Angus W. Stocking, L.S. is a licensed land surveyor who has been writing about infrastructure since 2002. Stocking can be reached at [email protected].
undefinedRehabilitation of corrugated metal pipe (CMP) storm water culverts is a common task in any sewer network, but several factors in a recent Waukon, Iowa, project presented unusual challenges. For one thing, this was a large culvert with a diameter of 72 in., and the CMP was badly degraded with an invert that completely rotted out.
The alignment of the pipe was in even worse shape. In 475 ft, 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 sliplining impractical, as bridging the gaps between offset sections wouldn’t have worked,” said Robb Johnson, P.E., vice president and project contractor for Engineering & Construction Innovations Inc. (ECI). “And for the same reason, CIPP was not an option.”
The culvert’s large size also was an argument against CIPP rehabilitation. In large diameter pipe, CIPP repairs are risky and prohibitively expensive.
At the suggestion of city engineers Fehr Graham, Waukon turned to centrifugally cast concrete pipe (CCCP). CCCP, also called spincasting, relies on a spincasting pump that is inserted into the pipe and withdrawn at precisely regulated speeds to apply thin layers of high-strength cementitious material that adhere to pipe substrates and cure quickly. In essence, the process casts a new concrete pipe within old pipes.
The new pipe is structurally sound, therfore substrate condition is not a factor except as it affects the casting process. Because the new concrete pipe is thin (about 1 in. in most applications) and smooth, reduction of storm sewer flow capacity usually is negligible. In Waukon, the spincasting system used was CentriPipe and the material applied was PL-8000, both are distributed by AP/M Permaform and used with the CentriPipe process. It relies on precisely graded quartz sands, non-metallic fibers and other complex admixtures to achieve a blend of strength and other properties desirable for horizontal pipe and sewer rehabilitation.
A Successful Solution
This was the first use of the system in Waukon, but not the first for ECI, which first applied CentriPipe in a 2009 pilot project for the Minnesota Department of Transportation’s (MnDOT) District 6. That project addressed a series of failing 36-in. CMP 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 in.,” Johnson said. “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 mostly was straightforward. Work took place in August and September, with no storm events, so minimal flows necessitated pump bypassing into a parallel section of the storm network. There was some groundwater inflow through the dilapidated and partially missing invert, however. To stem this inflow and provide a smooth surface for CentriPipe sled withdrawal, ECI crews used pumped PL-8000 to cast a new invert along all 475 ft of the culvert. In preparation for spincasting, crews securely covered just under a dozen inlets into the culvert, so they would not be blocked by the newly cast pipe.
Spincasting then proceeded section by section, starting from the downstream outlet into a shallow basin, which provided a staging area for the pump and winch. To achieve the specified design thickness of 1 in., 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 ft test section on a prepared section of pipe to verify and confirm equipment, material and application techniques so proper project parameters were met and satisfied the project’s engineer-of-record.
CCTV passes were made before and after the CentriPipe work, and depth gauges were used after each pass. Since PL-8000 arrives in bags and is mixed on site, the volume of material applied also can 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 approximately three weeks.
On their own initiative and per project specifications, ECI cast four 2-in. 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,” Johnson said. “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.
“The original contract called for just 374 ft of culvert to be rehabilitated, but after seeing the initial results the city awarded another 100 ft upstream that we were able to add on,” Johnson said.
