Building San Francisco's BART tunnels

Historical Essay

by Michael C. Healy

Text originally published in BART: The Dramatic History of the Bay Area Rapid Transit System Heyday Books: Berkeley CA, 2016

May 1971 View west from Market to southwest corner wnp25.2499.jpg

May 1971 view west on Market at Van Ness to southwest corner, boards covering trenches where BART/MUNI tunnel is under construction.

Photo: wnp25.2499


BART trench and tunnel construction on Market Street at 4th, June 1970. While Market Street was being torn up for cut-and-cover construction, one of the biggest challenges was relocation of the utilities, which one foreman referred to as an underworld of spaghetti.

Photo: Shaping San Francisco

4th and Market 1969 AAB-6647.jpg

Another view of Market at 4th, 1969.

Photo: San Francisco History Center, SF Public Library

On the bright, sunny morning of Monday, July 24, 1967, 125 or more children, out of school for the summer, lined up along San Francisco’s Market Street near Powell, poised with shovels in hand waiting for the signal. The children chatted among themselves with laughter and a general feeling of gaiety. They had a sense that something very important was about to happen, that history was about to be made in this “Baghdad by the Bay,” as iconic columnist Herb Caen referred to San Francisco. Along the curb of the street a path of hard surface composed of concrete and asphalt had been removed to expose the dirt underneath. Press photographers and television cameras were at the ready. On cue, the children began to dig, pushing their shovels into the soft earth for the official ground-breaking of BART’s Market Street subway. Following the ceremonial ground-breaking, the children were bused out to Contra Costa County and taken for a ride in a lab car on the Diablo Test Track.

This was another example of the public relations genius BARTD had come to be known for. The children symbolized the future, just as the transit project itself was striving to personify space-age ground transportation for current and future generations. BARTD referred to the event as a “dig-in,” presumably a takeoff on the jargon of the time. Market Street would be one of the most important segments of the 8.3 miles of the San Francisco BART portion of the 75-mile project. “Unquestionably, BART will become the central force in the wave of beautification, reconstruction, and new construction, some of which is already under way,” said Jack Barron, who managed the Transit Task Force created to coordinate the system construction with all city departments. Every aspect had to be negotiated.

Of course, not everyone was happy that morning. Many of the shopkeepers who owned businesses along that corridor balked, and understandably so. Some of the small businesses had been located there for more than fifty years. The owners knew that the coming of the BART trains was going to mean big short- and long-term changes for Market Street in terms of its new look and its economics. Some referred to the change as the “Manhattanization” of downtown San Francisco, a term meant as a criticism. There were always those who did not want to see change of any kind.

In the short term, the chaos of construction was going to be extremely disruptive, impeding general traffic flow in the vicinity of the Market Street corridor, and many business owners feared that customers would not want to fight the chaos and big machines that would come with the work. The street would more than likely be torn up for years, and they feared their shops would not be easily accessible. They were right. Many of the small businesses did close up shop, never to return. On the bright side, some of the major buildings, such as the Wells Fargo building and the Emporium-Capwell Department Store, planned to construct special entrances to the BART stations. At the Powell Street Station, a pedestrian tunnel was supposedly hollowed out that went all the way down to Mission Street. According to lore, this tunnel was intended to provide an underground walkway to the future Yerba Buena Gardens, but it was sealed up and never used.

The city saw the end of some of the anachronistic small shops as no great loss. Over the years a sleazy, honky-tonk element had mushroomed along Market Street, particularly at the lower end near the Embarcadero. Otherwise, the once-bustling east end of Market had been considered all but dead in recent years. Like the 1906 earthquake and fire that led to the demise of the old Barbary Coast, the coming of BART was considered a new turning point in San Francisco’s evolution. The city was anxious to revitalize Market Street, and, in fact, construction had already begun on high-rise developments along or close to the new BART line.

