Mitigation of Fault Rupture Hazard to Water Mains of a Major Metropolitan in the San Francisco Bay Area

Nisar, Ahmed, Paul Summers, Douglas Honegger, Alex Ameri, Christopher Hitchcock, Anna Liu, Henry Louie, and Jeffrey Bachhuber; "Mitigation of Fault Rupture Hazard to Water Mains of a Major Metropolitan in the San Francisco Bay Area;" presented at the 13th World Conference on Earthquake Engineering, Vancouber, B.C., Canada, August 2004.

The City of Hayward is located in an area of high seismic hazard, primarily because the Hayward Fault runs through the center of the city. The Hayward Fault is one of the most active faults in the San Francisco Bay Area, and has an estimated probability of 28% of a major earthquake in the next 30 years. The expected size of such an event is Magnitude 7.0 with fault rupture displacement ranging between 4 to 6 feet (1.2 to 1.8 meters). The City has a number of transmission and distribution water pipelines, with limited displacement and rotational capacity, which will be subject to fault rupture hazard.

The purpose of this work was to improve the reliability of water supply for critical tasks such as fire fighting and providing drinking water following a major earthquake on the Hayward Fault. The project focused on developing cost effective and practical solutions that would result in a reliable supply of water for emergency operations immediately following the earthquake. Because of the uncertainties associated
with earthquakes and the need for a highly reliable solution, the proposed mitigation scheme was based on accepting damage at the fault rupture locations and developing strategies and tools that the City could use rapidly in a time of emergency and chaos that typically follows a major natural disaster. To ensure a continued water supply immediately following the earthquake, the approach consisted of a combination
of pipeline replacement and developing a scheme to isolate and bypass damaged pipelines.

For the mitigation scheme to be successful, detailed information is needed on the amount and distribution of fault displacements at the fault crossing, uncertainty in the estimates of fault displacement, the number of fault traces within the fault zone, the width of the shear zone at the fault, the impact of long-term fault creep, and the depth and strength of surface sediments overlying the fault. Therefore, prior to developing
the mitigation scheme, detailed mapping of the fault was performed through field reconnaissance, evaluation of pre-development aerial photographs and an extensive literature search on fault trenching, historic mapping of the fault and geologic and geotechnical investigations within the fault zone.

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