Micropile Foundations | Geotechnical Construction
Sunrise Powerlink Project
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Project Description
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. In total, 13,200 tons of steel, 1135 miles of overhead high-voltage wire, 691,200 cubic feet of concrete, and more than 5.4 million labor hours went into completing the project. The alignment runs through various difficult-access and environmentally sensitive terrain, including desert, mountains, big horn sheep territory, and National Forest Land.
The first transmission tower was completed in March 2011 and the line officially went into service June 18, 2012. 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, requiring that 33 percent of all energy within the state come from renewable sources.
Team Members
San Diego Gas & Electric, PAR Electrical Contractors, Crux Subsurface, DCI Engineers
Crux’s Role
Crux provided design-build micropile foundation services for over half of the lattice tower structures on the alignment. 234 of the 438 total 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 minority of the micropile foundations, while a new, innovative steel cap was employed across many of the tower sites.
SDG&E had previously approved the use of steel pile caps for tubular steel pole projects, but Sunrise was the first application of Crux’s patent pending steel pile cap design for lattice tower foundations. 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 significantly reduced and totaled only 18 months instead of the originally planned 24. Each individual tower site was also subject to specific work windows depending on surrounding flora and fauna restrictions. The steel caps were constructed in an offsite facility and significantly reduced the amount of installation time in the field. They required an average of 15 field labor hours and 12 helicopter trips per tower, whereas concrete caps would have required an average of 51 field labor hours and 40 helicopter trips per tower.
Due to the lack of road construction, materials, equipment and personnel were transported to and from the drill site by medium and light-lift helicopter. Crux manufactures specialty lightweight drilling equipment to be easily transported by helicopter for projects such as this. Once onsite, the proprietary drill rig was set up on a platform and rotated around a centroid to efficiently install each micropile with enough precision to facilitate bolted connections between the micropiles and pile caps. An array of between 3 and 12 eight-inch diameter micropiles was installed to a typical depth of 35 feet at each tower leg. The micropile installation process consisted of placing an upper steel casing to a prescribed depth, drilling and placing high strength all-thread rebar into the lower bond zone, and grouting each pile with high strength grout from the bottom up. For foundations that were installed in soil, pressure was applied to the grout while casings were retracted from the bonded section. A minimum of two piles were tested to their maximum factored load at each tower site to ensure design criteria were met. Steel caps were then installed on the micropile arrays to prepare for the arrival of a lattice tower, which was also transported and erected by helicopter.
The use of Crux’s steel cap micropile foundation design on the Sunrise Powerlink significantly reduced the environmental impact of construction and was a key component to the owner and general contractor meeting an accelerated construction schedule. The line went into service just in time for the summer months, which typically take the greatest toll on the region’s electrical systems, and has the capacity to power 650,000 homes. This was the largest-scale and most challenging transmission project Crux has completed to-date.
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