Peachtree City Saves With Trenchless Storm Sewer Repair

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Peachtree City, GA, part of the Atlanta metro area, is an interesting place; it’s a designated foreign trade zone, and it’s also known for its 90-mile network of golf cart paths. About 9,000 households own golf carts, and they’re a major component of the city’s transportation infrastructure. The city must be doing something right–as recently as 2009, it was ranked eighth on Money magazine’s list of the 100 Best Places to Live in the United States.

Like many cities, however, Peachtree City is struggling to keep up with failing infrastructure.

“A little more than 95% of the city’s stormwater pipe network is corrugated metal pipe,” explains stormwater manager Michael Madison. “Almost half of our corrugated metal pipe currently in the ground is beyond its expected service life. We are routinely identifying pipe systems that are severely corroded, most to the point where the pipe invert is completely rusted through, leaving holes in the invert of the pipe.”

For rehabilitation of failing large-diameter storm sewer, Peachtree City is now routinely using a relatively new technique called centrifugally cast concrete pipe (CCCP), a process pioneered by AP/M Permaform. The company’s technology is called CentriPipe and is based on a concrete spincaster being pulled through pipe at precisely controlled speeds. The spincaster applies one or more coats of high-strength cementitious grout, essentially casting a new concrete pipe inside the failing pipe. The new pipe is structurally sound and does not rely on support from the existing pipe. And because the new pipe is smooth, and quite thin–engineered pipe thickness is usually well under 2 inches–flow capacities aren’t significantly reduced.

“The Fayette County Stormwater Department invited us to observe a CentriPipe project they were involved in, and we were impressed by the whole process, especially the compact and unobtrusive nature of the work,” says Madison. “We’ve used it on several projects since then, and it really works well for us.”

A Sensitive Project
A recent project gives an idea of how the process fits into Peachtree City’s stormwater system maintenance and rehabilitation program. A sinkhole opened up around a drop inlet on Hip Pocket Road. Inspection revealed that about 325 feet of 42-inch-diameter corrugated metal pipe (CMP) was severely corroded, with complete rust-through of the pipe invert in several locations. Several large voids had also developed below the pipe. The estimate for a conventional trenching and replacement was $230,000.

Why so high? Well, in several areas of Peachtree City, the sanitary sewer is constructed with a product called truss pipe. Truss pipe, introduced in 1965, is basically plastic pipe with a truss-like matrix between thin outer and inner layers. On paper, truss pipe has many advantages: It’s light, strong, and relatively inexpensive. But in Peachtree City, at least, it has proven to have a fatal flaw–it has become brittle with age, to the point where it can’t really be disturbed. “We try not to disturb truss pipe as much as possible,” says Madison. “Just bumping into it can cause small cracks to form and run along whole sections. Once cracked, it has to be replaced.” On Hip Pocket Road, the 42-inch CMP crossed the truss pipe sanitary sewer in several locations. An 8-inch-diameter truss pipe sanitary sewer was also co-trenched with the 42-inch CMP. Essentially, then, digging and replacing the storm sewer would also require replacement of multiple sanitary sewer lines and extensive bypass pumping.

Fortunately, CentriPipe requires no trenching, and it works from inside the pipe. Utility Asset Management Company (UAM), based in Byron, GA, is a CentriPipe contractor and began work by repairing the rusted CMP invert and voids. Because the spincaster needs to be withdrawn without jogging or bumping, CentriPipe projects often begin with the creation of a smooth new concrete “runway” along the invert, especially on CMP. On the Hip Pocket Road project, some voids below the pipe were quite large and required quite a bit of new concrete. And one area of pipe was badly bellied and required a spot excavation so that a section could be lifted and leveled.

With invert repairs accomplished, work went smoothly despite difficult access–one end of the storm sewer run ended in a 3-foot by 3-foot junction box, and the other discharged a few feet above Lake Peachtree. “It was a tight fit,” says UAM president Anita Clyne. “But we were able to do our work.”

All in all, the project took four days from start to finish. The thickness of the pipe and the number of layers was established by engineers contracted by AP/M Permaform, and in this case three passes that laid down less than an inch of new pipe were sufficient. The grout used was PL-8000, supplied by AP/M Permaform. PL-8000 is a fiber-reinforced cementitious grout specially developed for spincasting. It is strong, sets quickly, and adheres tightly to most pipe surfaces, including steel, clay, and brick.

Best of all, the CentriPipe project ended up costing Peachtree City only $72,000, a savings of $158,000 compared to the trench-and-replace estimate.

Quality control and inspection was accomplished by tracking the amount of PL-8000 applied, spot-checking layer thickness with simple gauges, and visual inspection of the cured layers and final pipe. Spraycasted PL-8000 was also used to coat and seal the junction box.

A Successful Solution
The Hip Pocket Road storm sewer was the fifth CentriPipe project to be completed in Peachtree City. Previous projects have typically been shorter, beneath areas of heavy residential development or, in one case, an ornamental golf cart bridge that would have been expensive to replace. The golf cart bridge project rehabilitated a section of arched pipe. “The invert of the arch pipe had completely rusted away, and there was about a two-foot-deep void below the pipe,” says Madison. “But CCCP repaired it completely and even kept the arch shape. We inspected the work after a year and found no deficiencies with the work or the product.”

CCCP has been so successful that it is now the city’s first choice for repair of storm sewer over 30 inches that for one reason or another cannot be open-cut and replaced; resinous linings are typically used for smaller-diameter pipes. “We inspect 20% of our stormwater system annually, and of course we respond to sinkholes and other evidence of failure immediately,” says Madison. “We know we’re going to be doing a lot of storm sewer rehabilitation, and CCCP is proving to be a very cost-effective method.”

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