By any measure, San Francisco's effort to seismically upgrade its water supply is big. The $4.765-billion Water System Improvement Program (WSIP) includes 83 separate projects spread throughout the Bay Area and beyond. Continuously under way for 10 years, construction isn't expected to wrap until 2018. Currently, 16 projects are under construction that total $2.7 billion in contract value.

Yet one of the most critical links in the chain of projects is a comparatively small, $75.1-million upgrade to a pair of water pipelines that cross the Hayward Fault in Fremont.

Despite its size, the upgrade to Bay Division Pipeline Nos. 3 and 4 is "one of the most challenging engineering efforts of the program and is symbolic of the success of the program overall," says Dan Wade, WSIP director for the San Francisco Public Utilities Commission (SFPUC).

In an earlier phase, SFPUC added shut-off stations on either side of the fault to close off or divert water between either of the two parallel pipelines.

But simply shutting off the water isn't enough. The agency also is charged with ensuring that its 2.6 million customers will have access to potable water within 24 hours after a major earthquake. To achieve this goal, at least one of the pipelines must survive a sudden horizontal offset of up to 6.5 ft when the right-lateral strike-slip fault ruptures. According to most seismologists, an earthquake along the Hayward Fault is overdue and could happen anytime.

Project designer Marty Czarnecki, senior vice president in the San Francisco office of URS, initially took inspiration from the Denali Fault crossing on the Trans-Alaska Pipeline. Perched on PTFE, or Teflon, "shoes" that slide on long steel beams, the pipeline successfully withstood 14 ft of lateral motion during a magnitude-7.9 earthquake in 2002.

Although this technique couldn't be replicated due to the California pipeline's underground location and tight right-of-way, the resulting solution calls for building a new section of pipeline No. 3 that rests on sliding supports within an articulated concrete "vault" box, which can flex like an accordion toward the direction of the fault movement. Inside the vault, two massive ball joints rotate up to 12° to allow the pipe to shift. On the north end, an innovative slip joint connects to a pipe clad with anti-corrosive metal panels to ensure free movement, whether the earthquake hits in five months or 50 years.

Steve P. Rados Inc., the project's Santa Ana, Calif.-based general contractor, began the pipeline upgrade in September 2012. Construction under the traditionally bid, $31-million contract will be substantially complete by October, with final testing and sign-off expected in April 2015.

Building the pipeline and installing one-of-a-kind components using uncommon materials underneath the intersection of a major freeway and state highway requires an extreme attention to detail, says Jim Pelletier, assistant northern California manager at Rados.