahlen. “With precast pipe, they can reuse the same form and make hundreds of them a day, depending on the size.”
Although the Salt Lake City area is flat and receives little rainfall, the amount of snowmelt from the mountains is significant, and a large volume of water can flow into the pipe. In addition, Wahlen says, this section of pipe is lower than the elevation of the river, so water will back up from the river.
In 1983, the city experienced a significant flood event. Record snowfall that winter, an enormous snowpack, and a sudden warm spell in late May led to flooding, and on May 28, the pipe clogged. City Creek overflowed into nearby streets and didn’t return to its normal course for weeks. Wahlen assisted in the condition assessment and the structural analysis of the pipe in the spring of 2010, which found debris on the ceiling.
Inspections also revealed that while some sections of the pipe looked brand new, others had spalling, reinforcing corrosion and scour. Ensign Engineering took cores of the pipe to determine the strength of the concrete. The compressive strengths ranged from 2,830 psi to 5,750 psi.
“Precast concrete pipe today typically tests out at 8,000 to 9,000 psi,” says Wahlen.
The pipe’s 7-inch wall thickness was the same as is required in the same size pipes today, but the wall had voids and aggregate larger than would be allowed today. In addition, Wahlen says, in some places there was no consolidated concrete at all. He suspects that it was mixed in mining cars, where there would have been no real control over the mixing. There even could have been some snow in it, which would have affected how well the concrete cured.
One of the main advantages of precast pipe is the quality control, he says. Computers control the mixes in the concrete, and some plants even use robotic technology.
The original pipes had single cage triangular steel reinforcement with an area somewhere between a #2 and a #3 bar, about a quarter of what is required today for pipes of the same diameter.
Some of the soil around the original pipe looked like clay. It was replaced with structural backfill.
Crews replaced the pipe in the summer of 2010. They closed one lane on each side of the street and channeled traffic to the remaining lane in each direction. The work took several weeks; one of the challenges was maintaining access to the businesses on the street.
“Dewatering was a challenge, too,” says Wahlen. First, because a constant flow came from City Creek, crews put pumps in the upstream storm drain to pump water around the construction. And although the weather was mostly dry, a storm shut down construction for a day or two. The pipe flooded and they had to divert the water by pumping it onto the street.
“In 1910 they had the same problem,” he says, “and they had to divert the water, too.”
On the other hand, a lot has changed since then. During the construction in 1910, workers unearthed a human skeleton–and just threw it away. Safety also has improved. In 1910, the trench collapsed because it had no reinforcement.
For this replacement project, crews dug down to the pipe with a track hoe. Some of the concrete was still very strong, and underneath, some of the circular wood forms were in surprisingly good condition, Wahlen says.
Crews replaced 11,028 feet of the existing pipe with precast pipe of the same 84-inch diameter, butted the existing pipes to the new ones, and sealed them together by pouring a concrete collar around the connection. The new precast pipe joints have a bell and spigot design, with a gasket on the spigot end. When they’re connected, they are completely watertight.
Once the pipes were connected, crews filled the trench with structural backfill and repaved the street.
“Looking back, both projects were incredible–just separated by 100 years,” says Wahlen. “I’ve always told people that concrete pipe will last 100 years.”
Culvert Over McLeod Creek
A corrugated steel culvert carrying McLeod Creek under the road between Lillington and Sanford in Harnett County, NC, has just been lined with a glass-smooth liner by Hobas pipe.
“It had seen its best days,” says Dennis Johnson, a bridge manager with the national contracting service Applied Polymerics in Mount Airy, NC, who was in charge of the project. The invert of the pipe was starting to rust and corrode, causing it to lose its strength. At the same time, the weight of the fill above the pipe was causing the top to bow.
The site was a perfect place for the liner, Johnson says. There was approximately 30 feet between the stream bottom and the top of the road. Replacing the pipe would have required a cut more than 100 feet long, which could have disrupted traffic for weeks and been very expensive. Instead, sliplining took less than a week, cost much less, and caused less disruption to traffic.
“I think sliplining has its place,” he says. “When there’s a lot of fill, a large volume of traffic, or major utilities, it’s definitely the way to go.”
