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Apr 4, 2014 6:47 PM by Jason Laird - Q2 News

Rock hazards weigh heavy on Zimmerman Trail closure

BILLINGS - Commuters are going to have to do without the convenience of Zimmerman Trail for the foreseeable future.

Landscapes are constantly changing. One of those changes happened on March 25 when a rock slide forced the closure of Zimmerman Trail.

The threat of recurring rock slides above and along the popular route has left city officials with no other choice but to close the roadway.

"What occurred is, we had about half of a fairly large rock block fail," said Dan Nebel, senior project geologist for Terracon Consultant Inc., the company the city hired to survey the slide area.

When the slide occurred, four- to six-foot slabs of rock fell onto to the roadway, causing damage to the guardrail and asphalt, said Nebal.

"At the time it failed, there's also a remaining large fragment there that we estimate weighs in the range of 250 to 300 tons," he said.

Company geologists said cracks in the rock on the top and sides in the slide area make another break-off a strong possibility.

With that risk looming, Zimmerman Trail cannot be reopened until it's deemed safe.

"What we have here is kind of a progressive failure of these rocks over time due to freeze (and) thaw. And what we're looking at is to see evidence of recent movement or movement that has occur to such a point that the rocks are over-hanging and look like gravity could take affect," Nebal said.

According to the geologists, the biggest concern lies beneath the earth in the form of a seismic event.

"If we were to have a major seismic event, we would see quite a bit of rock fall," Nebal said.

Geological crews will remain on scene along Zimmerman Trail to continue their investigation. The company hope to have a final estimate of the cost to break up and remove the rock hazards by the end of April. Finding a contractor to complete the work will be the next step.

The city hopes to reopen the road by early June.


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