The San Diego Gas & Electric Company (SDG&E) is proposing to construct a new 138-kV substation adjacent to an existing 138-kV substation and replace the existing 138-kV substation with a new 230-kV substation in San Juan Capistrano, California. Geosyntec was contacted by SDG&E to perform a geotechnical investigation as part of the as-needed geotechnical contract between Geosyntec and SDG&E. The geotechnical investigation was performed to characterize the subsurface at the site, perform engineering analyses, and provide geotechnical engineering recommendations related to earthwork, grading, retaining wall, and foundation design for the proposed substation.
The project site is underlain by the landslide-prone Capistrano Formation and the potential for landslides was evaluated as part of the geotechnical and geologic hazards investigation. The proposed new graded pad for the new 138kV substation involved cuts and fills on the order of 25 feet with a new retaining wall planned along the new access road on the site and within the proposed new substations.
Geosyntec's Scope of Services
For this project, Geosyntec performed a site reconnaissance and field exploration program, geotechnical laboratory testing, and engineering analyses. The field exploration program consisted of 10 exploratory test pits excavated with a backhoe and seven borings advanced using hollow-stem auger drilling techniques. Geotechnical laboratory testing was performed to characterize the subsurface soils encountered during the field exploration program. Geosyntec provided grading and foundation recommendations for the proposed development of the new SDG&E 138-kV Substation pad as well as engineering recommendations regarding the suitability of the existing 138-kV substation pad for expansion as a new 230-kV substation. Geosyntec engineers provided design recommendations for retaining walls and foundations being considered for the site. Wall types considered included mechanically stabilized earth as well as conventional and soil-nailed. Foundation considerations included recommendations for soil parameters to be used in the EPRI Moment Foundation Analyses and Design program for foundation designs involving electric power structures.
As part of the geotechnical investigation, Geosyntec performed a geologic hazards analysis of the site. This included evaluation of the potential for landslides, fault rupture, liquefaction, and flooding, along with other geologic evaluations.
The field investigation protocol for the new 230-kV substation on the site of the existing 138-kV substation was modified to allow for performance within an active substation without shutting down power to the facility. Geosyntec applied its broad expertise in conducting field investigations, particularly in locations with overhead and lateral constraints, to develop a highly controlled program using specialized equipment. As California is a seismically active environment, Geosyntec's geotechnical engineers provided ground improvement recommendations to stabilize the potentially liquefiable subsurface without requiring excavation below the existing groundwater, a potentially significant cost savings to SDG&E.