Landslide Triggering for Transmission Power Lines in Southern California

Power transmission and subtransmission lines in California often face unpredictable climate change, high seismic activity, and geohazards such as slope instability. One such line is a transmission line that traverses over 50 mi of mountainous terrain from Ventura County to Santa Barbara County. Most of the transmission line structures along this line are located in remote mountainous locations and are founded on sedimentary bedrock formations. Some are in landslide terrain, adjacent to mapped landslides, on steep slopes, or on ridgetops. The instability risk for these structures increases with significant precipitation and/or seismic events. Loss or damage to any of these structures due to ground deformation would result in disruption of service to all subtransmission and distribution lines in the service territory. Since the structures’ remote locations make prompt repairs impractical, preventative mitigation is often necessary. However, mitigation is expensive, so measured and targeted mitigation becomes essential. Through a geological desktop study and site visits, the locations of structures that are most prone to landslides were identified. Groundwater mounding modeling, field investigations, and other geotechnical and hydrologic studies were conducted at these sites. The project team estimated peak ground accelerations and spectral accelerations (PGA and SA), with 2% and 10% probability of exceedance in 50 years. They used the historic precipitation data to generate landslide-triggering precipitation criteria and create a monitoring tool to predict rainfall. The probability of exceedance depends on the project life cycle. The team first calculated the factor of safety (FOS) under saturated conditions due to precipitation, and then they calculated the seismic load. The seismic vulnerability was assessed, excluding the impact of storm events. The FOS was calculated based on the project life cycle, where line resiliency is determined using relationships between FOS and the probability of exceedance, considering storm recurrence intervals. In short, this transmission line is a single-point service interruption system prone to landslides, and due to its remote location, it is difficult, time-consuming, and expensive to fix. Mitigation measures were developed and implemented. The approach requires determining which locations pose the highest risk, defining the circumstances that require mitigation, and selecting efficient mitigation measures based on the structure’s resilience.