Huge tunnel to be built under San Francisco Bay

Hoping to protect one of the Bay Area's main water supplies after the next major earthquake, construction crews will soon embark on a job that sounds like something out of a Jules Verne novel: building a massive, 5-mile-long tunnel underneath San Francisco Bay.

Using a giant boring machine, workers will carve a 14-foot high corridor through clay, sand and bedrock from Menlo Park to Newark as deep as 103 feet below the bay floor. They'll then run a 9-foot-high steel water pipe through the middle.

"All the experts tell us that within the next 30 years, there is a 63 percent chance of having a major earthquake in the Bay Area," said Ed Harrington, general manager of the San Francisco Public Utilities Commission, which is in charge of the project.

"By building extra tunnels and strengthening our pipelines, it means we have much greater assurance that we'll have water after the next earthquake."

Only 12 companies in the world are certified to perform the job, which is estimated to cost $347 million. Digging will start next spring on the Menlo Park shoreline just south of the Dumbarton Bridge, and head eastward, with work scheduled to be completed in 2015. An additional 16 miles of pipe connecting to the tunnel on either side of the bay also will be replaced.

The job is part of a $4.5 billion renovation by the San Francisco PUC to upgrade its water system. Commonly known as the Hetch Hetchy System, the network of tunnels, pipes and reservoirs delivers water 167 miles through gravity-fed pipes from Hetch Hetchy Reservoir in Yosemite National Park to Crystal Springs Reservoir along I-280 in San Mateo County.

The largest water system in the Bay Area, it provides some or all of the drinking water to 2.5 million people from North San Jose through the Peninsula to San Francisco, along with Fremont, Hayward and other parts of the East Bay.

Another agency, the Santa Clara Valley Water District, provides water to 1.8 million people in Santa Clara County from groundwater and the delta.

An engineering marvel, the Hetch Hetchy system was built following the 1906 earthquake, when San Francisco burned after its water system failed. Today, much of its equipment is antiquated and at risk of collapse in the next major quake.

The tunnel, for example, will replace two large steel pipes built in 1925 and 1936 that sit on the floor of the bay, and could easily break in a major quake, cutting off water for weeks.

"Being buried deep in stronger, tighter materials, there is much smaller vulnerability to being pulled apart from shaking and liquefaction," David Schwartz, a geologist with the U.S. Geological Survey in Menlo Park, said of the proposed tunnel. "From an engineering point of view, it's much stronger."

Schwartz noted that since the 1989 Loma Prieta Earthquake, a 6.9 magnitude event that killed 63 people and did $6 billion in damage, other Bay Area agencies have been hard at work.

Racing to beat the next earthquake, Caltrans has retrofitted dozens of freeway overpasses and is rebuilding the Bay Bridge. Pacific Gas & Electric has upgraded gas lines and substations. BART is retrofitting the Transbay Tube, a 3.6 mile-long cylinder that sits on the floor of the bay, connecting Oakland and San Francisco.

Many of the region's hospitals have not been retrofitted. And thousands of old buildings, including homes and unreinforced masonry buildings, remain at risk.

USGS scientists say there is a 63 percent chance of a quake of 6.7 magnitude or larger hitting the Bay Area by 2036. Geologists are most concerned about the Hayward fault, which runs from San Jose to Richmond.

With that backdrop, the San Francisco PUC won approval from San Francisco voters in 2002 to upgrade its water system. Funding is coming from revenue bonds, financed by a near-doubling of residential water rates in San Francisco from $23 a month now to $40 in 2015, with similar hikes expected in other communities that receive Hetch Hetchy water.

The project also will rebuild pipelines, water treatment plants and Calaveras Dam, north of San Jose, over the next five years so that they can withstand a quake of up to magnitude 7.9 on the San Andreas fault and 6.9 on the Hayward fault.

Despite the sensitive politics of anything involving the bay, environmentalists did not oppose the new tunnel.

"The environmental effects of a tunnel would be less than if they built new pipelines across the marshes," said Florence LaRiviere, co-founder of the Citizens Committee to Complete the Refuge, in Palo Alto.

Still, LaRiviere said she wants the PUC to remove the old pipes when the tunnel is done to restore the shoreline to its natural state. For now, the agency plans to leave them in place as a backup.

