Minneapolis Tackles Tunnel Project to Reroute Water Main Under Mississippi

Minneapolis Tackles Tunnel Project to Reroute Water Main Under Mississippi

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The City of Minneapolis Public Works Department has an ambitious new plan for a large-diameter water main that is currently suspended from the 10th Avenue Bridge spanning the Mississippi River. They’re going to reroute it under the river.

The $24 million project might sound pretty run-of-the-mill until you understand how they are doing it.

Peter Pfister, professional engineer, and project manager for the Water Treatment and Distribution Division of Public Works, says that it’s an incredibly interesting project, and one that engineers in his position do not often get the chance to be involved in. 

A microtunneling technique will be used to install the water main underneath and across the Mississippi River. First, a 30-foot wide working shaft will be excavated for the purpose of lowering the machine to the tunnel depth. On the other side of the river will be a 12-foot wide machine retrieval shaft.

Once lowered, a remotely operated microtunnel boring machine begins its work of excavating a five-foot diameter tunnel through St. Peter sandstone that lies under the river. As the robot makes its way across the river, a conveyor belt takes the excavated earth up to the staging area where it is then trucked away. The new tunnel will consist of a 48-inch water main inside a 60-inch diameter steel casing.

Why are they doing it?

The city had excellent reasons for undertaking such a project. The current water main attached to the bridge was installed almost 70 years ago, and it’s not an ideal situation. It gets incredibly cold in Minneapolis, and there is a risk of the main freezing during low-flow periods or shutdowns.

And then there’s the constant exposure to road salts that causes corrosion-based damage to the pipe.

In addition, inspection and maintenance of the pipe is difficult to perform over the river and requires involved safety procedures and equipment. While Pfister was not aware of any records detailing the cost of maintenance on the line over the years, he did say that when evaluating the costs of a design alternative, it is important to consider the cost of all the repairs and maintenance that will or will not need to be made for that given alternative.

After an inspection of the main and consideration of rehabilitation alternatives, it was decided the main should be replaced. The Minneapolis Public Works Department and its design team did explore other options such as horizontal directional drilling and placing the main back on the bridge before settling on the microtunneling as the best solution.

“You want to conduct adequate engineering studies when making a decision like this and make your evaluation in large part based on the life cycle costs of the alternatives, but also on factors such as reliability and non-economic costs,” says Pfister.

An often-assumed lifecycle of a new main is about 100 years, but the project team thinks they can get a longer run out of it, making the economic lifecycle cost of the microtunneling option favorable to other methods with lower estimated initial costs.

Because this project will be running parallel to significant rehabilitation of the bridge, the project team is keeping the public informed through a website dedicated to the project and through community meetings. The project is already underway and will continue through the summer of 2020.

If this technique of microtunneling has sparked your interest, Pfister says that’s because the technique requires substantial design effort and specialized crews and equipment. Its use for pipes of this type and size is usually limited to projects where traditional open trenching is not a practical option, such as crossing water bodies, difficult terrain, railroads, freeways, or areas with sensitive surface features such as historic or densely built up areas. Another thing that engineers in other municipalities might be disappointed to learn is that the department’s employees don’t get to operate the robot. That job goes to the experts trained to manipulate this very specialized piece of equipment. 


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