Geotechnical Construction for Transmission Line Project
San Diego Gas & Electric’s (SDG&E) Wood to Steel Replacement Program will replace approximately 5,500 existing wood transmission poles in San Diego County with tubular steel poles. The purpose of the program is to reduce maintenance expenses, improve fire safety, and reduce damage to SDG&E infrastructure during fire season. The project is being completed in various segments over the course of several years.
San Diego Gas & Electric
Hard rock and close proximity to residential homes necessitated a foundation alternative to excavated or drilled shafts at a number of sites. Crux was selected to provide design-build micropile foundations, typically for dead-end or angled structures. Specialty compact equipment was used to successfully install foundations under energized lines and within six feet of existing wood poles.
To date, Crux has designed and installed 172 foundations on eight separate segments. Access constraints and geotechnical conditions have varied significantly, requiring a combination of track vehicle, crane, and helicopter. Crux’s customized micropile installation equipment can be broken into 2,500-pound components, allowing for more efficient transportation. The drills can also operate with as little as 10 feet of overhead clearance, which was beneficial as extended power outages were not permitted.
Micropiles were installed in battered arrays of between 3 and 16 piles, depending on site-specific geotechnical conditions and tower loads. Steel pile caps were installed on all but two foundations, which employed concrete pile caps. The piles at these two locations needed to be spaced further than usual to bridge an underground utility, which would have resulted in steel caps too large and heavy to be feasible. Steel caps were installed at the remaining 168 structure locations. The caps were fabricated in a controlled, offsite facility and transported to the sites for installation.
Crux’s largest Wood to Steel scope included installing 87 steel cap micropile foundations for SDG&E’s TL 637. The project required close coordination with other SDG&E subcontractors, completing alignment sections out of sequence and erecting poles immediately following foundation completion. Select foundation sites were located in protected habitats and required enhanced mitigation measures.
Upon completion of the project, Crux received a letter from the SDG&E Environmental Programs team expressing their gratitude at the extra effort put forth to avoid environmental impacts.
Completed Segments: TL 678 (23 structures – 2010), TL 685 (32 structures – 2010), SP 820/821 (2 structures – 2010), TL 689 (20 structures – 2011), TL 6927 (2 structures – 2011), TL 635 (4 structures – 2011), TL 6913 (2 structures –2013), TL 637 (87 Structures – 2014)