The water agency also procured the 72-in. ball joints, which are the largest known to be manufactured in the world, according to fabricator EBAA Iron Sales in Eastland, Texas. The company's line previously topped out at 48 in. SFPUC also purchased a full-size prototype and subjected it to rigorous testing.

Composed of ASTM A536 ductile iron, each ball joint weighs in at over 17 tons and is tested hydrostatically to 200 psi, higher than the pipeline's normal operating pressure of 125 psi. The ball joints were tested at 12° displacement, which exceeds the 8° of movement expected during an earthquake, says Czarnecki.

Between the ball joints, two sliding supports allow the pipe to move freely. Mirror-finished stainless-steel plates on the bottom of the pipe sit atop PTFE-coated supports anchored to the floor. Between the supports over the main fault hazard zone, 1-in.-thick unsupported steel pipe stretches 147 ft.

A cladded-pipe element that connects the north ball joint with the slip joint was procured by Rados. Held in place by steel frames, the 50-ft-long continuous piece of pipe is free to compress into the slip joint with "absolutely no binding," Czarnecki says.

Planning and installing the 46-ton cladded pipe was one of the most difficult tasks on the project, Pelletier says. Fabricator XKT Engineering, Vallejo, Calif., welded a ribbed steel frame on four quadrants of the pipe exterior to provide flat platforms upon which long sheets of highly polished stainless steel could be attached using flush bolts. The apparatus allows the pipe to slide freely through the frame. On the 12 ft of cladded pipe closest to the ball joint (where the largest loads will occur), the stainless-steel sheets are replaced by Hastelloy, an expensive alloy of nickel, molybdenum and chromium.

"One of the problems we faced was that nobody could make a prediction of what the coefficient of friction for stainless steel or Hastelloy is going to be 50 years from now, when it needs to work," Czarnecki says. Since stainless steel can corrode under certain conditions, the team added the sections of Hastelloy, "which is about the best corrosion-proof material you can get," he adds.

Shape-Shifting Vault

Without something to isolate the pipe from the movement of the fault and the surrounding ground, the joints and other components aren't enough to ensure that the pipeline will survive an earthquake. Therefore, the entire 305-ft stretch of new pipe is housed inside a 20-ft-wide by 18-ft-tall concrete "vault" box. But this is no ordinary culvert: Nine articulated segments make up the bulk of the vault's length. Each 20-ft-wide segment is independent from the others, separated by 6-in. gaps that run diagonally—in the same direction as the fault—at a 45° slant. The gaps "will allow each segment to move and rotate independently without coming in contact with or damaging the segment adjacent to it," Dessaure says.

According to Czarnecki, this vault concept has never been attempted before in this application. So, URS journeyed to Cornell University's Bovay Laboratory Complex in New York to perform a 1:10- scale model test. Data from the test was incorporated into a soil structure model using FLAC, or Fast Lagrangian Analysis of Continua, software for further analysis.

The vault segments sit atop a "pool- table-level" concrete base slab that allows the articulated segments to slide easily. Rados was required to cast the 305-ft-long, 26-ft-wide base slab to within a tolerance of no more than 1/8 in. variation over 10 ft in any direction. "We probably didn't realize when we bid, as much as we realize now, that the tolerances that we are building far exceed anything done normally in the [heavy construction] industry," Pelletier says.