Needing to replace a portion of its water main, St. Louis Park, Minn., was not about to dig for a solution. The project site situated near a gas main and beneath a heavily traveled roadway made excavation prohibitive.
"We had a lot of traffic, so we wanted to get the job done quickly, with as little disruption as possible," said City Engineer Maria Hagen. "Plus, the street is paved in concrete. Removing and replacing the concrete would not only have been time consuming and disruptive [but] very expensive as well."
The street Hagen refers to is Excelsior Boulevard, which accommodates more than 21,000 vehicles each day. It is one of the main thoroughfares for St. Louis Park, a city comprising approximately 11 square miles west of Minneapolis.
The city has more than 140 miles of water main, built in the 1940s. In 2001, a planned, multi-use development threatened to tax capacity in a portion of the pipeline, prompting the city to explore methods of replacing the affected infrastructure.
Recognizing that excavation and replacement of the concrete pavement was too costly, the city chose to use static pipe bursting rather than open cutting for the project. Working with New Richmond, Wis.-based Tjader & Highstrom, it planned to replace approximately 1,700 feet of 6-inch cast iron pipe with 8-inch high-density polyethylene (HDPE) pipe using the trenchless method.
As the pipeline project got under way in May 2002, crews were, at the same time, repaving Excelsior Boulevard. Two of the boulevard's four lanes already were closed to accommodate paving, and the bursting runs would not require additional traffic diversion. However, they would require the contractor to establish a temporary bypass providing water to area businesses for the duration of the project.
The contractor divided the project site into three sections ranging from 475 feet to 810 feet. It excavated launch and exit pits at the beginning and end of each run and, using the Grundoburst 800G system from Aurora, Ill.-based TT Technologies, burst the existing pipe.
As part of the bursting process, a bladed roller was pulled through the existing pipeline by a hydraulically powered bursting unit. As the host pipe split, expanders attached to the rollers forced the fragmented pipe into the surrounding soil while the rollers simultaneously pulled the HDPE pipe into place.
The runs were interrupted only once, when crews encountered a bend in the project's longest segment. Not visible on any of the project records, the bend forced crews to excavate and investigate. Ultimately, they divided the segment into two runs and proceeded as they had with the other segments.
"When contemplating trenchless projects, [you need] very accurate as-built plans both from the city and the private utilities to minimize interruptions and conflicts with other infrastructure," Hagen notes.
Completed in 10 days, the Excelsior pipeline project cost St. Louis Park $152,000. According to Hagen, estimates for open-cut replacement reached $200,000. Pleased with the project's outcome, St. Louis Park will consider static pipe bursting for future projects.
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