Nonlinear 3D Dynamic SSI Analyses of a Caisson Wall to Protect Filter Outlet Conduit at Diemer Plant in Yorba Linda, California

The Metropolitan Water District (MWD) owns and operates the Robert B. Diemer Water Treatment Plant in Yorba Linda, California. The plant, which serves approximately 4 million people in Los Angeles and Orange Counties, was constructed in 1963 in accordance with standards in place at the time. The plant is in a seismically active region and is approximately 0.5 mile from the Whittier Fault, capable of generating Mw6.8 earthquake. The Diemer plant was constructed by cutting off the top of a ridge and placing the excavated materials in the adjacent ravines to produce a level pad. The plant’s 121-in. diameter filter outlet conduit (FOC) is buried about 7 ft while crossing a 400-foot length of the northeast fill slope (NFS). The fill materials were placed without removing colluvium of very low shear strength, and were compacted to the practices of the early 1960s. As such, it was not benched into a competent sedimentary rock as modern practice would dictate. In addition, the fill slope has an inclination of 1.5H:1V, which is steeper than the current practice of 2H:1V. Previous studies and field explorations have indicated that the fill slopes placed in ravines around the plant site including the NFS are only marginally stable under static loading conditions and do not have adequate stability under the MCE event. This led to concerns about seismic slope instability which was confirmed by various analyses. Seismic performance of the FOC was evaluated by performing dynamic soil-structure-interaction analyses using the FLAC 3D computer program. Based on nonlinear seismic deformation analyses, constructability, and consideration of minimum interference with plant operation, a caisson wall (CW) with a length of about 400 ft was selected to laterally support the fill slope housing the FOC. Analysis results indicated that 6-foot diameter caissons spaced at 8-foot on center would seismically stabilize the upper portion of the NFS supporting the conduit. This paper presents the analysis results in terms of lateral displacement, moment, and shear distributions for the CW, the maximum shaking-induced displacements of the FOC and the top of caissons. The effectiveness of the CW in terms of ensuring the structural integrity of the FOC during earthquake shaking is also provided. The results indicated that shaking-induced axial stresses along the FOC are well below the best-estimate buckling stress and the envelope of maximum principal wall stresses of the FOC remain in the elastic range.