Type 2 RHRS Slope on Highway 26

The Idaho Transportation Department (ITD) was experiencing regular rockfall along a 12-mile section of US Highway 26 in south-eastern Idaho. ITD maintenance crews performed regular rockfall patrols of the roadway to conduct safety inspections and remove rockfall debris affecting the travel lanes.  Rockfall occurrences were most active during periods of rainfall, springtime freeze-thaw cycles, and snowmelt runoff.  However, rockfall also occurred during drier times of the year due to wind erosion undermining rock blocks supported by a soil matrix in those slopes comprised of colluvium.  Rockfall quickly filled the roadside ditch and required regular cleanout.

Landslide Technology (LT) was retained to perform the Rockfall Hazard Rating System (RHRS) to assess and prioritize the rockfall hazards along the roadway so ITD could allocate limited mitigation funds.  LT performed a geologic reconnaissance to delineate rockfall sections based on slope conditions and rockfall mode.  The rock slopes were divided into structural domains and the relative rockfall hazards were rated using the RHRS with additional comments added to convey the relative rockfall hazard.  A total of 15 sites were rated and evaluated. 

Following the hazard ratings, conceptual mitigation measures and cost estimates were formulated to determine where ITD funds could reduce the most risk for the least cost.  LT evaluated the following rockfall mitigation measures including scaling, trim blasting, pattern or key block rock bolting, draped gabion mesh or cable mesh, shotcrete, and various rockfall barrier systems.  The conceptual cost estimates included mitigation quantities and unit prices for the rock slope stabilization and rockfall mitigation measures.