The new tunnel and pipeline provide redundancy to the existing Crystal Springs Bypass Pipeline built in 1969. The original pipeline and soils in this area were subject to failure during heavy precipitation or a major earthquake.

The 1969 Crystal Springs Bypass Pipeline had been a critical, unique link in the transmission system conveying Hetch Hetchy Regional Water System water from the East Bay to the Peninsula.

The alignment of the 1969 pipeline, an-8-foot-diameter shallow buried pipeline, is below the hillside along Polhemus Road in San Mateo.

A major landslide in the area occurred in the winter of 1996-97 and raised concerns about future system reliability. If a landslide or earthquake had damaged the pipeline, Hetch Hetchy water supply to the Peninsula could have been cut off.  

The SFPUC determined that replacement of the pipeline with a deep underground tunnel and pipeline would most effectively ensure reliability.