Enhancing Chances for Water to Cross the San Andreas Fault in the Elizabeth Tunnel

The designer of the Los Angeles Aqueduct was aware of the location and importance of the San Andreas fault, and that it was crossed in the Elizabeth Tunnel. Fairmont Reservoir, which provided storage at the upstream end of the tunnel, was taken out of service after the 1971 San Fernando earthquake because its design was the same as a dam at the southern terminus of the aqueduct that failed. A new dam was constructed to provide head with piping that included a bulkhead in the tunnel to prevent backflow into the reservoir. The 2008 Great Southern California ShakeOut scenario revealed that all three aqueducts bringing water to Southern California could be damaged in a single earthquake. The Los Angeles Department of Water and Power considered constructible “enhancements” that might allow water to flow through the tunnel after fault displacement. A short section of high density polyethylene (HDPE) pipe placed across the fault zone inside the tunnel was considered because of its excellent performance in the 2010–2011 Christchurch earthquakes. Some unstable locations were indicated in 1907–1911 as-built tunnel construction geology notes, but most of the concrete-lined tunnel had no notes and it has not been tested by nearby earthquakes. Considerations began in 2012 for anchoring an HDPE pipe in the semicircle invert. The existing bulkhead was to be replaced with a removable-door bulkhead to allow construction access from the out-of-service reservoir. Design alternatives were studied; steel ribs were added for structural support across the 146-m-wide zone of most recent faulting; shotcrete overlays were added in very poor rock quality zones identified with a seismic refraction survey inside the tunnel. HDPE pipe was eliminated from the design concept during final design, which was completed in 2020 by Los Angeles Department of Water and Power engineers. Construction was planned to begin in late 2021 and be completed by 2026.