Project Specifics

As part of a new 120-mile long transmission line, San Diego Gas & Electric Company (SDG&E) constructed an approximately six-mile, twin circuit 230-kV transmission line, placed underground in the Alpine area of San Diego County, California.

The project entails construction of 40 vault structures, two cable poles, and an approximately 225-foot long trenchless (tunnel) segment beneath Interstate Highway 8. The majority of the underground segment involved excavation of two utility trenches extending along the major thoroughfare of Alpine Boulevard, sub-parallel to and south of Interstate Highway 8, from Star Valley Road to just west of Peutz Valley Road.

Geosyntec Scope of Services

The purpose of our geotechnical investigation was to provide geotechnical engineering recommendations for the design and construction of the proposed underground segment of the transmission line through the Alpine area. Geosyntec developed and implemented a field exploration and laboratory testing program consisting of a site reconnaissance, geologic mapping, geophysical seismic refraction surveys, exploratory borings, potholing explorations, thermal resistivity testing, and a geotechnical laboratory testing program. This allowed the firm to provide engineering recommendations for the design and construction of the proposed project including unique energy industry design recommendations for Moment Foundation Analysis and Design and thermal resistivity. In total, 25 borings, 18 seismic refraction surveys, and 227 pothole explorations were performed along with subsurface characterization of the entire approximately six-mile alignment. Geosyntec also provided engineering support services to assist in the development of the design drawings.

Notable Accomplishments

Geosyntec worked closely with SDG&E’s design engineering consultant to identify existing utilities along the alignment by field potholing with a vacuum excavating technique. This to significantly reduced damage to existing utilities when compared to conventional mechanical excavation techniques. Applying Geosyntec’s extensive experience with high-voltage transmission lines, the firm was able to make recommendations on the application of trenchless technology (tunneling) as a construction strategy to pass a 138-kV power line beneath Interstate Highway 8. Our recommendations included consideration of thermal resistivity issues were based on our specialized thermal resistivity field and laboratory testing experience.

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