Those tight tolerances caused headaches when it came time to cast rows of concrete secant piles along both sides of the vault box. Half the 3-ft-dia piles, which protect the vault from the surrounding soil, are 65 ft deep and cast from reinforced high-strength concrete. Alternating between each of these sturdy piles are 35-ft-deep, unreinforced low-strength concrete piles, which act like seismic fuses. During a seismic event, the low-strength piles are intentionally designed to break. It's almost like a zipper coming apart to help distribute the load over the length of the articulated vault, says Todd Cockburn, principal with construction management firm EPC, San Francisco.
Developing the right batch mix for the fuse piles "caused everybody a lot of grief," Cockburn says, because the mix was designed to be no weaker than 1,000 psi, or else the piles would be too fragile, and no stronger than 1,500 psi, or else they might not fail during the earthquake.
The batch plant had never developed a mix with such a narrow tolerance range before, and when trial batches were first tested, the samples broke prematurely. "It was difficult because, usually, concrete design is minimum strength," Pelletier says. "The long-term strength creep that occurs in concrete is something that is not a predictable number because no one has ever focused on holding the top end down, which [URS] had us do here." Eventually, it was determined that the samples, which are normally handled after just two days of curing, had to remain undisturbed in a controlled environment for seven days in order to reach the proper strength threshold.
The existing section of pipeline No. 3 that is being replaced will be abandoned in place. Traditional methods to deal with fault-line crossings—such as strengthening and sliplining the pipe—are being used to upgrade pipeline No. 4. These upgrades will help the line withstand the 6-in. to 12-in. movements expected from Hayward's subfaults but not the larger movements of the main fault line.
In the early stages of the project, work was delayed by several months after excavation for the vault trench revealed geologic features that seismologists studied to better pinpoint the location of the fault zone. But Pelletier says the team was able to schedule around it, and the project is currently on schedule and budget.
While no one can say with 100% certainty that such a novel system will perform exactly as expected during an impending natural disaster, Wade says SFPUC has gone to great lengths to reduce risk by obtaining proper warranties to insure against latent construction defects while performing rigorous model and full-scale tests, extensive quality control and independent third-party reviews. "The real insurance policy is the diligent planning, design and construction processes that are in place to build the WSIP projects in such a manner to reduce the system's overall risk," he says.
