NASSCO Report: Pipeburting
Deep Pull Saves Janesville, Wis.,Time, Money
Oct 29, 2013
East Conde Street is the only industrial park access to two businesses in Janesville, Wis., where they experience steady schedules of truck traffic throughout the day. Shutting down East Conde Street to replace a 660-ft section of 12-in. sewer lying 19 ft beneath it would be a tremendous financial burden on these companies.
A camera report showed that nearly a quarter of the sewer pipe’s bottom portion had disintegrated. Replacement to prevent any more collapses became an urgent priority, and pipe bursting was the recommended replacement technique. The job was awarded to Speedway Sand & Gravel of Middleton, Wis., which would perform a 12-on-12-in. pull of fused HDPE pipe in a single pipe bursting run. Speedway selected a 15.75-in. concrete expander to run behind a pilot pulled by a 125-ton static bursting machine for this job.
Speedway used the static pipe bursting technique vs. pneumatic because of uncertainty about a 12-in. water main’s integrity. The water pipe was lying near the entrance manhole, perpendicular to and above the sewer line. Troy Dominy of Speedway said a number of factors made this a challenging burst. First was its length. In a Class A Project, IPBA recommends burst lengths of 350 to 400 ft; this run would be more than 50 percent longer. Second, the pipe was 15 ft deep, and the entry and working pits needed to be excavated roughly 17 ft.
The static technique consists of paying out 3-m sections of threaded rod from a working (or receiving) pit to the entry pit. In this case, the expander and trailing HDPE replacement pipe were attached to the pull rod. Then the static bursting machine pulled the string through the existing pipe as the crew retrieved the pull rod one length at a time. A cone-shaped burst head was used to fracture pipe and shove aside the fragments as it progressed. Placing the burst head behind a pilot shaft of about the same diameter as the pipe’s I.O. ensures that as the burst head fractures and expands the pipe uniformly, and is consistently compressed outward as the head progresses. In this way pipe is not allowed to bunch up in front of the head as it fractures, which it would then have to plow ahead of itself.
The burst head, or “expander,” is always larger in diameter than the replacement pipe. Having extra space around the pipe decreases friction of replacement pipe against the ground and existing pipe, which increases in proportion to the advancement of the replacement pipe. The formula used to calculate the optimum annulus in this case determined that a 15.75-in. burst head be used for the 12.75-in. O.D. HDPE replacement pipe.
As work began, crews staged such things as the backfill, pull rods, and fused HDPE pipe along the road. A crew was assigned to each pit at the manholes: a 12-by-10-ft working pit for the static bursting machine and 45-by-6-ft entry pit for the fused HDPE, whose actual length was 670 ft to be trimmed after positioning for connection in the installation pits.
The city did not want to cut the existing, active, high-pressure water main, so the machine pit crew installed a 12-ft long trench box beneath it, allowing the water main to be supported on the end of the box. The pulling machine was then set inside the box, under the pipe. When the machine was finally in place, the crew set a second 10-ft box on top of the first and shored up the exposed areas of the pit with metal sheeting. The tight configuration of the excavation made preparation of this pit the most difficult part of the entire project, because about 10 percent of the original 4-ft wide pipe trench had been dug in existing sugar sand type soil conditions. This made it difficult to retain the trench walls, during excavation of the insertion pit, at the manhole in the intersection of East Conde Street. The rest of the digging was in clay and was not as difficult, Dominy said.
Dominy said the pipe bursting and pipe pull went perfectly, and although he had wanted the extra confidence a 125-ton static bursting machine would provide, the pull at its peak required only about 50 tons of pulling force. Most of the pull, during the installation, was accomplished with much less force. The 660-ft pipeline installation took only five hours of the total project time. As the pull was being completed, Speedway’s crews were excavating the lateral pits to attach the laterals to the newly installed mainline pipe, with connection kits.
Steve Sage, P.E., a Janesville Engineering Division senior engineer, said pipe bursting this project had saved the city about 20 percent of anticipated cost, with the job coming in both ahead of schedule and under budget.
Sage noted that Janesville’s sewers will continue to need attention as they age. When future projects present themselves, he will be able to use this project as further proof that pipe bursting is not only viable but a fast, cost-effective alternative to replace additional pipe beneath Janesville’s city streets.
This article was provided by the International Pipe Bursting Association (IPBA), a Division of NASSCO.