A long-term solution was found to protect a Minnesota city's storm sewer system from river flooding
It pays to be progressive. Just ask the city officials of Mankato, Minnesota. This forward-thinking southern Minnesota city of 32,000 was the first municipality in the state to purchase a television system for assessing the internal condition of its sewer lines.
It is now one of the first cities in the country to use the video documentation to secure federal funds for the repair of a flood-damaged storm sewer. In addition, it is using those funds for a long-term solution that will better protect the storm sewer system the next time the Minnesota River overruns its banks.
As the waters from the Great Flood of '93 receeded, the City of Mankato quickly began investigating soil problems occurring around River Front Drive, a major thoroughfare located near the river. The City retained MSA Consulting Engineers, Minneapolis/St. Paul, to evaluate the storm sewer lines running under the street. These pipelines carried storm water runoff from the city's underground pipeline system to the Minnesota River.
"When the river rises, its elevation is higher than the storm sewers in parts of the city. Therefore, sewer outlet gates must be closed and all stormwater pumped into the river," explained Tom Roushar, vice president and project engineer for MSA.
"During the flood, pump station operation caused fluctuating water levels and pressures in the storm sewers, which contributed to soil migration and resulted in the formation of voids in the older storm sewers with cracked or bad joints. Additional pressure from the water-saturated soil surrounding the pipe also may have caused problems."
"Some of the damage showed up as sink holes on the surface," said Dale Tranter, MSA vice president. "For example, the foundation of a recently restored historic railroad depot on River Front Drive started to sink."
Below ground, video taken inside the sewer lines showed a significant number of leaking joints.
"To obtain FEMA (Federal Emergency Management Agency) funding for renovations to the sewer system, we needed to be able to document that the flood had actually caused the damage," explained Mankato Public Works Director Paul Baker. "Fortunately, we had televised the storm sewer the year before the flood, when the line was intact. When you compare the tape with the deterioration shown in the video shot after the flood, the conclusion is obvious."
The Engineer's Challenge
Not quite so obvious, however, was how the city was going to repair the damage. The storm sewer was very difficult to access.
MSA's evaluation pinpointed a three-block-long section of pipeline originating at a storm drain on Walnut Street and extending under River Front Drive as the primary trouble spot. The pipeline was buried deep, averaging 15 ft below the surface.
The line ran under a railroad depot and train tracks, a ramp for a 480-car parking garage, a four-lane city street and a significant amount of electrical and telephone conduit. Any repairs needed to be coordinated so as not to disturb the adjacent construction of a $25 million civic center, part of Mankato's Riverfront 2000 urban renewal program.
Complicating matters further were the area's high water table and fine sand soil that require sheeting and dewatering for excavation. These conditions placed severe limitations on the city's repair options.
"For most sewer renovation proj-ects, engineers have several alternatives to choose from," said Tranter. "We didn't have that luxury."
"Both the risks and the costs associated with replacing the pipe through a traditional open-cut solution were unacceptable," said Roushar.
Under these conditions, two approaches were considered, cured-in-place (CIPP) and sliplining. Both of these approaches are trenchless processes for reconstructing damaged pipeline. With CIPP, a seamless custom-made felt tube injected with resin is inverted (turned inside out) through a manhole into an existing damaged pipe. The tube later cures into a structurally sound "pipe-within-a-pipe."
With sliplining, a high density, polyethylene material is pulled through an existing pipe. The sliplining process usually requires two access pits. However, on this project, access could have been achieved without the use of pits by going in through the flood wall where the pipe empties into the river.
When analyzing the approaches, the City of Mankato looked at some immediate concerns:
Levels of Risk-"When you're working around major utilities, there's always a chance for a mishap," said Tranter. "We also had some fears of building foundations settling if we excavated the unstable soil. So we were really leaning toward a solution that didn't require digging, that could leave everything in place."
Suitability of Application-In addition to alleviating safety concerns, the solution had to be installed within a short time frame, because of the amount of traffic and other activity along Walnut Street.
Value-It is impossible to calculate the savings achieved by not disrupting business. It is also difficult to assess what other unforeseen costs might surface when you start digging trenches around utility lines.
These short-term savings were important to Mankato officials. However, they still were second in priority to the city's long-term objectives.
"We were looking at the long-term and what was best for the city," said Baker. That is why Mankato officials were most interested in a solution that would stand up through the next great flood.
Made of non-woven polyester material laminated to a layer of polyurethane, CIPP material is designed as an independent replacement pipe. This material adds strength to a partially deteriorated sewer line. In addition, with CIPP the storm's line would gain flow.
"The installed product is jointless and has a smooth finished surface ,which normally increases the flow capacity of the line," said Brad Mirkovick of Insituform Central, Inc., (ICI) a wholly owned subsidiary of Insituform Mid-America.
The design and installation of this technology are not as simple as they may sound. The installer needs the design and construction knowledge to adapt the technology to specific site conditions.
While sliplining is also a jointless process, and hydraulically smoother than the existing pipe, its use results in a smaller pipe than CIPP. However, this was not of particular concern in Mankato.
MSA continued with the development of project specifications for rehabilitating 842 ft of 36 in. pipe, 62 ft of 24 in. pipe and associated manholes, and restoring sidewalks and streets where sinkholes had caused deterioration. With all the documentation in place, FEMA committed to paying 90 percent of the project's costs.
The specifications called for either CIPP or HDPE sliplining. The contractor, Mankato Plumbing and Heating, elected to use CIPP furnished and installed by ICI.
Almost exactly one year after the flood, ICI, MSA, the city engineer and contractor assembled at the Walnut Street site. ICI brought the custom-made polyester felt tube and boiler trucks needed to complete the inversion.
As if to remind everyone of the reason they had all gathered there that day, a steady rain began and fell for most of the day.
"We don't like it to be raining when we're completing an inversion on a storm sewer because the line can fill up," said Mirkovick. On sanitary sewers, where flow is continuous, ICI creates a bypass for the line while the inversion is being completed. The bypass is omitted on storm sewers, which become active only during inclement weather.
After the rain had stopped, workers began by closing the flood wall at the Minnesota River where the storm sewer drained into the river. They had already coordinated closely with Mankato City Engineer Ken Saffert and Utility Superintendent George Rosati to make sure no utilities would be inadvertently disturbed.
Enhanced polyester resin and dye-treated catalyst had already been used to saturate the polyester felt tube. The clear, polyurethane laminate and light-blue dye allowed every square inch of the material to be visually inspected to make sure the tube was thoroughly saturated with resin before the tube was inverted into the pipeline.
With everything in place, the force of water was used to invert the 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 were connected to the boiler trucks, and the water was continuously recirculated until it reached a temperature to trigger the catalyst. The heat starts the exothermic reaction that changes the resin into a thermoset pipe.
When the material had cured into a finished pipe, the water was allowed to cool before it was drained. The construction crew cut and sealed the ends of the new structural pipe at the manholes and the job had been completed-quietly, safely and without disruption.
Exactly what awaits Mankato once its renewal project is complete is uncertain. However, the leaders of this forward-thinking city know one thing: it pays to be progressive.