Possible Stillwater slide solutions

An 18-page draft report by the engineering firm hired to look at the rockslide that has been blocking the Stillwater River Road near Midnight Canyon Road since June 3 arrived at the Stillwater County Commissioners office on Dec. 23.
The draft report by Kami Deputy, an engineer with Shannon & Wilson Inc., offers four options and recommends additional evaluation work. A county committee dealing with the rockslide met to discuss the report on Dec. 29 (see sidebar).
According to Deputy’s draft report, the solutions sought by the commissioners include protecting the traveling public on the road, nearby residences and the river channel while keeping taxpayer costs down.
The rockslide occurred following a wet spring with multiple freeze-thaw cycles that likely forced cracks in the rock face to open more, the draft report said. People reported seeing water seeping from the failure zone, and increased hydrostatic pressure in the rock face could have contributed to the event.
The failure zone measures about 5-10 feet wide, 5 feet deep and 50 feet long. Rockfall entering the river drove water onto the opposite shore, and some fallen rocks left divots on residential lawns.
Additional minor rockfall was reported through the spring, the draft report states, and Deputy noted that Stillwater County Road & Bridge Department Superintendent Mark Schreiner said a large rockfall event occurred at the same site about 50 years ago.
“Based on the morphology of the slope, it appears that similar events have occurred along similar intersecting discontinuities within the same geological formation,” the draft report said. “It can be assumed that similar future events will occur. The timing of future events cannot be predicted, although increased risk for rockfall events can be attributed to significant rainfall (or snowmelt), particularly in the spring after a series of freeze-thaw cycles, as well as during earthquakes.”
According to Shannon & Wilson’s modeling, a rock less than 5 feet in diameter falling from the top or middle of the face could travel across the road and the river, while a similar-sized rock falling from about 30 feet up could cross the road and enter the river. A rock larger than 10 feet in diameter would likely end up between the base of the cliff and the river.
The safest solution to the problem would be to stop rocks from falling in the first place, the draft report said, and an August report by GeoStabilization International proposed using drilled rock bolts to hold the rock face together. The GSI recommendations, however, were considered “economically infeasible,” Deputy’s draft report states, so Shannon & Wilson “focused on alternatives that would be less costly to construct.”
Option 1 calls for slope scaling, with workers removing rocks from the face that had been loosened by freeze-thaw cycles.
Scaling is most effective if done every two to five years and should be done in conjunction with the other three options, Deputy said.
“Scaling cannot prevent large failures from occurring in the future, but removing the smaller loose rocks will decrease the likelihood of having small rocks falling on the road and impacting traffic,” Deputy said.
Option 2 calls for erecting a 250-300 foot long rockfall fence between the road and the river capable of stopping rocks up to 5 feet in diameter from entering the river. The fence would not protect traffic on the road, and it could be damaged by a rock more than 5 feet in diameter, Deputy said.
Option 3 calls for erecting a 200-250 foot long rockfall fence about 30 feet above the road on the cliff face. The fence would protect traffic on the road, but the rock size it could handle would be limited and the fence would require regular maintenance, Deputy said.
Option 4 calls for erecting a 200-250 foot long rockfall detection fence that could automatically close a gate on the road or signal the county road department. The fence would be unable to accurately judge the magnitude of rockfall and would require routine inspection and maintenance, Deputy said.
In addition to these options, Deputy’s draft report recommends that the county conduct a risk analysis similar to those done by state highway departments, conduct more evaluations using crews on rope rappels, survey the cliff face from the opposite shore using Light Detection and Ranging (LiDAR) remote sensing equipment, or further evaluate the cliff face with core drilling and laboratory strength testing.
Deputy suggested that funding might be available from the Federal Highway Administration’s Western Federal Lands division because the Stillwater River Road provides access to federal lands. The same type of funding is being used to rebuild the West Rosebud River Road.

Commissioners want more information on rockslide
The county committee dealing with the rockslide on the Stillwater River Road wants more information before it chooses any mitigation options that could possibly get the road open again.
The committee, composed of the three Stillwater County commissioners, Road & Bridge Superintendent Mark Schreiner, County Attorney Nancy Rohde, and Disaster & Emergency Services Director Carol Arkell, met on Dec. 29 to discuss the recent draft report on the rockslide prepared by Shannon & Wilson engineer Kami Deputy.
The committee members agreed that further evaluation of the rockslide by remote sensing or by crews rappelling down the rock face, as recommended by Shannon & Wilson, should be the next step.
“We need to see what the rock at the top is doing,” Schreiner said. “Otherwise we’d be putting up a rockfall fence without knowing what we really need to do.”
He also noted that the public might oppose installation of rockfall fencing for aesthetic reasons, and he was concerned that the road beneath the cliff was too narrow for fencing.
“There’s not enough room there for fences,” Schreiner said. “We’d have to cut the road down to one lane.”
Rohde expressed concern about some of the language used by Deputy in the report. Defining the event as a “rockfall” and not a “rockslide” could affect funding, she said.
Rohde also said Deputy’s characterization of GeoStabilization International’s August mitigation proposals as “economically infeasible” was “too conclusory.”
“We’re probably no further ahead than we were,” she said about the draft report.
Arkell said the draft report “brought up more questions,” and Schreiner said it provided a different point of view.
How soon additional evaluation work can be conducted could depend on winter conditions and access. Consultants will need to get equipment to the top of the cliff, which would need the approval of landowners in the area.
The committee agreed to arrange a conference call with Shannon & Wilson in the second week of January to discuss the draft report and how to move forward with further evaluation of the rock face.