Burnsville Interceptor Rehabilitation
With a budget exceeding $100 million a year in wastewater pipe capital improvements over the next several years, Metropolitan Council Environmental Services takes its job of protecting the wastewater infrastructure of the greater Minneapolis-St. Paul metropolitan area's nearly 3 million residents very seriously. MCES operates eight wastewater treatment plants and 600 miles of interceptor sewers in the region, and maintains a near-perfect track record of compliance with federal and state clean water discharge standards.
MCES' service area is home to the Minnesota Valley National Wildlife Refuge, a 14,000-acre protected habitat for migratory birds, bald eagles, and a number of unique species such as prothonotary warblers. Easily accessible from the Twin Cities, the Preserve is a treasured natural resource to the thousands of Twin-Cities residents and tourists who visit each year.
Protecting this land is also important from a historical and cultural perspective. Dating back to 1750, the site was a summer village of the Mdewakanton Sioux Indians led by Chief Black Dog, namesake of Black Dog Lake.
MCES's 40-year-old interceptor sewers within the Black Dog Preserve area of the Refuge were suffering from severe hydrogen sulfide gas corrosion; a pipe failure would have devastating effects on the surrounding environment.
According to Harold Voth, PE, consulting engineer on the project for Brown & Caldwell, MCES takes a proactive approach to its pipe system maintenance. “With pipe corrosion, there aren't immediate consequences – however eventually they would run the risk of collapse,” he said. “That's something you would never want to see, with the project running almost entirely through the Refuge.”
MCES contracted with Hugo, Minn.-based Lametti & Sons to renew the 4.66 miles of interceptor sewer pipes within the Refuge using the Inliner Technologies method of cured-in-place pipe. The contract, valued at approximately $18,579,000, also entailed rehabilitating 14 manholes and installing two new manholes, construction of access roads and restoration/native replanting upon completion of the two-year project.
Inliner® CIPP is a minimally invasive technology that is used to restore aging and damaged underground sewer pipes. A felt or fiberglass tube saturated with a resin/catalyst mixture is inverted or pulled-in to the host pipe, inflated tightly against the walls of the pipe, then heated with water, steam or UV light to cure the resin–creating a “pipe within a pipe.” Inliner CIPP gives pipes increased flow capacity due to the liner's smooth surface, meeting or exceeding the product's design life of 50 years.
In the spirit of the Sioux Indian Proverb, “To touch the earth is to have harmony with nature,” the project was carefully planned to minimize disruption to the wildlife that inhabit the Reserve.
A major constraint was an Eagle Protection easement. During nesting season, all work was prohibited within a 660' perimeter of any active Eagle's nest. Therefore, it was necessary to stage and schedule the project phases so when the CIPP installations reached the nesting area, nesting season had passed enabling the team to install pipe lining within the area. The area was also home to a Calcareous Fen, which had an even higher level of restriction than the eagle's nest.
Addressing environmental issues was only one challenge faced by the project team. There were also significant logistical issues. The project sites could only be accessed from the West end of the Preserve, and with no roads along the route of the existing pipeline, Lametti had to create access roads.
Initially, the temporary access road was created using HDPE access mats. After the first 5,000 feet, however, Lametti discovered that the HDPE road would be no match for the combination of a high water table, unstable soil, and the trucks and equipment needed to perform this large-scale rehabilitation effort.
“Road failures were starting to occur beneath the excessive weight of the trucks delivering the pipe liners and resin, necessitating the buildout of the remaining 15,000 feet of access road as rock roads,” said Dan Banken, vice president, Lametti & Sons.
With parts of the project located along waterways and a rail line, easements were tight. “There were a lot of features to avoid, including existing pipes below ground that would not have been able to withstand the load from above,” said Banken. “We had a narrow area to work with.”
MCES specified the water-curing method for this project, but no municipal water was available on-site. To solve this problem, Lametti installed a temporary water main to pump water for curing from Black Dog Lake.
Meeting the project's two-year timeline, the schedule required Lametti to work non-stop, in all weather conditions. In total, the project entailed 26 installations. The 12”, 36”, 42” segments and one segment of a 60” liner were wet-out in Lametti's wetout facility in Hugo, Minn. That left 20 segments of 60” and larger liners requiring over-the-hole wetouts, in which the resin was catalyzed, mixed, and pumped into the liners as they were being inverted into the pipe.
Over-the-hole wetouts are performed inside a climate-controlled tent. With temps ranging from sub-zero to over 100 degrees throughout the year, tents were heated during the winter and cooled during the summer to maintain a stable temperature for the resin. The tent and tower setup was constructed near the middle of the section being renewed, then Lametti would complete the inversion/lining in both directions from the same access point.
Northern Dewatering was the subcontractor that set up the temporary conveyance systems, pumping a range of 6,000 – 14,075 gallons per minute. Each bypass was approximately 6,000 feet long, or one-fourth of the total project length of 23,000 LF.
The final installation on the project covered a span of 66” diameter pipe over 1600 feet long. The shipping height of the liner alone would have exceeded highway shipping regulations, so the eight-layer liner was manufactured in two sections each measuring over 800' in length.
Maintaining the integrity and strength of the seam attaching the two sections of this extremely large and heavy liner was critical. The eight layers of heavy duty felt within the tube were filleted open, and layers were spliced with heavy-duty needle and thread. It was important to distribute the splices of the layers over a distance of approximately 120' to ensure no two layers were spliced at the same location. A team of four specialists dispatched from the manufacturing plant worked around the clock for 24 hours to complete the field splice.
“This was one of the most complex CIPP installations that Lametti has performed,” said Banken. “After the sewing was complete, the section took 48 hours to wet out and get it into the pipe, and another four days of cooking to cure the liner.”
The CIPP subcontractors and suppliers included Liner Products for the felt tubes and Interplastic Distribution Group for the polyester resin. Fourteen manholes were rehabilitated using FRP manhole inserts along, with two new FRP manholes by L.F. Manufacturing. Hobas provided RPMP pipe for sliplining a section of the project that was not suitable for CIPP.
It's easy to see why CIPP is a “go-to” technology for MCES, which Voth said considered sliplining as an alternative to CIPP but was concerned with the reduction in cross-section and reduced hydraulic capacity.
Both Brown & Caldwell and Lametti said that they have done many extensive rehab projects for MCES, and this was one of the largest in terms of the diameters of the pipe, overall length and project cost.
“Met Council does a lot of interceptor rehab work,” said Banken. “They believe in implementing fixes that will last a long time.”
Role / Responsibility
Lametti & Sons was the prime contractor for this multifaceted project.