Motorists who use Interstate 35W in Bloomington have encountered lane, ramp and shoulder closures over the past few weeks as Minnesota Department of Transportation crews make repairs to wooden noise walls on the side of the freeway.
The repair project between 82nd and about 106th streets began last fall and is expected to be completed this month, said Natalie Ries, MnDOT’s Metro District Noise and Air Quality Program supervisor.
Drive reader Dick has seen workers taking down the old barriers and installing new wood in their place. He wanted to know why the walls were being replaced since it seems like they “should last a lot longer.”
The walls he is referring to were some of the first installed in the metro area when they became part of the landscape in 1978. As things tend to do in Minnesota’s harsh climate, the walls have weathered since then. The concrete pillars are still in good shape, but the wooden planks have splintered and holes have appeared, leaving gaps that allow noise to sneak through into nearby neighborhoods and business areas the walls are meant to buffer from sounds, Ries said.
The same thing was true along the segment of Hwy. 52 that runs through West St. Paul, South St. Paul and Inver Grove Heights. MnDOT this spring completed fixes on failing walls and concrete posts and removed vegetation.
Solid barriers block the direct path of sound waves from the highway but don’t totally eliminate it. Noise can go over the walls and around them, Ries said. But for those within 500 feet of a highway, “they are effective,” she said.
The constant hum of freeway traffic can generate noise levels in the 65 to 70 decibel range. According to the Minnesota Pollution Control Agency, that is akin to the volume of a normal conversation with a person 3 feet away (on the low end) or a vacuum cleaner (on the high end).
With the noise walls, Ries said, “we can see a seven to 10 decibel decline. It’s easily noticeable.”
As of last year, MnDOT had about 140 miles of noise walls throughout the state, with most of them in the metro area.
This summer and next, residents in mobile home parks along I-35W in Mounds View and Blaine will see new walls going up. The agency will put up seven new 20-foot walls as part of the I-35W North MnPass project that includes a new High Occupancy Vehicle lane between County Road C in Roseville and Lexington Avenue in Blaine.
Next year, MnDOT will put up noise walls on the east side of I-35E in the vicinity of Royale Drive and Safari Trail in Eagan.
MnDOT has two main types of walls: concrete, which costs between $35 and $40 per square foot, and the standard wood walls that cost between $25 and $30 per square foot.
In a few places, such as on the north end of the Nine Mile Creek bridge on northbound Hwy. 169 in Edina, walls are made of lighter-weight acrylic panels with metal posts to meet crash-testing standards, Ries said.
MnDOT budgets about $2 million a year for noise mitigation, Ries said.
The agency installs new noise walls in areas that exceed state or federal noise limits and where significant noise reductions can be achieved.
MnDOT tries to incorporate noise wall repairs or new construction with road construction when possible, she said.
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