When the Wright County Highway Department in Minnesota decided to improve approximately 1 mile of County State Aid Highway 18 (CSAH 18), one goal was to improve the structural strength of the roadway. Another goal was to enclose the drainage ditch running along the road, which would be done using large diameter corrugated polypropylene drainage pipe.
Completed in October 2018, the project was designed to improve safety and capacity through structural and functional improvements of the roadway. Some advancements included widening the roadway for left-turning traffic, providing right-turn lanes and improving intersections to meet current state aid design standards for the future capacity needs of the corridor. As the most heavily traveled two-lane road in the county, just west of Minneapolis, it is used by some 5,100 vehicles a day. Approximately 1 mile in length, the new section of CSAH 18 was open to traffic in October 2018.
Storm sewer and watermain pipes were re-constructed as part of this project along with relocation of utilities by several private companies. The storm water drainage pipe was buried at variable depths of 3 to 8 feet deep along the roadway to capture runoff and reduce saturated conditions in the ditches along the road. Previously, large amounts of runoff were causing problems with roadway ponding and flooding of nearby houses and business. Nearly 6,000 feet of storm water pipe was used, ranging from 15 to 30 inches in diameter. Exceeding the requirements for the project, corrugated dual-wall polypropylene (PP) pipe from Prinsco, Inc. was selected as an alternative to reinforced concrete pipe (RCP). Veit & Company, Inc. was the prime contractor and WSB & Associates, Inc. provided inspection and construction management services.
“The robust polypropylene material has a high stiffness coefficient and is well suited for a variety of applications,” said Daniel Currence, P.E. director of engineering for the drainage division of the Plastics Pipe Institute, Inc. (PPI). “This inherent benefit improves joint performance by creating a solid connection in less time and effort than RCP. Because both RCP and PP pipe were approved for the project, the bids had highly competitive pricing, which meant substantial cost savings for the project and taxpayers. This is an example of allowing open competition on a state road project, which provided many benefits for Wright County. Many areas of our country, however, have restrictive specifications that do not allow thermoplastic pipe to compete, creating virtual monopolies for legacy products and driving up costs. When all vetted products are allowed in a bid specification, that saves taxpayers a lot of money. Plus, when that alternative is thermoplastic pipe, installation is faster, which cuts the amount of time drivers might be inconvenienced by the construction of projects like the CSAH 18.”
Because the Prinsco GOLDPRO Storm pipe is manufactured in 20-foot lengths compared to a typical 8-foot length for concrete pipe, it allowed the project to be installed much quicker. Moving the pipe into the ditch and properly aligning the joints required less time, and because the polypropylene pipe has an inline bell, additional excavation for bell holes was not necessary as would be needed for RCP pipe. The Prinsco pipe is also lightweight making handling easier.
Kory Eichhorst, project manager on the job site of WSB & Associates, said, “The installation went quickly due to the light weight of the pipe, especially in tight spaces.”
“Pipe weight is a critical consideration in the cost and use of fuel to deliver the product–a component of the carbon footprint,” Currence said. “An 8t-foot length of 30-inch RCP pipe weighs more than 3,000 pounds. A single 20-foot length of 30-inch diameter polypropylene pipe weighs 348 pounds, approximately one-tenth the weight of RCP. Additionally, three sections of RCP pipe would be needed to construct the same 20-foot run of one ‘stick’ of polypropylene pipe. That means a 20-foot run of RCP pipe would take 7,500 pounds of pipe compared to 348 pounds for the polypropylene pipe.”
Currence continued by saying that “ weight differential means a great deal in transportation costs and greenhouse emissions. The PPI data library shows that most states allow a maximum load of 80,000 pounds when trucks have from four to seven multiple axles, and the length of the truck bed is long enough to spread out the weight. So, the largest single delivery for RCP would be restricted to just 64 feet of pipe. As a comparison, even a tandem trailer can deliver 320 feet of corrugated thermoplastic pipe with multiple diameter sizes nested inside each other and be way below the highway weight restrictions. While this makes roads safer, the reduced fuel use reduces the greenhouse impact.”
After installation of the four different pipe sizes, each run was inspected by deflection testing to 5% of the diameter. All the pipe, nearly 6,000 feet, passed the test with Prinsco’s custom mandrels pulled through the pipe.
“Performance of large diameter corrugated thermoplastic pipe is dependent on quality of installation,” Currence stated. “The majority of the structural backfill settles around the pipe within 30 days after installation–very little settlement occurs after that time period.”
Metal mandrels were pulled through over a mile of pipe, manhole to manhole, ensuring the pipe had not deflected beyond allowable bounds after installation. Each test was completed 30 days after installation without flaw. Pulling the mandrels through each run verified the structure of the pipeline.
“The whole project, including the mandreling, went very really well”, said Nick Decker of Veit & Company. “I was impressed with the polypropylene. From the results of the deflection testing, the CSAH 18 project far exceeded requirements and expectations set by the county.”
Currence added: “As contractors and engineers gain experience through projects such as CSAH 18, more people in the industry can feel confident specifying and installing corrugated plastic pipe products.”