Minnesota city uses trenchless sewer repair

When the flood of 1993 damaged storm sewer lines in Mankato, Minn., city officials were able to repair them using a trenchless, “in situ” technology.

As the waters from the flood receded, Mankato investigated soil problems occurring around a major thoroughfare near the Minnesota River. Pipelines under the street carried stormwater runoff from the city’s underground pipeline to the river.

When the river rises, its elevation is higher than the storm sewers in parts of the city. Sewer outlet gates must be closed and all stormwater pumped into the river. During the flood, pump station operation caused fluctuating water levels and pressures in the storm sewers, contributing to soil migration and resulting in the formation of voids around older storm sewers with bad joints.

Underground video taken the year before enabled Mankato to secure federal funding for the repairs.

“To obtain Federal Emergency Management Agency (FEMA) funding for renovations to the sewer system, we needed to be able to document that the flood had actually caused the damage,” says Public Works Director Paul Baker. “Fortunately, we had televised the storm sewer when the line was intact.”

But the storm sewer was difficult to access, and city officials were unsure how they would repair the damage. Engineers identified a 3-block-long section of pipeline 15 feet below the surface as the primary trouble spot.

In addition to a nearby railroad depot, the line ran under train tracks, a ramp for a 480-car parking garage, a four-lane street and a large amount of electrical and telephone conduit. Repairs also had to be coordinated to avoid disturbing the adjacent construction of a $25-million civic center.

Additionally, the area’s high water table and fine soil, which required sheeting and dewatering for excavation, further complicated matters.

Wary of potential problems arising from working close to utilities, contractors also had some fears of building foundations settling if the unstable soil was excavated.

Officials chose a trenchless process for reconstructing damaged pipeline called “cured-in-place-pipe” (CIPP). With CIPP, a jointless, custom-made felt tube, impregnated with resin, is inverted through a manhole into the damaged pipe. The tube later cures into a structurally sound “pipe within a pipe.”

Contractors proceeded with the development of project specifications for rehabilitating 824 feet of 36-inch pipe, 62 feet of 24-inch pipe and associated manholes. With all the documentation in place, FEMA committed to paying 90 percent of the project’s costs.

Made of non-woven polyester material laminated with a layer of polyurethane, CIPP material is designated as an independent replacement pipe. City officials selected a material with a design life of 50 years — the same as a new pipe.

Workers began by closing the flood wall where the storm sewer drained into the river, then used the force of water to invert the felt tube through an existing manhole and into the pipe being reconstructed. Water pressure kept the tube pressed tightly against the walls of the old pipe.

After the entire tube was inverted through the pipeline, hoses connected to heat-exchange trucks kept water recirculating continuously until it reached a temperature to trigger the catalyst. The heat starts an exothermic reaction, changing the resin into a thermoset pipe.

After the material cured, the water inside was allowed to cool before draining. The construction crew cut and sealed the ends of the new structural pipe at the manholes, and the job was complete.