Phase two: The plates have interior flanges about 1.5 inches thick that give purchase to the bolts holding the arced plates together and allow sections to be bolted together.
To create a smooth, watertight, structurally sound pipe within the new tunnel liner plates and to smooth out the corrugations created by flanges, Utility Asset Management used a relatively new trenchless repair technology, developed by AP/M Permaform, called CentriPipe. CentriPipe is centrifugally cast concrete pipe (CCCP) that relies on a computer-controlled spincaster that’s inserted into round, large-diameter pipes, spraying on thin layers of high-strength cementitious grout as it’s pulled back through the pipe at precisely calculated speeds. (For more information, see page 28 of the 2013 Public Works Manual.)
The contractor began by hand-troweling grout to fill in flange gaps, and to create a smooth invert so that the spincaster could be pulled back without jerking. An engineered, 1-inch thickness of AP/M Permaform’s PL-8000 high-strength grout was then applied, completing rehabilitation.
The process basically created a new concrete pipe within the new tunnel liner plates. Since both are structurally sound, and the PL-8000 adheres to steel and most pipe materials, the combination of tunnel plates and new pipe is exceptionally sturdy and will last indefinitely.
Preventing future failure
There was a third phase. The dramatic failure of the pipe was largely due to extremely unstable surrounding soil.
“This was basically the unregulated piping of a live stream,” Moy explains. “The lack of engineering led to a lot of water flow around the pipe and large soil voids.”
To prevent similar problems for the rehabilitated culvert, county engineers called for stabilization of the surrounding soil with cementitious grout injected until refusal. (For more information on cementitious grout, see page 68 of the 2013 Public Works Manual.)
“It came to a lot of grout,” says Moy. “Picture a box big enough for a pickup truck. About three of those were pumped in.”
To ensure quality, the county had very tight project specifications, applied normal inspection procedures, and contracted with an independent geotechnical firm for onsite representatives. After a year, the project is performing well.
“It’s hard to say you’re happy with an unplanned project that ran to almost $400,000,” says Moy. “But we recognize the value of this solution over others that were considered, and we’re happy that we were spared possible construction impacts, such as trenching.”
The work was procured as a request for proposal (RFP) and was performance-based to allow innovative approaches that minimized construction time and site impacts to businesses.
“Centrifugally cast pipe is proving to be a viable solution,” he says. “Together with the tunnel plates and grout injection, it was certainly effective for us.”
Angus Stocking is a licensed land surveyor who has been writing about infrastructure since 2001. E-mail firstname.lastname@example.org your experiences with centrifugally cast concrete pipe.