US-95 is the main north-south route through Idaho, serving as a major economic and community connector. The route passes through Riggins, ID, in Idaho County, where the terrain is characterized by steep canyons and rockfall hazards. Idaho Transportation Department maintains the route and contracts slope stabilization construction to reduce the frequency of rockfall and risk of route closure. Most recently, scaling and reinforcement work was completed above a sharp bend in the Salmon River, just 1 mile (1.6 km) north of the town of Riggins. The subject slope is approximately 250 feet tall (76 m) and measures several hundred feet across. The slope is characterized by a prominent spine of rock where large, dilated blocks of schist rock are underlain by steep geologic structure.
Project work involved the installation of fully grouted rock dowels to stabilize the dilated rock mass and scaling to remove loose detached material across the slope area. Reinforcement was also installed through a dip-slope formation across the southern half of the work area. In total, approximately 3,300 linear feet (1,006 meters) of reinforcement was installed. Delve Underground performed reconnaissance and design for the slope stabilization program. Rope access techniques, drone-based imagery, and 3D modeling were critical to project success.
Delve Underground also provided construction management services and on-slope engineering support throughout construction. Slope work was completed in December 2024, and paving through the project area is scheduled to be completed in Spring 2025.