The project, which took place in 2010, was a team effort, Johnson says. North Carolina DOT bridge engineer L.L. “Sonny” Upole came up with the idea and saw it through, even dropping in on the site to see how work was progressing.
Crews poured about 3 inches of concrete to cover bolts in the bottom of the pipe and nailed long, narrow strips of wood alongside two lines of bolts to prevent the liner from catching on them.
Supplier Pomona Pipe Products in Greensboro, NC, suggested the liner from Hobas Pipe USA. Hobas is the ISTT (International Society for Trenchless Technology) No-Dig Award 2011 Winner for project installation.
“I’ve been in construction all my life and I’ve worked with all kinds of structures,” says Johnson. “But Pomona Pipe Products knows all kinds of pipe and sliplining. They also had a rep, Don Joyce, onsite, who assisted me when we were sliding it in.”
Because it’s centrifugally cast and made of a fiberglass-reinforced polymer, the liner is glass-smooth, which can maintain or even increase flow capacity, according to Hobas. It’s also nonporous, resilient, and abrasion resistant, and can be used for new installations as well as for rehabbing existing ones. Each section of liner has a bell end and a spigot end with a rubber seal. When they’re pushed together, the spigot end fits into the bell end and forms a leak-free joint.
According to Hobas, the design service life is up to 100 years or more. “By using the liner, you’ve basically replaced the pipe,” says Johnson. “You don’t have to worry about it rusting or any maintenance down the road.”
Applied Polymerics installed six sections of 72-inch-diameter liner, for a total length of 110 feet. “The existing pipe was 90 inches,” he says. “Had it not been bowed, we would have gone with a larger liner.”
Because the liner was 18 inches smaller than the existing pipe, crews attached three stacks of 2-inch by 6-inch by 3-foot lengths of lumber to the top of each joint to prevent the liner from floating up during installation.
It’s crucial not to let the liner catch on anything as it slides in, he says, and the existing pipe was held together with bolts that protruded from the walls.
Crews first washed the culvert out, then built a dam with sandbags upstream to dewater the pipe. They poured about 3 inches of concrete to cover the bolts in the bottom of the pipe so the liner would slide in more smoothly.
“That was my first time installing this type of liner,” says Johnson. “Next time, I may just put boards on the bottom instead of concrete.” Once the concrete had cured, crews removed the dam, and for most of the rest of the project, they worked in about 6 inches of water.
Two lines of bolts were spaced closely together at approximately 7 and 5 o’clock along the entire length of the pipe. Long, narrow strips of wood were nailed alongside the bolts to prevent the liner from catching on them. Because the liner was 18 inches smaller than the existing pipe, crews also attached three stacks of 2- by 6-inch by 3-foot-long pieces of lumber to the top of each joint to prevent the liner from floating up during the installation.
It is also important to protect the front end of each section of liner while it’s being pulled through the pipe, Johnson says. Pomona Pipe Products provided a metal nosecone that fit inside exactly and allowed workers to pull evenly all around the pipe.
Applied Polymerics attached a steel 12-inch beam to the concrete headwall at the inlet end of the existing pipe and attached a winch to the very center of the pipe. Workers set the first section of the liner at the outlet end of the pipe, ran a cable from the winch through the liner, hooked it to the nosecone, and pulled the liner inside and through the pipe into place at the inlet end.
They repeated the process with the remaining sections of liner, pushing them together to form leak-free joints.
Once the sections were connected, they injected grout in the space between the pipe and the liner until it was completely solid and grouted both ends of the culvert and both faces were smooth. The project took about three days to install the liner and another couple of days to grout.
“After the grout was injected, it was pretty much a done deal,” says Johnson. “I think this is a great product. Once you install it, you have a new structure.”
Swann Avenue Pipe Rehabilitation
The video camera sees all–and sometimes it isn’t pretty. Stormwater pipes can be rusted and corroded, filled with trash and sediment, bowed from the weight of the fill above them. It might be years before anyone notices the damage.
But the city of Tampa, FL, which spends millions of dollars every year to repair, improve, and expand its stormwater management system, videotapes inside its pipes, so it can repair damage before it becomes too severe. This is what happened in 2010 with the pipe that runs under Swann Avenue.
