By: Angus W. Stocking
During a routine annual inspection in 2013, Grottoes, Va.—a town of 2,600 noted for its proximity to Grand Caverns, America’s oldest show cave—discovered severe corrosion in portions of its storm water system.
A large set of elliptical CMP culverts did not pass inspection—they were in poor condition with severe corrosion. Individual sections were failing and misaligned, and the town’s consultants recommended replacement.
Complicating the issue, the failing pipes were four parallel culverts, which are all are quite large—70 in. by 44 in.—running directly underneath Dogwood Avenue, one of Grottoes’ two main thoroughfares.
The town obtained cost estimates for trench-and-replace from Brunk & Hylton Engineering Inc. and, as expected, the price was high and the plan called for significant and lengthy traffic disruptions. Fortunately, Grottoes Town Manager Jeff Nicely had another idea.
“At a Rural Water Association conference, we’d seen a process called CentriPipe that looked like it could be useful in this situation,” he said. “The CentriPipe contractor in our area, Mike Shepherd, explained it to us, and we were very interested and asked Brunk & Hylton to look into it for us.”
The CentriPipe process is a centrifugally cast concrete pipe (CCCP) solution based on spin-casting technology developed by AP/M Permaform. While pulling a spin-caster through failing pipes, a very strong, highly adhesive, fiber-reinforced cementitious grout is sprayed onto the pipe in thin layers. As the layers build up, typically to a design thickness of around 2 in., they form a new, structurally sound concrete pipe within the old pipe.
The new pipe is intrinsically structural and thin, and it adheres tightly to the existing pipe or culvert with only minimal impact on sewer flow capacity. And because no annular space is left between the old and new pipes, no ground or storm water flow exists in that area. The material used—PL-8000 from AP/M Permaform—adheres to metal, clay, brick and HDPE. CentriPipe is also cost-effective: It generally costs less than other large-diameter rehabilitation methods.
But most importantly for Nicely, CentriPipe is a trenchless solution.
“Compared just on project cost, using CentriPipe was 15% cheaper than digging up the old sewers and replacing them,” he said. “But that doesn’t even account for the savings gained by not disrupting traffic for weeks.”
Mike Shepherd’s crew at D&S Contractors, as well as Arold Construction—both licensees of AP/M Permaform—is performing the work in two phases.
The crews cleaned the culverts and then made multiple passes, pulling the spin-caster on skids and pumping PL-8000 that was mixed on site (the material is dry and delivered to the staging area in bags) to build up a final thickness of 2 in. This dimension and other specifications were established by consulting engineers contracted by AP/M Permaform.
Quality control was monitored in two ways. To ensure the correct thickness was applied, “We measured the old CMP, from the top of the corrugations, and compared that to the finished product,” said Nicely. “And we also drilled several holes along the new concrete, to verify thickness. Everything was to specification.”
The new culverts are smooth, seamless, watertight and structurally stronger than the old CMP, with a longer projected service life.
“We’re quite happy with the results, and we expect to do the second phase—about 262 lineal feet—this year,” said Nicely. “Best of all, we never had to stop traffic.”
