Design of Metropolitan Water District’s Replacement of Casa Loma Siphon Barrel No. 1 Project, San Jacinto, California

Abstract

The Metropolitan Water District of Southern California (Metropolitan) initiated the Replacement of Casa Loma Siphon Barrel No. 1 (Project) to improve the seismic resilience of the Colorado River Aqueduct (CRA) facilities. The Casa Loma Siphon Barrel No. 1 is part of the CRA that conveys water from the Colorado River to Metropolitan’s water supply system and is a critical lifeline facility. The existing siphon crosses the Casa Loma Fault near San Jacinto, California (within the San Jacinto fault zone) and is susceptible to damage due to fault rupture and ground subsidence. The siphon was originally constructed in 1935 and is a 148-in. diameter concrete pipe. The most critical objectives of the Project are to retrofit the existing siphon for the anticipated fault displacement and long-term settlement due to groundwater withdrawl and to allow Metropolitan to quickly return the pipeline to service should damage occur during an earthquake. The Project will replace the existing siphon with dual 104-in. diameter, earthquake-resistant ductile iron pipelines (ERDIP). The project team evaluated several alternatives during design including different pipeline alignments, alternative backfill materials, and various pipeline joint layouts and configurations. Because of the large anticipated fault deformation, the preliminary analyses led to a design that incorporates ERDIP with Expanded Polystyrene (EPS) Geofoam backfill material to distribute the fault displacement across many pipeline joints. The Project team developed a novel approach to modeling the ERDIP joints by utilizing the finite element analysis program ABAQUS to simulate the pipeline joint deformations along axial and rotational degrees of freedom combined with traditional soil-pipe interaction modeling to examine the pipeline performance. This project has several unique challenges, including: (1) design for large fault displacements estimated at 12.8 ft horizontal and 2.6 ft vertical (coseismic surface displacement) and 3.3 ft of settlement over a period of 50 years; (2) novel application of EPS Geofoam as pipe backfill at the fault crossing to distribute displacement and reduce stress in the pipeline joints; (3) sophisticated finite element analysis using a three-dimensional model of the fault and pipeline interactions; and (4) full-scale testing of the 104-in. pipe joint performance to verify the maximum rotation and moment capacities.