The Drum-Rio Oso project involved there build of a PG&E northern California transmission line to replace existing aging infrastructure. Lattice towers would be replaced with new, steel structures for the updated 115 kV line.
Burns & McDonnell
Crux was selected to design and construct four specialty foundations for two H-Frame structures with significant access challenges. The structures were located on steep, rocky slopes, requiring light-lift helicopter access for all equipment and materials. Personnel hiked to and from structure locations, as the terrain was too steep and rugged to allow for a helicopter landing zone.
Steel cap micropile foundations were designed and constructed for both structures. The equipment and materials required for micropile installation can be lightweight and helicopter portable, and the use of steel pile caps in place of concrete alleviated the need for concrete transport. Each foundation consisted of three micropiles installed in a battered array to approximately 35 feet, for a total 430 feet of drilling.
The combination of rocky terrain and approximately 50-degree slopes resulted in some of the most challenging setups Crux has seen to date. Leveling platforms were set to support the drill, with support legs on the downslope side reaching 13 feet. Additional challenges included working in close proximity to existing overhead lines and safely performing helicopter operations at approximately 4,500 feet. Equipment was broken down further than usual to accommodate weight restrictions, and the steel caps were set with medium-lift helicopter as they were the heaviest component of the foundation.