Idaho’s State Highway 55: Emergency Response, Rock Slope Design, & Construction

From 2020 to 2023, the Idaho Transportation Department (ITD) District 3 oversaw the widening of State Highway 55 between Smiths Ferry and Round Valley, Idaho, approximately 60 miles (97 km) north of the city of Boise. This route provides a critical transportation connection between Boise and the town of McCall and is one of the few north–south connector routes in Idaho.

At the project location, the highway follows a deep incised canyon formed by the North Fork of the Payette River. ITD’s project goal was to improve roadway geometry and sight distance by widening and realigning the roadway using a series of rock cuts and downslope wall systems. Within the 1-mile (1.6 km) project limits, work included nine major rock cuts ranging between 60 and 200 feet high (18–61 m).

On March 15, 2021, a series of significant failures occurred in the newly constructed rock slopes, damaging several pieces of heavy equipment and shutting down the road. ITD immediately contacted Delve Underground to mobilize to the site, assess what caused the slope failures and the risk to the traveling public, provide emergency stabilization design and construction services, and assess and redesign the remainder of the rock cuts for the project. ITD’s mandate was to maintain the original construction schedule even though the project had to be redesigned. Delve Underground was given broad latitude with respect to coordinating the redesign, and construction phasing and staging to meet this objective.

Over the next two construction seasons, the Delve Underground team provided blasting consulting for over 80 blasting plans; developed plans, specifications, and estimates for the rock-cut redesigns; and oversaw installation of rock dowels, mesh drape, pinned mesh stabilization systems, and rock slope drains. To facilitate rapid redesign of the project, the team used rope access safety techniques* and drone/UAV technology extensively to collect data that was utilized both to establish the new cut designs and for design of supplemental rock reinforcement required as each cut lift was constructed. Initial slope mitigation and redesign were delivered to ITD within just 30 days of the emergency route closure.

*See “The Benefits of Rope Access in Challenging Terrains” (July 2023) for more about Delve Underground’s rope access services and training program.

The success of this project relied on tight integration of the project delivery team, including the ITD project engineers, ITD management, the contractor, and Delve Underground. The close working relationships established early on during construction allowed design work to be closely coordinated with construction sequencing and phasing such that delays were substantially avoided. Not only was the project completed within the original schedule—in fall 2023—it was awarded the Idaho Association of General Contractors Top Gold Award for Excellence in Construction Partnering.