Ed Kemmick/Last Best News
A couple of workers in yellow vests are visible just east of the aptly named Monkey Rock above Zimmerman Trail Thursday.
People living near Zimmerman Trail probably don’t need to worry about the prospect of earth-shattering explosions to bring down dangerous rock formations.
Paul Rieger, district operations engineer and project manager for the Montana Department of Transportation, said explosives will be used on only one formation, and it will be a “super-light charge” designed to fracture the rock and let it fall away from the Rims.
He said the technique is known as “trim blasting,” as opposed to “production blasting.” City engineer Debi Meling put it in even simpler terms, saying the explosion would go “poof” instead of “boom.”
“Every conversation we have revolves around how do we best do this safely,” she said.
The Rims overlooking Zimmerman Trail, which climbs from Rimrock Drive to Highway 3 at 32nd Street West, were going to undergo some trimming anyway, in preparation for larger improvements to the connector road.
The project was put on the fast track after falling rock damaged Zimmerman Trail on March 25. On Monday, the day that crews from GeoStabilization International arrived in Billings from Colorado to work on the Rims, there were two major rockfalls just west of Zimmerman Trail.
The one formation where explosives will be used is known as Monkey Rock, for obvious reasons. Rieger said GeoStabilization crews drilled two vertical holes in the top of the rock and three more at 35- to 45-degree angles on the east side of the rock.
Ed Kemmick/Last Best News
A front-end loader removes a freshly fallen boulder from Zimmerman Trail Thursday afternoon.
The charges will be dropped into those holes and exploded, most likely toward the end of the project, in mid-June. Monkey Rock is already mostly separated from the main body of the Rims, and it leans noticeably to the west.
Rieger said the hope is that the explosions will drop the top half of Monkey Rock, which is about 80 feet high, straight down. Crews will build dirt berms to catch falling rock and protect the roadway.
There are six problem areas where rocks are being removed. Except on Monkey Rock, workers will use pry bars or expanding air bags to remove dangerous rocks and formations. Asked if they would use hydraulic jacks, as crews did three years ago when huge slabs of sandstone were brought down at two locations in Billings, Rieger said they might.
Jacks were not included in the project proposal, he said, “but if they can’t drop them with bars and bags, they could use hydraulic jacks.”
Six houses directly below Zimmerman Trail will be evacuated when work is being done on Monkey Rock and a formation just east of Monkey Rock, Rieger said.
Meanwhile, the city is still working with GSI and consultants to determine whether a tall, freestanding block of sandstone towering over Sixth Avenue North at North 10th Street needs to be removed. GSI is working under a state contract, for $718,000, to do the work on Zimmerman, and the city hopes it will also do any work needed above Sixth Avenue before the company leaves town.
The sandstone column doesn’t look all that large looking up from Sixth Avenue, but it is massive. Dan Nebel, senior project geologist with Terracon of Billings, which has been doing the preliminary engineering work on all the problem areas on the Rims, said the formation is 25 to 35 feet high and weighs an estimated 150 to 200 tons.
Meling said the assumption is that the formation will be removed, unless further study shows it to be “more stable than I think it is.”
