Static Pipe Bursting in Minnesota

The success of a recent water main replacement in Moorhead, Minn., backed by positive readings from a digital pressure and pullback monitor, helped change some minds about restrained-joint PVC pipe’s resilience in static pipe bursting applications.

Last year, Moorhead Public Service, the City’s community-owned water utility, opted to replace a 360-ft section of aging 6-in. cast iron water main that had experienced several breaks over the past few years. Due to high traffic and limited workspace in the busy retail area in which the water main was located, it was agreed that static pipe bursting with its minimal disturbance would be the best way to complete the main replacement.

The region’s corrosive soils and previous problems with cast iron and ductile iron pipe mandated the specification of a thermoplastic pipe for corrosion resistance. The City had frequently used fusible pipe for past water main replacements, but the space limitations made that option unfeasible.

“This water main section was in a busy part of town and there was no room to lay down 360 ft of pre-fused pipe,” said Patrick Hockett, CPC, senior project manager for KPH Inc, Moorhead Public Service’s general contractor. “We also did not have enough space available to provide the pit size needed for fused pipe. Since the utility preferred PVC as their replacement pipe material, we looked at cartridge-loaded, restrained-joint PVC pipe.”

Upon the recommendation of pipe distributor DCMC Sales Inc., the project team agreed to try a new product from North American Specialty Products LLC (NASP) — Certa-Lok C900/RJIB Restrained Joint Integral Bell PVC Pipe. The product features the Certa-Lok mechanical pipe joining system, which uses a combination of precision-machined grooves and a nylon spline to provide high tensile strength in pulling or pushing installations. A flexible elastomeric gasket within the integral bell provides a solid, pressure-certified hydraulic seal.

KPH subcontracted the pipe bursting portion of the project to Red Pederson Utilities (RPU) Inc., a frequent project partner. At first, the trenchless contractor was unsure about using restrained-joint integral bell PVC pipe in a static pipe bursting application.

“We’ve been in the trenchless construction business for the past 15 years and have installed plenty of fused HDPE and PVC pipe via pipe bursting, but this was the first time that we were bursting with a cartridge-loaded, integral bell PVC pipe,” said RPU owner Red Pederson. “We were concerned because, unlike the fused pipe we’d used in the past, it isn’t a heat-welded product and doesn’t have a consistent outside diameter. So, we didn’t know how well it would hold together during a pull.”

In response to the trenchless contractor’s concern, NASP offered onsite support services for the installation and recommended using a TensiTrak Pullback and Pressure Monitoring System from Digital Control Inc. A device that is connected to the drill-head and the new pipe, TensiTrak is used during the pullback process to measure the pull force and annular mud pressure on the pipe to help identify any problems before they occur.

KPH planned the project for a five-day timeframe. On the first day, the crew closed down a portion of the roadway nearest to the main and began clearing the area and disconnecting two fire hydrant tie-ins along the line. Insertion and receiving pit excavations were made the next day, using a Komatsu 300 Excavator and a Komatsu PC138 Excavator with a concrete breaker and plate compactor attachment. The insertion pit was dug to be 9 ft deep, 24 ft long and 6 ft wide. The receiving pit was the same depth and width, but only 12 ft long. A New Holland Skid Steer was used for dirt removal. Later that day, the RPU crew set up its pit boxes and lowered its TT Technologies GrundoBurst 800G static pipe bursting system into the insertion pit.

Pipe bursting began the next morning, with a crew of four from Red Pederson Utilities operating the pipe bursting machinery from the insertion pit and a three-person KPH crew in the receiving pit to load and assemble the pipe. RPU burst through 360 ft of cast iron main in clay soil with no trouble and changed out the drill head for a cutting head, expander and tracer wire. The TensiTrak system and first joint of 8-in. Certa-Lok pipe were then attached for pullback.

Each 20-ft pipe joint was lowered into the receiving pit by an excavator and quickly attached to the pipe assembly as pullback continued. Pederson was impressed by how fast the pipe assembly moved.

“It was a very smooth operation, in terms of pipe assembly and timing — comparable to the pace you have with pre-fused pipe, where you don’t have to stop at all,” Pederson said. “We found that there was very little down time with the pipe joint assembly. Using fused pipe would have been very difficult in this application, due to our space limitations, so the Certa-Lok pipe provided an ideal solution.”

“The integral bells went together very nicely — possibly even easier than regular C900/RJ pipe,” Hockett added. “The spline was easy to insert, as well.”

NASP and TT Technologies representatives were onsite to provide technical support and monitor the pull force on the pipe. Representatives from the City of Moorhead and Fargo, N.D., also observed the installation. Throughout the burst and pullback, TensiTrak readings showed that there was no excessive pressure to threaten the tensile strength of the PVC pipe. The pull force peaked at just more than 11,000 lbs — well below the 24,300-lb limit for 8-in. restrained-joint PVC pipe — when the pipe assembly pulled through the remnants of stainless steel repair joints and a tee that fed a fire hydrant.

All told, the water main replacement ran smoothly and was completed in about three hours. KPH then made all necessary connections on both ends, replaced the two fire hydrants and filled in the pits. The new pipe passed pressure testing, and the contractors and customer were happy with the results.

“This is probably one of the smoothest jobs we’ve ever done,” Hockett said. “We had good weather, and we didn’t run into anything we hadn’t anticipated. We can attribute the success to a combination of quality products and an excellent contractor partner like Red Pederson Utilities.”

The project was also an eye opener for the municipality and project team, proving the resilience of segmented restrained-joint PVC pipe as a viable and cost-effective solution for municipal water and sewer infrastructure upgrades.

“The pull forces were very low and the TensiTrak showed that we had an extremely low drag pressure,” Pederson said. “I drove away from the site with the realization that the pipe bursting world for us had just changed. We’ll be using more restrained-joint PVC pipe in our projects from now on.”

Mark DaSilva is with North American Products, a Westlake Chemical company.