US-12, Greer to Kamiah, Idaho, Rock Slope Assessment and Design

US-12 enters Idaho at Lewiston, Idaho crossing the Snake River from Clarkston, Washington and climbs along the original Lewis and Clark trail next to the Clearwater River past Orofino and upwards to Lolo Pass at the Montana border, approximately 170 miles in length. From Greer to Kamiah, US-12 snakes between high granitic cliffs and the river creating dangerous blind corners and obscuring potential rockfall debris in the highway. Between approximate MP-52 and MP-58, Idaho Transportation Department (ITD), District 2 has been reviewing concepts to widen the road, reduce the blind corners around the major rock slopes with the highest potential for rockfall, and avoid soil slopes which would require retaining walls. The project was divided into two phases. Phase 1 includes four slopes with the highest kinematic potential for rockfall between MP-52 and MP-54 which range in length from about 100 to 1900 feet and are upwards to 200 feet high. During the fall of 2014, the authors, using limited rope access rock climbing techniques, mapped and characterized the geology of the rock slopes. They followed up with analysis of the kinematic stability and provided recommendations for design cuts. Recommendations included regrading and flattening of the rock slopes to eliminate rockfall problems and or installation of rock bolts on unstable blocks where grading was not practical or there was right of way (ROW) problem. Challenges affecting the constructibility of the project include: pioneering access roads to the top of the proposed cutslopes, presplit drilling of thin sliver cuts on the steep rock faces, control of fly rock, blast vibrations and over pressure at the river’s edge because of the prime fish habitat. In Phase I, the four rock slopes are presently in design review. Phase II includes at least 13 slopes exhibiting similar rock slope problems.


  • English

Media Info

  • Media Type: Digital/other
  • Features: Figures; Photos; References;
  • Pagination: pp 251-265
  • Monograph Title: Proceedings of the 66th Highway Geology Symposium (HGS 2015)

Subject/Index Terms

Filing Info

  • Accession Number: 01638294
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jun 21 2017 5:16PM