Mile Rock Tunnel, Under Forty-eighth Avenue from Cabrillo Street to San Francisco Bay at Point Lobos, San Francisco, San Francisco County, CA

Significance: The Mile Rock tunnel was constructed in 1914 and 1915 and is an example of the whole-scale reconstruction and reconfiguration of the City of San Francisco's public works system that occurred following the 1906 earthquake. This reconstruction effort drew worldwide interest and support, and offered challenges and opportunities to engineers all over the nation. Designed by M.M O'Shaughnessy, a renowned civil engineer best known for his design of Hetch Hetchy Water System, the tunnel was the first constructed in the city that used a combination of open cut timber cribbing and boring through solid rock and represented a technological and engineering innovation in the city. It served as the storm drainage facility for the Sunset and West Mission districts and portions of the Richmond and Ingleside districts. The tunnel has experienced little modification through the years. Ongoing, occasional maintenance has occurred, but there has been no major reconstruction or redesign and the tunnel retains a high degree of integrity of location, design, setting, and workmanship.
Survey number: HAER CA-162
Building/structure dates: 1915 Initial Construction