Seismic Retrofit of the Hetch Hetchy Aqueduct at the Hayward Fault

Abstract

In the early 1990s, the Utilities Engineering Bureau (UEB) of the City of San Francisco inspected the pipelines at the fault crossing locations. BDPL 3 and 4 were found to be wrinkled, but not yet leaking. BDPL 1 and 2 had originally been outfitted with expansion couplings either side of the fault, and while pipeline bending had occurred, no serious local distortions were observed. The distortions in all four pipes were due to the ongoing creep of about 5 mm per year on the Hayward fault at the pipe locations. Engineering studies confirmed that if left unmitigated, the ongoing creep (or future fault offset) would eventually break the BDPL 3 and 4 pipelines (excessive wrinkling), and use up remaining slip in the expansion couplings for BDPL 1 and 2, causing those pipes to rupture open. Consequences of pipe failure include the following. If all 4 pipelines failed at the same time due to fault offset, all water supply from the east (Hetch Hetchy reservoir and Calaveras reservoir via the Sunol water treatment plant) would be lost until repairs were made. The BDPL pipelines serve many suburban municipalities in the east bay, south bay and peninsula areas of the greater San Francisco Bay Area, as well as the City of San Francisco. The City of San Francisco also has its own terminal raw water reservoirs located near the Pulgas Tunnel, which could support the City's population of 800,000 until repairs for the BDPL pipelines could be made. However, about 2,000,000 people in the east bay, south bay and peninsula areas are served by local water distribution systems, some of which have no alternate water supplies, and some of which have only a few