The Bonneville-Hood Transmission Line was originally completed in 1941, at the tail end of the Great Depression and as part of Franklin D. Roosevelt’s New Deal program. BPA commissioned a rebuild of a 22.5-mile section running from the Bonneville Dam Powerhouse to Hood River, crossing rugged, remote terrain.
Bonneville Power Association
Crux was selected to provide micropile foundation design and construction for replacement structures located in extremely steep, rocky terrain. The alignment traverses challenging geotechnical conditions featuring talus slopes, and there was very little data available prior to construction. Crux worked with BPA to develop a verification testing program, involving the installation of test piles to establish bonding capacities in the talus.
In total, Crux installed steel cap micropile foundations for 25 steel poles and one H-Frame structure, as well as guy anchors at seven micropile sites and one direct embed. Foundations consisted of either 4, 6 or 8 pile layouts, installed to depths ranging from 18 to 80 feet. Helicopter transport was required for all equipment and materials at 24 of the 25 structure locations, and crew access involved a combination of helicopter and hiking in from the nearest drivable access point.
The most challenging aspects of the project derived from the talus slopes, creating difficult access, setup and drilling conditions. Close to 600 helicopter hours were recorded, requiring increased planning and communication. Crux incorporated temporary helipad platforms at select locations to reduce hiking requirements. Extensive fall protection was implemented along access trails and at select sites as an increased safety measure. This included hand lines, working harnesses, and handrails. Custom leveling platforms were also employed to create flat, stable work space in the extreme conditions. To accommodate the challenging and unpredictable drilling conditions, Crux was prepared with a variety of tooling and installation approaches to select from as conditions changed.
Crux’s integrated design-build approach allowed for efficient design modifications as necessary when subsurface conditions varied from what was expected. This was especially valuable given the limited geotechnical information and challenging terrain.