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MDT temporarily shelves plans for removing Bonner bridge piers

MISSOULA – Faced with some major engineering questions and the potential of damaging a critical railroad bridge, the Montana Department of Transportation is postponing plans to remove dangerous bridge piers on the Blackfoot River.

But MDT says the intention is still to eventually remove the piers entirely.

The original plan for replacing the I-90 bridges over the Blackfoot River was to remove the massive bridge piers in the center of the river. The piers have become an increasing safety concern now that floaters and fishermen can float through that stretch of the Blackfoot down to the Clark Fork. That work was to have happened this summer.

But further engineering has run into a major problem. Removing the highway bridge piers would dramatically increase the flow of the river, and could cause severe damage to the Montana Rail Link bridge.

“What MRL designed for was about 6-to-8 feet per second, which is what we saw with our structure as mitigated by the Corps and the dam removal,” said Montana DOT Chief Engineer Dwane Kailey. “However, once we pull those piers out of there, the velocity is pretty much doubled. They jump up around about 16-feet per second. And again, they did not mitigate for that level of velocity, so we’re trying to figure out how best to achieve that.”

Kailey says the hydraulics problem is bringing everyone back to the drawing board to find what MDT calls a “cost-effective and sustainable solution.”

“We immediately sat down with MRL and started discussing the issues with them and trying to find other options, other alternatives. And we’re still working with them as well as the resource agencies to find a solution that I know we all want to get to.”

Those agencies include the U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, the Federal Highway Administration, the Montana Department of Environmental Quality, and Montana Fish, Wildlife and Parks. The problem is a complicated one because the free flowing river has already been “scouring” a deeper channel, and issues like fish habitat have to be addressed, but with an affordable plan.

“We have identified a mitigation that would work for them. Unfortunately that adds about $17 million to the project cost.”

Kailey says the state hopes to remove the piers next year. In the meantime, the state will be posting warning signs to remind people to use caution when the river isn’t already closed by the interstate bridge construction.

Dennis Bragg

Dennis Bragg

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