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-to-access and environmentally sensitive terrain, including desert, mountains, big horn sheep territory, and National Forest Land. 

235 of the total 438 tower locations were deemed as helicopter-only construction sites and required an alternative foundation solution, thus micropile foundations were selected. Micropile foundations for lattice towers had traditionally utilized cast-in-place-concrete pile caps. For this particular project, steel pile caps were contemplated as an alternative to promote schedule advantages for tower sites constrained by short environmental windows of opportunity.

The project crossed three major regions; desert, mountain, and coastal marine from El Centro, to El Cajon, CA. Design specifications required a corrosion protection method comparable to that of concrete cover, sufficient to endure the potential of aggressive soils in the desert as well as the corrosive salt air of marine environments. Specifications also necessitated a design life of 75 years. The corrosion protection method also needed to provide resistance in the case of partial burial for short periods of time, as many of the foundations are located on steep slopes. The difficulty of access the remote sites in mountainous regions and the environmental restrictions along the entire alignment required a foundation design and corrosion protection method that could remain maintenance-free on a longer scheduled interval.

Corrosion Protection Selection Process

Paint and other coatings were deemed insufficient for the following reasons:

• Rigors of transport and installation Pile caps were manufactured at facilities across the United States. The best coating applications were applied in a controlled environment and transported to the project site. Each cap weighed between 2500 and 4500 lbs, and transportation without creating breaches in the coating was challenging. Once delivered to the project site, the caps were flown by external load under helicopters and set precisely onto groups of micropiles. Conflicts during the fit-up between the piles and the cap presented challenges in identification of breaches in the coating, as well as limited the ability to repair. Surface applied coatings did not meet design life requirements. None of the paints, epoxies or flame spray processes contemplated allowed for a design life of 75 years of service without routine maintenance.
•  Inspections were scheduled on intervals, and minor limited maintenance was expected, but major routine scheduled maintenance was determined to be unacceptable.

Weathering steel was eliminated from the selection process for the following reasons:

• Micropiles are sensitive to long, unsupported free lengths projecting above ground surface. This creates the need to keep pile caps as low to the ground as possible. Many of the towers were located on steep slopes, prone to surficial erosion, and there was a possibility soil could come in contact with the cap by erosional deposit. Weathering steel relies on a surface patina which is formed and maintained by wet/dry cycles, and soil burial traps moisture against the steel, damaging the ability to form patina and exposing the steel to excess corrosion. Inspections could identify burial or soil deposits, but would require additional maintenance to mitigate.
• The extent of corrosion is difficult to quantify. If soil contact was identified, testing to determine the significance would be complex and would necessitate skilled technicians. 
• Repair or replacement of corroded components would be restricted by the same limitations as original construction. Grinding or welding require special permits and environmental constraints impacted the opportunity to conduct maintenance or repair. 

Hot-dip galvanizing was chosen as the method of corrosion protection for this component of the alternative foundation design for the following reasons:

• Design life sacrificial steel and HDG provides a design life in excess of 75 years.
• Resistance to short term burial Steep slopes and unstable soil conditions may create partial burial for durations between inspections.
• Durability during transportation and assembly Intermetalic layers and cathodic qualities of the zinc allow minor imperfections in the coating to remain untreated or are easily repaired with field applied cold galvanizing compound. 
• Environmental The slow degradation of zinc is preferable over the deposit of surface coatings from flaking, peeling, or mechanical stripping for coating maintenance. 

Benefits of HDG Selection

• Steel Caps with HDG reduced field labor hours significantly over cast-in-place pile caps, creating construction schedule savings in an accelerated schedule.
• Steel fabrication and galvanizing are completed inside fabrication facilities, increasing quality control, and decreasing variables associated with weather and other outside factors. 
• Steel Caps with HDG significantly reduced helicopter hours as compared to cast-in-place pile caps. The reduction in helicopter loads created economic savings and a reduction in environmental emissions, as well as reduced worker exposure to safety incidents.