Stillwater River Road Rockfall
Stillwater County, Mont.
Best Project

Owner: Stillwater County Road and Bridge
Lead Design Firm & Contractor: GeoStabilization International

A spring 2015 rockslide completely blocked Stillwater River Road, spilling boulders and debris across the roadway and into the river. The ensuing cleanup was split into two phases. The first phase cleared rocks from the roadway; a second phase in 2018 reinforced the site and removed enormous boulder columns that still towered above the roadway after the rockslide. The second phase also scaled other loose rock from the cliffside and installed wire netting and bolts to secure the remaining rocks to the slope face.

Though the original plan called for stabilizing the remaining rock  with anchors and a high-tensile mesh, drone photos revealed that the safer approach was to remove them. The site had limited access, with no way to bring in large equipment for rock removal, so the team decided to dislodge the towering rocks by “pushing” them away from the cliffside using air pillows. This innovative approach cut costs and allowed the county to remediate four additional areas of rockfall. 

The team was constrained with using readily available standard rockfall barriers and a minimal catchment area between the slope’s toe and the roadway to keep falling material out of the environmentally sensitive river areas next to the road.

Engineering calculations showed the impact energy of 500 tons of falling rock was more than twice what the standard barriers could handle, so rather than spending the time and money on custom barriers, the team decided to dissipate that energy by building a soft berm in front of the rockfall barrier. This ensured the standard barriers could withstand the post-impact energy of the scaled rock and the fly rock and further enhance safety for the traveling public by removing potential fall hazards.

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