CORRELATING SURFACE GEOLOGY WITH 1913-VINTAGE AS-BUILT GEOLOGY ACROSS THE SAN ANDREAS FAULT FOR SEISMIC ENHANCEMENT OF THE ELIZABETH TUNNEL, LOS ANGELES AQUEDUCT

Vulnerability of the five-mile-long Elizabeth Tunnel to earthquake hazards was recognized by the City of Los Angeles in 1913 when it was constructed across the San Andreas Fault Zone (SAFZ) as part of the initial Los Angeles Aqueduct (LAA). In 1970, the second LAA went into service, in the same tunnel crossing the SAFZ. Currently, 50% of the City’s water is imported through this tunnel. Viable mitigation alternatives are difficult to identify and to date no hazard mitigation measures have been implemented, However, the City plans to install a 30” HDPE pipe in the 9x11’ concrete-lined tunnel to enhance the likelihood that water will get through following a SAFZ earthquake. The initial pipe segment will cross the 2000’-wide SAFZ, with additional segments added during annual maintenance shutdowns. As-built tunnel records revealed reasonably good rock quality in the SAFZ , whereas poor-quality rock was 3500-4000’ to the south. A laser survey in 2012 demonstrated no creep deformation. Four subparallel faults cross the tunnel, including the SAFZ which is near the tunnel midpoint. Published geologic maps show gneissic and granitic rocks along the alignment. “Quaternary-Tertiary crushed granitic rocks” is mapped in fault contact with Cretaceous quartz monzonite and quartz diorite above the poor-quality rock in the tunnel about 900’ below ground. Field observations revealed minor differences between crushed and non-crushed granitic rocks, suggesting most rock in SAFZ proximity has been tectonized. One small zone of gneiss mapped against crushed granitic rock is a reminder of the scale of faulting and potential for variable conditions at depth. As-built geologic conditions within the tunnel are consistent with surface geology. The highest hazard to the tunnel is fault displacement, followed by roof collapse in the poor-quality rock. These considerations are guiding seismic enhancement to allow water flow following a major SAFZ earthquake.