The city of St Louis Park, MN is experiencing significant commercial and residential redevelopment. As a result, the Public Works Department is always looking for cost-effective means for upgrading utility infrastructure. A recent water main replacement project gave city officials their first look at trenchless pipe bursting. The results were impressive and more pipe bursting work could be in the city’s future.
The city of St. Louis Park covers 10.8 square miles just west of Minneapolis and is home to over 44,000 residents. The city’s water system is comprised of over 140 miles of water main, six treatment plants, three water towers and four reservoirs. Over two billion gallons of water flow through the city’s system each year and 6.5 million gallons of water are used on an average day.
The planned development of a $115 million multi-use commercial and residential complex threatened to tax the area’s aging water system beyond its capacity. According to St. Louis Park City Engineer, Maria Hagen, the location of the existing water main prompted the city to explore trenchless construction methods rather than traditional open cut. Ultimately static pipe bursting was specified for the project.
Hagen said, “Trenchless pipe bursting was a good application for this area because of a combination of factors. We had a lot of traffic so we wanted to get the job done quickly with as little disruption as possible. Plus, the street was paved in concrete. Removing and replacing the concrete would not only have been time consuming and disruptive, it would have been very expensive as well.” The close proximity of a natural gas main to the water main also made trenchless pipe bursting a preferable replacement option.
Utility contractor Tjader & Highstrom, New Richmond, WI was contracted for the project. The Grundoburst 800G static pipe bursting system from trenchless equipment manufacturer TT Technologies, Aurora, Ill. was utilized.
Static Pipe Bursting
Over the last several years static pipe bursting has been gaining popularity and favor among public works officials, engineers and pipe replacement contractors. During the static bursting process, a special bladed roller is pulled through the existing line by a hydraulically powered bursting unit. As the bladed rollers are pulled through, they split the host pipe. An expander attached to the rollers forces the fragmented pipe into the surrounding soil while simultaneously pulling in the new pipe, usually High-Density Polyethylene (HDPE).
The specially designed bladed rollers are essential to the bursting process. The blades actually split the host pipe instead of ripping or tearing it. This makes the process very clean and prevents potential damage to the product pipe. It also makes bursting ductile iron and steel pipes possible, a first for pipe bursting equipment.
The water main replacement project took place under the heavily traveled Excelsior Boulevard, which carries over 21,000 vehicles per day. Plans called for the replacement of approximately 1,700 feet of existing 6-inch cast iron water main with 8-inch HDPE. The project was divided into four sections each ranging between 410 and 475 feet.
Exit and launch pits were excavated at the beginning and end of each pipe bursting run. A 6-inch temporary bypass was established to provide water service to six area businesses during the project. With the exception of one line sweep that did not appear on any records and required excavation, the static bursting equipment had no difficulty with the 6-inch cast iron pipe. Bursting went very quickly with times ranging from one hour and 15 minutes to three hours per run.
Hagen said, “The project was innovative for several reasons. First, the pipe bursting method is innovative in and of itself. Second, this was the first static pipe bursting project for a water main in the State of Minnesota. Everyone was very pleased with the results. However, when contemplating trenchless projects, very accurate as-built plans, both of City and private utilities, are needed to minimize interruptions and conflicts with other infrastructure.” Hagen said the city is looking at other projects where static pipe bursting could be used in the future.
by Jim Schill
American City & County, November 2002