Exploration for the Trans Bay Tubes

The most expensive portion of the original BART system were the 3.6-mile long twin transbay tubes, originally estimated to cost $133 million. The actual $180 million price tag was paid by motor vehicle tolls on the S.F.-Oakland Bay Bridge (Godfrey, 1966). In mid-1960 Parsons, Brinkerhoff, Quade & Douglas (PBQD) drilled five exploratory borings in the Bay as part of feasibility studies (Swain, 1960) to confirm the general soil profile where it diverged from the S.F. Bay Bridge, where the underlying geology was already well documented (Trask and Rolston, 1951).

During 1964-65 PBQ&D (1965) drilled another 25 borings in the bay, more or less following the track of water-borne ferries that had plied the Bay prior to completion of the S.F. Bay Bridge in November 1936 (Fig. 3 upper). Of foremost interest to the design team were potential settlement problems, stability of the temporary excavations to be made in the bottom of the bay, and the likely presence of rock along the proposed alignment, shown in Fig. 4.

Settlement of the tubes was to be handled by employing compensated excavations, which would seek to remove more soil load overlying compressible strata than originally existed before placement of the tubes. The design team was most concerned about future settlement in vicinity of the east portal, where the Port of Oakland planned on extending their existing mole, across the tube’s proposed alignment. The construction of this mole would increase the effective pressure bearing on compressible strata beneath the tube. This was handled by ....

The alignment was unique in that engineers sought to avoid good foundation materials, opting to keep the tube on as loose material as possible, so that its flexibility could distribute relatively large oscillations over its long length, and thereby avoid zones where bending stresses might be concentrated. Seeking to avoid the Yerba Buena rise in the Franciscan basement underlying central S.F. Bay, the design profile changed alignment both horizontally and vertically, dipping as much as 135 feet beneath the Bay, as shown in Fig. 3 lower. Grades were limited to a maximum of 3%, and a minimum of 0.3%, to facilitate drainage.
Design of the Trans Bay Tubes

When the first phase of the Chicago Subway was being built in between 1939-42, soil loads were measured on test sections of two types of tunnels. One simulated a flexible circular lining, and the soil pressure on this structure was estimated by measuring interior deflections of the steel lining. Soil mechanics pioneer Professor Karl Terzaghi reasoned that soft clay needed to deflect modestly in order to develop fairly uniform stresses around the tunnel, after which it shouldn’t deflect any more. Test results from the flexible Chicago test lining made by Ralph Peck suggested that, even with consolidation, the ultimate deflection never amounted to very much in soft clays.

But, the Second World War interrupted progress on the Chicago Subway before they had a chance to build tunnels with thinner sections. In the intervening 23 years between Chicago and BART, this information lay untapped, and subway designers continued to specify heavily reinforced tunnels. Believing that just enough steel was needed to handle the ring stresses because the soil loads arch around the flexible lining, University of Illinois Professor Ralph Peck found himself in the influential position of being one of the external consultants hired by Parsons-Brinkerhoff to review the overall plans and provide advice to the design team, between 1964-73. Peck convinced PBQD engineer Tom Kuesel that a great deal of money might be saved on BART by using flexible circular linings designed only for ring stresses only, ignoring bending. Kuesel (1968) succeeded in convincing others on the design team thin wall circular steel linings should work well in the soft clay soils that dominated the transbay tube alignment, due to its flexibility (to withstand differential settlement) and high tensile strength (200 psi) at the segment connections, which would offer greater performance redundancy in event of an earthquake.

The design team ended up specifying three different thickness for their circular steel lining, for differing overburden conditions (3/8 inch, ½ inch and 5/8 inch). All were designed as flexible linings, with only enough strength to hold themselves up, with acceptable deflections of 3 or 4 inches under their own weight. BART contracted with Kaiser Steel in Napa for the all their steel tunnel lining and the transbay tube shells.
Placement of the Transbay Tubes

In April 1966 BART awarded the initial $90 million construction contract for the transbay tubes to Trans-bay Constructors, a joint venture of Kiewitt, Raymond, Tidewater, and Healy-Tibbets. 19,113 feet of twin tubes in 57 sections were placed between the Oakland and San Francisco ventilation structures. Unique up until that time for a trench-type tunnel, were the horizontal and vertical curves built into the alignment, which required 15 horizontally curved segments, 4 with vertical curves and 2 with both.

