Rehabilitation of corrugated metal pipe stormwater culverts is a common task in any sewer network, but several factors in a recent Waukon, IA, project presented unusual challenges. For one thing, this was a very large culvert with a diameter of 72 inches, and the CMP was badly degraded with an invert that was completely rotted out. The alignment of the pipe was in even worse shape; in 475 feet, five sections had been displaced horizontally and vertically, with gaps or offsets between sections ranging from several inches to more than a foot.
This combination of factors ruled out two traditional methods of trenchless repair. “The out-of-roundness made slip-lining impractical, as bridging the gaps between offset sections wouldn’t have worked,” explains Robb Johnson, P.E., vice president at project contractor Engineering & Construction Innovations Inc. (ECI). “And for the same reason, CIPP was not an option.” The culvert’s large size was also an argument against CIPP rehabilitation; in large-diameter pipe, CIPP repairs are risky and prohibitively expensive.
So, at the suggestion of city engineers Fehr Graham, Waukon turned to centrifugally cast concrete pipe (CCCP). CCCP, also called spin-casting, relies on a spin-casting pump, which is inserted into a pipe and withdrawn at precisely regulated speeds, applying thin layers of high-strength cementitious material that adheres to pipe substrates and cures quickly. In essence, the process casts a new concrete pipe within old pipes. The new pipe is completely sound, structurally, so substrate condition is not a factor except as it affects the casting process. And, because the new concrete pipe is thin (about one inch in most applications) and smooth, reduction of storm sewer flow capacity is usually negligible. In Waukon, the spin-casting system used was CentriPipe from AP/M Permaform and the material applied was PL-8000, which is distributed by AP/M Permaform and used with the CentriPipe process and in other situations where very high strength is needed along with good adherence. It relies on precisely graded quartz sands, non-metallic fibers, and other complex admixtures to achieve a unique blend of strength and other desirable properties.
A Successful Solution
This was the first use of the CentriPipe system in Waukon, but not the first for ECI, which first applied CentriPipe in a 2009 pilot project for the Minnesota Department of Transportation (MnDOT) District 6. That project addressed a series of failing 36-inch corrugated metal pipe culverts and was judged a success by both MnDOT and ECI. “Since then, we’ve done two to three CentriPipe projects annually, usually in larger sewers, up to 90 inches,” says Johnson. “It’s often a good choice for very large diameter sewers or for projects that are a bit more geometrically complex, as in Waukon.”
In Waukon, the rehabilitation began with dewatering, which was mostly straightforward. Work took place in August and September, with no storm events, so minimal flows could be bypass-pumped into a parallel section of the storm network. But there was some groundwater inflow through the dilapidated, or actually missing, invert. To stem this inflow, and to provide a smooth surface for CentriPipe sled withdrawal, ECI crews used pumped PL-8000 to cast a new invert along all 475 feet of the culvert. Also in preparation for spin-casting, crews securely covered just under a dozen inlets into the culvert, so that they wouldn’t be blocked by the newly cast pipe.
Spin-casting proceeded section by section, starting from the downstream outlet into a shallow basin, which provided an adequate staging area for the CentriPipe pump and winch. To achieve the specified design thickness of 1 inch, each section was sprayed three or four times, with care taken to sufficiently overlap the pipe offsets and gaps. To ensure layer thickness and project quality, ECI started with a 10-foot test section on a prepared section of pipe to verify and confirm equipment, material, and application techniques, to ensure proper project parameters were met to the satisfaction of the project’s engineer-of-record. CCTV passes were made before and after CentriPipe work, and simple depth gauges were used after each pass. Since PL-8000 arrives in bags and is mixed on site, the volume of material applied can also be tracked for a redundant method of ensuring thickness. PL-8000 cures quickly, so section passes can be completed on successive days. In all, five adjacent sections were rehabilitated in about three weeks.
ECI cast four 2-inch cubes of concrete each day for testing by an independent laboratory. “It wasn’t called for in the project specifications, but we feel it’s good practice,” says Johnson. “It was a way for us to show that PL-8000’s compressive strength, which is 10,000 psi at 28 days, exceeded project specifications.”
Waukon inspectors were impressed with the CentriPipe process. Johnson says, “The original contract called for just 374 feet of culvert to be rehabilitated, but after seeing the initial results, the city awarded another 100 feet upstream that we were able to add on.”
After a couple of dozen projects in the Midwest, ECI is getting used to results like this. “The CentriPipe system has been an excellent addition to our company, and we’re seeing a lot of interest from municipalities. For challenging situations like this one in Waukon, it’s a high-quality, cost-effective solution, and we’re glad the word is getting out.”