Rockslide fix?

The cost of the Stillwater County Commissioners’ preferred alternative for dealing with the rockslide blocking the Stillwater River Road could be $325,000, the county’s rockslide committee learned May 25.
That includes design engineering, scaling to remove loose rocks, and drilling weep holes to drain water from behind the rock face, according to a May 23 geotechnical engineering report from consultant Shannon & Wilson.
An additional $52,000 could be spent on special instrumentation mounted on the side of the rock face to alert the county when significant rock movement occurs above the road.
The rockslide has blocked the Stillwater River Road near Midnight Canyon since June 3. Shannon & Wilson field crews investigated the rock face by rappelling on April 5-6 – despite strong winds – and used a drone to take photos and video on April 6-7.
The rockslide scar is 135 feet high, 5-10 feet wide, 5 feet deep and 50 feet above the roadway. The debris field extends 90 feet along the road and 100-125 feet away from the rock face.
Fallen rocks range up to 20 feet long, and many fell into the Stillwater River. Additional rockfall along the quarter-mile long rock face was reported after the June 3 event.
In an 18-page draft report sent to the county commissioners on Dec. 23, Shannon & Wilson offered four options – scaling, the least expensive; installing rockfall detection fencing, the second least expensive; installing rockfall fencing along the river side of the road to keep rocks out of the river, the second most expensive; and installing rockfall fencing 30 feet above the road to protect travelers, the most expensive.
The county commissioners chose scaling as the preferred alternative. Shannon & Wilson recommends scaling is most effective if repeated every 2-5 years.
“While scaling cannot prevent large failures from occurring in the future, removing loose rocks will reduce rockfall hazard to the traveling public for a period of time,” the consultants noted in their report.
Shannon & Wilson recommended scaling 200-300 feet of slope length, focusing on the slide scar area.
A temporary catchment would be needed during scaling to prevent rocks from going into the river, but just in case, the consultants suggested talking to the Army Corps of Engineers ahead of time.
Shannon & Wilson also recommended drilling 10 weep holes, each 3 inches in diameter and 30-35 feet long, to drain water that could build up behind the rock face and create hydrostatic pressure.
Geotechnical instrumentation installed on the side of the rock face could include two tilt meters, each capable of measuring a 1/1,000-degree change, and two joint meters, each capable of measuring a 1-millimeter displacement. The equipment would communicate via satellite, because the site is out of cellphone range, and be powered by a solar panel. The instruments would need to be inspected and maintained annually, which would require additional rappelling.
On May 31, the Stillwater County Commissioners unanimously approved applying for a $1.8 million Pre-Disaster Mitigation Grant from the Federal Emergency Management Agency to deal with the rockslide. The county will need to come up with a 25 percent match, or $620,000.
In a cost-benefit analysis accompanying the application, the county estimated damages before mitigation at about $16.8 million based on 953 vehicle-trips per day traveling an extra 34 miles around the rockslide at the federal rate of 54 cents per mile.
Stillwater County Department of Emergency Services Coordinator Carol Arkell said the calculations are based on 100 addresses in the affected area with an estimated 2.5 people per address.