Chaumox Road is a vital transportation link connecting residents along the west bank of the Fraser River to Boston Bar and providing essential access to First Nation lands. The roadway supports year-round access to safety and health services, cultural connection, and economic activity in a remote and terrain-constrained region.

The November 2021 Atmospheric River event caused widespread slope instability and infrastructure damage. A complete washout at the steep Gowen Creek culvert crossing occurred, severing community access and highlighting the vulnerability of infrastructure in this terrain.

Emergency works completed by the maintenance contractor and directed by McElhanney restored access through the installation of temporary twin culverts and construction of a realigned embankment. However, the substandard temporary alignment and the increased frequency of extreme weather events necessitated a permanent and resilient solution.

The BC Ministry of Transportation and Transit retained Thurber Engineering to complete geotechnical investigations and hazard assessments, and to design of foundations, anchors, retaining walls, and embankments. McElhanney led the hydrotechnical, structural, transportation, and environmental components. Collaboration with the Boothroyd Indian Band supported site works and environmental stewardship throughout construction.

Through close collaboration, the team advanced a 32 m single-span bridge supported on reinforced concrete abutments. The structure features four steel girders with a partial-depth precast concrete deck and cast-in-place abutments and retaining walls. The straight, widened configuration selected simplified construction within the narrow corridor while improving hydraulic capacity and debris passage. The team incorporated prestressed rock anchors and a geogrid reinforcement layer to control wall movement and mitigate potential soil washout, enhancing long-term stability and resiliency.

Delivered through a collaborative, multidisciplinary approach, the project addressed complex site constraints and met performance objectives. The steep gully, erodible soils, and high scour potential created challenges for stability and drainage. Roadway geometry also had to accommodate long, heavy logging trucks, requiring flatter grades and larger curve radii.

Because full global stabilization of the downslope terrain was not practical, the team adopted a performance-based design approach. Rather than attempting to completely buttress the slope, the strategy focused on maintaining roadway and bridge integrity even if soil loss were to occur.

The permanent solution incorporated a bridge founded on rock-socketed piles to provide deep support. Constructed with reinforced concrete and tie-back anchors, the approach fills and abutments will resist lateral loads in the steep terrain. The developed geogrid detail will retain backfill behind the walls in the event of potential retrogression in front of the piles, adding redundancy to the system. Careful evaluation of the bridge configuration balanced cost and constructability. A shorter bridge with tall retaining walls in the ravine would have been difficult and costly, while site access and girder delivery limits ruled out a long-span bridge without walls. The team selected a straight, wider bridge accommodating the curved roadway rather than a custom curved structure to simplify construction and use readily available materials.

The completed 32 m single-span steel girder bridge, founded on deep steel pipe piles, provides a resilient, permanent crossing resistant to shallow slope instability. Reinforced abutments, anchored retaining walls, and integrated geogrid reinforcements mitigate erosion and soil loss, limiting deformation under extreme events. The optimized straight alignment improves constructability, hydraulic capacity, debris passage, durability, and long-term climate resiliency.