The contractor excavated nearly 6 million cubic yards of Young Bay Mud, Merritt sand and undifferentiated organic silt encountered on the floor of the Bay along the proposed alignment, using a 13 cubic yard clamshell. This trench was between 33 and 133 feet deep, 60 feet wide at the base, with side slopes between 1.5:1 and 3:1 (horizontal to vertical), as depicted in Fig. 5. Excavated material was dumped into 2,000 cubic yard bottom dump barges, and disposed of in the tidal draw channel west of Alcatraz Island (no longer allowed).

The twin tube sections were between 273 and 366 feet long and averaged 11,000 tons apiece. The 3/8-inch thick raw steel pipes supplied by Kaiser Steel were fabricated into 57 binocular shape twin tubes 48 feet wide by 24 feet high at the Bethlehem Shipyard in South San Francisco. After launching into the bay, they were outfitted with 24-inch thick interior concrete linings, designed to combat buoyancy, as well as concrete floors, walls and walkways. An emergency accessway and exhaust air ducts were constructed between the 17-feet diameter tubes, which were spaced 8 feet apart. The tubes were sealed at either end and sunk

After fitting out, the tube sections were sealed at either end and floated to a catamaran placing barge, from which they were suspended (Fig. 6) for lowering. Contrary to popular belief, the tubes were never filled with water, they were sunk by dumping 500 tons of gravel onto ballast pockets built atop the tubes, gradually lower them into position (Murphy and Tanner,1966, Engineering News Record, Nov 16, 1967). The tubes were set on 2 feet thick gravel blankets placed within 0.15 feet of design grade, across the floor of the dredged channel (Fig. 5). The goal was to place each tube about 2 feet from the existing line of tubes, then bring them within 1 inch of design alignment using 50-ton hydraulic jacks with 39-inch strokes, connected to four railroad car couplers installed at each end.

Once positioned, water was trapped between the two end bulkheads, between neoprene gaskets. This trapped water was bled from the joint and pumped out, creating some buoyancy, but the pumping also brought the sections closer together, under vacuum. The temporary bulkheads on both sides of the new joint were then removed and reused on other sections. Liner plates were welded across the new joint from the inside to make a permanent connection capable of transmitting 200 psi in tension. A 2 feet thick concrete lining was then placed within the transition (the remaining lining having already been placed at Bethlehem’s dock.

Once attached, a specially configured screed barge was placed over the newly placed tube and fixed into place by means of four 140-ton anchors attached to positioning winches. Two 17 feet diameter floatation tanks could then be flooded to submerge the 16 feet deep barge to only 4 feet of freeboard, steadying the barge. The screed barge used 3 inch diameter pipes to place sand and gravel backfill with relative position around the sunken tubes (Fig. 5). The tunnels were also provided with cathodic corrosion protection.

Beginning in September 1966 the first tube was placed against the west side of the Oakland Ventilation structure. 26 sections were then placed, stretching 2 miles into the Bay by April 12, 1968. Upon completion of the more extensive San Francisco ventilation structure, work shifted to that side of the Bay, preceding easterly across the Bay. A special earthquake joint was attached to the west end of the westernmost tube, adjoining the ventilation structure. This joint was fabricated and attached to the 324-feet long section at the Kaiser Steel Yard in Napa. About one tube section was placed every two weeks, and placement was completed in early April1969 (ENR, April 10, 1969).

There have been five leaks of the transbay tube during the initial 30 years of operation. On several occasions pin-sized holes have developed in exterior steel lining, which have quickly been detected by observation of the accumulated seepage. Following the October 17, 1989 M 7.0 Loma Prieta Earthquake BART engineers discovered seepage emptying into the tube and the line was shut down for day of inspection. In 1977 a fire in the eastbound tube caused the death of one Oakland fireman because of toxic fumes given off by burning polyurethane seat cushions.

Up until that time the BART sunken tube tunnels were the longest and deepest ever built (Warshaw, 1968). The success of the BART tubes led Parsons-Brinkerhoff to design similar structures for the Hong Kong Cross Harbor Tunnel (completed in 1972), the East River 63rd Street Tunnel in New York City (completed in 1974) and the Second Hampton Roads Bridge-Tunnel (completed in 1976). These projects were described in Kuesel (1974).

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