Swann Avenue is a two-lane collector road with a center left-turn lane. The 5-block stretch from South Howard Avenue to a retention pond at South Packwood Avenue is a commercial area with restaurants, banks, and a park in a very urban setting. In June 2008, it had a peak volume of 395 vehicles per hour in the afternoon, according to the city.
The land is flat, so there isn’t much velocity to the stormwater, but the volume can be high. The annual rainfall in Tampa is approximately 52 inches, and during the rainy season, especially during tropical storms, 2 to 3 inches of rain can fall in one day.
“We noticed the street was experiencing depressions,” says Michael T. Miller, the city’s stormwater engineer on the project. “We figured the soil must be going somewhere. We’d done a video inspection the prior year and saw very little. The structural integrity was still there; there were just some leaky joints.”
Digging up the street and repairing the joints would have caused too much disruption to the traffic. Miller considered sliplining, but couldn’t see the advantage because the pipe was structurally intact.
John Compton inside a pipe with a completed joint seal, an extrawide Weko-Seal held in place by three stainless steel bands
“I had been to a trade show and had seen Miller Pipeline (no relation),” he says. “It was my first introduction to Weko-Seal, and I thought it looked pretty promising. I asked Miller for a demo and our field crews were very impressed with them; they really liked them.”
Weko-Seal is a flat, typically 6-inch-wide cylinder made of EPDM rubber that seals the full circumference of pipe joints from the inside. It can be manufactured to fit pipelines that are round, square, elliptical, and many other configurations, in sizes ranging from 18 inches to more than 216 inches. It is manufactured and installed by Miller Pipeline Corporation, headquartered in Indianapolis, IN.
In the case of Swann Avenue, the pipe was a 43- by 68-inch elliptical reinforced concrete pipe, 1,680 feet long. The city decided to use Weko-Seal on all 210 joints.
Most of the manholes along Swann Avenue are in the left-turn lanes, and these are where Miller Pipeline staged its equipment, to divert traffic to the remaining lane.
“The traffic was kind of a nightmare,” says John Compton, Miller Pipeline’s foreman on the project and part of the company’s road crew that travels across the world installing Weko-Seal. “We were right in the middle of a busy street. We set the truck in such a way that drivers would have to hit the truck before they hit the personnel.”
The crew used a jet truck to clean the majority of the dirt and debris and scoured the rest by hand. Some areas were more than half full of sand, he says. Workers built dams to keep it clear of water; however, although the project was done during the dry season, it rained two to three times every week, and sand and stormwater infiltrated through the leaky joints. The crew hand-shoveled the sand out, but the stormwater could have been dangerous.
“I watched the weather every day, hour by hour, so I knew in advance what was going to happen,” says Compton. “If it started to sprinkle, I pulled everyone out to see what would happen. When a quarter- to a half-inch of rain falls above ground, it can fill one to two feet inside the pipe. There were a couple of incidents when the pipe got three-quarters full. It took 20 to 30 minutes to happen. It’s like a flash flood, in a way.”
In addition, there was always some groundwater seeping into the pipes, Miller says. Workers used pumps to keep the water down, and when it backed up too much, they opened up the downstream dam to let it through, then rebuilt the dam.
They started at the upstream end, prepping the joints by scraping down to the concrete, filling them with oakum–a waterproof mixture of tar and hemp–and resurfacing them with a portland cement and sand mixture.
“Then we popped in the Weko-Seal so it straddled a joint,” says Compton. Workers placed a stainless steel band on each side of the 6-inch seals and three bands on the extra-wide ones. They used a spinner bar to expand the bands to 4,500 psi to compress the seals against the pipe wall, then drove a metal wedge into each band to hold it permanently in place. Finally, they did a pressure test to ensure that the seals were airtight.
Some 178 of the 210 seals were the standard 6-inch width. Workers used 32 extra-wide 9-inch seals to seal disjointed pipes and wrote “EW” on them to identify them. Before they finished the job, they discovered that there was considerable infiltration from about 20 joints to the west of the project, so they sealed those as well. They videotaped the completed job with a robotic camera, gave the tape to the city, and removed the dams.
“This is usually a two-week project,” says Compton, “but it took five weeks because of the sand and the rain. Besides that, it went very well.”
