Seismic Slope Stability Analysis and Mitigation for Transmission Power Lines in Southern California

Many of the high voltage transmission line towers in Southern California are located in hilly terrains and in a high seismic hazard environment which requires assessment of these towers during a major seismic event. Some of these towers are located within historic landslide features making them quite vulnerable for large deformations during seismic shaking. Permanent ground deformations due to slope failures during and after a seismic event could result in losing some of these towers and their functions. Therefore, in this study, a seismic assessment in terms of slope stability was performed for several high voltage towers along a transmission power line corridor from Tejon Pass to Cajon Pass in Southern California. Incidentally, this corridor runs along the Southern San Andreas fault. Based on an initial study including review of geology maps and site visits, out of 2,269 structure locations along the corridor, 15 locations were selected for further evaluations for seismic slope stability. Field investigations consisting of borings were performed at some of these sites and soil/rock samples were obtained for laboratory testing. At some locations, field survey was performed to establish elevation contour lines in order to develop appropriate cross sections. In addition, at some locations, towers were assessed for both local slope stability and global slope stability. Local slope stability concerns were raised because some towers are located next to a cut slope which was made primarily for access roads. The analyses involve developing ground motions using UCERF3 seismic source model, developing cross sections (both local and global), assigning strength properties to each layer, and performing seismic slope stability analysis. Ground motions were developed for probability of exceedance of 5-, 10-, 20-, and 50-percent in 50 years and parametric studies were performed to establish design level earthquake. Seismic deformations were estimated using simplified method. For locations where deformations exceeded a certain threshold, different mitigation methods were proposed including but not limited to soldier pile and lagging walls and soil-nail walls. Challenges for this study include high ground motions resulting in about 1 g for PGAs, anticipated large slope deformations, and difficult terrain to implement mitigation methods. This paper will present the thought process to select the design level earthquake based on the parametric studies, results of analyses in terms of FOS and seismic deformations, and details about mitigation methods used at certain locations.