Location: Inyo National Forest
Southern California Edison (SCE) commissioned the rebuild of the Rhinedollar Overhead Powerline, located within the Mono Lake Ranger District of the Inyo National Forest. The rebuild was necessary to meet current State and safety standards, and to increase reliability of the line. 13 lattice tower structures were replaced with tubular steel pole (TSP) structures to support an upgraded 12 kV distribution circuit. The 1.6-mile alignment runs from the Poole Power Plant to Ellery Lake Dam, and required helicopter only support to replace because of the limited access to tower sites. Helicopters were employed to remove existing structures, transport personnel and equipment, and deliver all foundation materials and replacement structures.
Southern California Edison, Crux Subsurface, Quanta Subsurface, DCI Engineers, Mountain Air Helicopters, PJ Helicopters
Crux was selected by SCE to serve as the general foundation contractor on the Rhinedollar replacement project, providing 100% design-build micropile foundation services for all 13 replacement poles. All sites required helicopter support because of limited access caused by rugged hillside terrain. Elevations along the alignment ranged from approximately 7,800 feet to 9,670 feet, which created challenging helicopter conditions and limited construction to a very small window in the summer months. Higher than average temperatures added to the challenging helicopter conditions, and required innovative scheduling techniques and load adjustments. Processes were altered specifically for the altitude conditions on this project, requiring equipment to be moved in different sequences and broken down further than usual to accommodate weight restrictions.
Another challenging aspect of the project was the close proximity to public campgrounds and Highway 120, which provides the East entrance to Yosemite National Park. Yosemite is the third most visited National Park in the country and sees large amounts of traffic during the summer months when the rebuild took place. In order to ensure public safety throughout the construction process, flight corridors to transport external loads were chosen specifically to avoid campground areas. Flight corridors were also chosen to avoid crossing highway 120 where possible. In the few areas where it was unavoidable, traffic control was employed to ensure vacant roadways when helicopters were crossing with external loads attached. These safety measures were implemented successfully, and the project was completed without incident.
Site excavation on Rhinedollar was minimized through the use of Crux’s platform drilling technique. Customized drill rigs were set up on platforms, and arrays of between three and six 7-inch micropiles were installed depending on geotechnical conditions and TSP foundation loads at each site. After grouting activities were completed, proof load testing was performed at each foundation location. Proof load tests were completed to the maximum factored load to ensure design criteria were met. Crux provided a steel pile cap design for this project to reduce helicopter flight hours associated with transporting supplies and concrete for cast-in-place concrete caps. Steel caps were set in place by helicopter and bolted to the micropile array to act as a transition plate between the piles and steel poles. Crux has utilized a similar design on past projects, including San Diego Gas & Electric’s Wood to Steel Replacement Program.