Steel Cap Micropile Foundation Design-Build
The Sunrise Powerlink is a new 117-mile, 500 kV transmission line capable of carrying 1,000 megawatts of renewable energy from Imperial Valley into San Diego. The alignment runs through a variety of difficult-access and environmentally sensitive terrain, including desert, mountains, big horn sheep territory, and National Forest Land. The San Diego Gas & Electric (SDG&E) project was completed on time despite an accelerated schedule, and will contribute to California’s Renewable Energy Mandate.
San Diego Gas & Electric
PAR Electrical Contractors
Crux provided design-build micropile foundation services for over half of the lattice tower structures on the alignment. 234 of the 438 tower sites were located in difficult-access and/or environmentally sensitive locations, including 74 within the Cleveland National Forest. These tower sites required helicopter-only construction methods as road construction was not permitted or feasible.
Concrete caps were utilized for a small number of the micropile foundations, while a new, innovative steel cap was employed for the remainder. Crux had been developing the technology and introduced steel as an alternative to concrete micropile caps when the project schedule was accelerated. Due to environmental and other factors, construction schedules were reduced from 24 months to 18. Each tower site was also subject to specific work windows depending on the proximity of protected species. The steel caps were constructed in an offsite facility and significantly reduced the amount of installation time in the field. An average of 15 field labor hours and 12 helicopter trips were required per tower, whereas concrete caps would have required an average of 51 field labor hours and 40 helicopter trips per tower.
All materials, equipment and personnel were transported by helicopter. A proprietary drill rig was set on a levelling platform, and arrays of between 3 and 12 eight-inch diameter micropiles were installed to a typical depth of 35 feet at each tower leg. Pile depth and quantity varied depending on tower load and geotechnical conditions at that site. Steel caps were then installed on the micropile arrays using a bolted connection. The use of Crux’s patented steel cap micropile foundation significantly reduced the environmental impact of construction, and was a key component to meeting an accelerated construction schedule. All construction work was completed within schedule and the line is currently in service.