Iowa town chooses 'once-in-a-lifetime' solution for piping challenge
When Council Bluffs' public works officials began examining sewer rehabilitation options, one of the city's largest sewer outfall lines was at stake, a combination of 48" and 42" pipe that carried about two-thirds of the city's wastewater. The pipe was cracked, had offset joints and was corroding away from the inside. The solution they chose was HOBAS pipe, which can be jacked under existing structures and comes in 20' lengths...
COUNCIL BLUFFS, IA, April 8, 2005 -- Council Bluffs has been a popular place to visit since Lewis and Clark arrived in the area in 1804. Today, it's part of the greater metropolitan area that includes Omaha and has a population of only about 60,000 but welcomes more than 10 million visitors a year. The many parks and historical sites or the three gambling casinos may attract them. Or they might be attending a convention.
They arrive by plane, train, car or bus. But in any case, the dynamic city needs to accommodate them. Obviously, infrastructure is vital to Council Bluffs and the city's leadership knows it.
The city's buried infrastructure is just as important as the highways, bridges and railroads. Like any modern city, Council Bluffs needs efficient, reliable sewers to remain vibrant and growing. Although unseen, the city's sewers share top priority with its aboveground structures. So, when Council Bluffs' public works officials began examining sewer rehabilitation options, they were very deliberate in their selection process.
One of the city's largest sewer outfall lines was at stake, a combination of 48- and 42-inch pipe that carried about two-thirds of the city's wastewater. The pipe was cracked, had offset joints and was corroding away from the inside. In some places, the six-inch wall thickness had deteriorated to only a couple of inches.
"We have some real challenges with regard to sanitary sewer flows," admitted Gregory Reeder, P.E., city engineer for Council Bluffs. "We have a high water table and very flat terrain in some areas." To illustrate the problem, he noted that the new sewers in Council Bluffs often have as little as 3/4-inch fall every 100 linear feet.
"Our pipes sit underwater in some places," he continued. "There are places where the sewage just sits there in the pipe from time to time. That creates hydrogen sulfide that attacks the concrete pipes. So we have had corrosion problems as well as installation issues due to our flat terrain and high water table."
With hydrogen sulfide eating away at its main sewer line, the Council Bluffs' public works department knew that a major sewer failure was inevitable. The line had already experienced several problems. Leaky pipes had caused soil erosion, offset joints, sink holes, soil infiltration and a number of other worries.
In 1998-99, the city began exploring repair and replacement options, but finding the right solution was complicated. The pipe's size, burial depth and high ground water made the project extremely difficult and potentially very expensive. Engineers also had to consider that the line ran under an interstate highway, a street and a levee.
The city hired a team of consultants from George Butler & Associates and HGM Associates of Council Bluffs to study the sewer and make recommendations. Terry Smith, P.E., of HGM, was the lead engineer on the project.
"We conducted a study, reviewed grades and pipe capacities, and we considered several alternatives," said Smith. "We looked at sliplining, cured-in-place pipe and a partial replacement. We even considered replacing the entire line. The city's position was to do it right, to make sure all the problems were fixed and that the capacity of the line would be there for future growth."
"Doing it right" in this case could have been incredibly expensive and disruptive, given the size of the line and the dewatering that would be necessary to control the ground water and prepare the site for new pipe installation. Dewatering alone would have cost about $35 per linear foot of pipe. But the city placed a priority on making this project fail-safe in the future. They wanted a new line that would last 50 to 100 years, so they considered every method.
Reeder, the city engineer, explained why avoiding sewer leaks and pipe problems was such a priority in Council Bluffs, "A pipe failure where a deep sewer repair is needed can cost $50,000 to $100,000. That's why watertight joints were extremely important for this project. We knew much of the line would be under the water table and we didn't want the joints taking on groundwater. But with larger diameter pipes, your options are limited. We looked at a lot of alternatives but we kept coming back to a product called HOBAS pipe. Of all the products we looked at, theirs was the most corrosion resistant and their joints are extremely water-tight."
After careful analysis, the city opted to conduct a series of five projects to replace the old sewer with a new 60-inch line made with HOBAS pipe. The cost of the projects ranged from $780,000 to $2.6 million. In total, the city now has almost 17,000 linear feet of new sewer line ranging from 42 inches to 60 inches in diameter, much of it parallel to the old sewer but in a more efficient layout and designed to accommodate future growth.
HOBAS pipes, which come in sizes up to 110 inches in diameter, are centrifugally cast of glass-fiber reinforced polymer mortar in a precise, computer-controlled process that produces a pipe of high strength and corrosion resistance without add-on linings, coatings or cathodic protection. It has excellent long-term hydraulic characteristics due to the smooth, non-corroding interior surface and is highly resistant to abrasion.
Another reason Council Bluffs chose HOBAS is that the pipe can be jacked under existing structures. Because the old line ran under several of them, finding a non-disruptive solution was important.
Gene Lea, president of B&L Construction Co. in Omaha, Neb., was an early proponent of HOBAS pipe. Lea's company was the low bidder for all five Council Bluffs projects.
"HOBAS has FWC coupling joint connections," said Lea. "You can cut the pipes to shorter lengths in the field and utilize the push-together FWC couplings. It's very simple. That gives you a lot of installation flexibility. Plus, the joints are absolutely watertight. They will not leak, and that was an extremely important factor."
From a cost-efficiency standpoint, Lea and his installation team also benefited from the fact that HOBAS pipes come in 20-foot lengths. Concrete pipes, which are heavier, typically come in lengths of 8 or 12 feet.
"Our production rates were very good, much better than with other materials," said Lea. "It takes a certain amount of time to lay a length of pipe. If you have to lay two to two-and-a-half times as many joints, the job will take a lot longer."
Lea said that there were no unusual installation requirements for the new pipe. The pipe was enveloped in sand so no special backfill was needed. "We had very few questions about the installation. It was just real easy," he noted.
Construction of the new line stretched over five years. In retrospect, the engineers said that they would do only one thing differently about the project -- utilize HOBAS Tee Base system for all the manholes.
"If we had to do it all over again, we would use the HOBAS Tee joints for the manholes rather than the lined concrete manholes we used in Phase One," said Reeder. "We just weren't happy with those. After Phase One, we used Tee Base sections with fiberglass riser cones. Both of these products resist hydrogen sulfide attack. I have total confidence that these products will give us 50 to 100 years of service without any problems."
"HOBAS pipe was an excellent choice for Council Bluffs," said Reeder. "Just about any other option runs a much greater risk of failure in the future. We wanted this to be a once-in-a-lifetime solution."
HOBAS Pipe USA (www.hobaspipe.com), based in Houston, Texas, is a producer and supplier of corrosion resistant HOBAS centrifugally cast fiberglass reinforced polymer mortar pipe systems with product and process technology under license from HOBAS Engineering AG of Switzerland. The pipe is manufactured in sizes from 18 inches to 110 inches in pressure and non-pressure classes.