An addition to the Golden Gateway Redevelopment Project was the ambitious four-block Embarcadero Center. Beginning a half block from BART and running west at a slight angle parallel to Market Street, the Embarcadero Center was to include four high-rise office buildings comprising about 3 million square feet of office space, and three levels for restaurants and shops. Part of the development would be the new Alesa Building at One Maritime Plaza and the 840-room Hyatt Regency hotel, located a few steps from the entrance of the eventual Embarcadero Station. The city, which agreed on the need for the added station, determined that the tax increments from anticipated higher property values would help fund Phase One of the unplanned Embarcadero Station through the sale of bonds. City supervisors allocated $15 million to pay for the basic station box.

The emergence of the Market Street Development Association, an organization devoted to bringing about a renaissance to the street that was once the spirit and spine of San Francisco, was also a strong signal that change was on the doorstep. One of the early primary goals of the association was to continue to raise money to build the Embarcadero Station at Davis Street. At the association’s behest, several private developers kicked in $500,000 for initial design work a few months after the ground-breaking.

Problems between BART and the city over the aesthetics of the Market Street stations popped up when the city demanded that the stations have skylights. This concept created several issues. It would have required the unscrambling and additional relocation of utility conduits, an assortment of pipes and sewer lines crossing Market Street not far below the surface and just above the roofs of the stations, and indeed the structures themselves would need to be redesigned, thus causing delays and adding to unplanned costs. The city’s position on the skylight issue was that they would open up the stations’ concourse levels to natural light. Further, the city would not allow BART to build protective canopies over the entryways, nor to post station-identifying signs, again citing aesthetics, particularly after the Market Street beautification plan was adopted. It involved widening sidewalks, landscaping, and general sprucing up. Interestingly, that plan would eventually influence the city’s quest for the skylights.

Meanwhile, soil studies had begun under Market Street in 1963 with the drilling of 5-inch bores to 100- and 200-foot depths. The engineers weren’t sure exactly what they were going to find, but they had a hunch it was not going to be good. The permeability of the soil, particle size, settlement behavior, and other factors gave clear warning of the complex construction problems ahead for PBTB engineers and project contractors.

Actual work on the Market Street subway did not begin until the day after the “dig-in,” on Tuesday, July 25. With the first dig, contractors encountered a maze of utilities of one kind or another about ten feet below the surface. Over the past hundred years, more than two hundred utility companies had planted lines, many of which had been abandoned long before and did not even show up on plans provided by the city’s public works department. Threading through them, contractors found live high-voltage wires, conduits of all shapes and sizes, high-pressure fire hydrant lines, and steam pipes, all of which presented an ever-present danger. They would have to be moved or suspended in place while the work went on around them. It was critical that the lines be kept intact so that service to surrounding office buildings would not be interrupted.

The work consisted of building twin double-decked tunnels for BART and Muni, which followed Market Street to Van Ness and then split up into single-deck tunnels. The BART tunnels would swing south along Mission Street; the Muni tunnels would continue along Market. The San Francisco line would include twelve completed stations, including three specifically for the San Francisco Muni system. There was no money at that time for Phase Two work—the fully detailed completion of the Embarcadero Station’s interior. The shell was a placeholder until additional funding could be found.


Mission and 23rd Street, cut and cover construction of BART tunnel underway, c. 1970.

Photo: BART

The contractor for that portion of the Mission–Market Street line was a joint venture made up of Morrison Knudsen, Brown & Root in conjunction with the Perini Corporation. Morrison Knudsen also got the contract for excavation of the 700-foot space for the Embarcadero Station.

Cut-and-cover construction along San Francisco’s Market Street was the primary method used to build the mammoth subway stations. Contractors first had to excavate enough to provide a staging area for their work, and then they decked over the hole so that streetcars, buses, and general traffic could continue to move freely. Two Calweld oscillating tunnel-boring machines were used for soft-earth tunneling. Eighteen feet in diameter, they were employed to bore twin mile-long tunnels between the Mission Street lines and the Market Street lines. The late Harre Demoro, an acclaimed transit reporter and writer, once described the shield boring machine as something like a giant cookie cutter.

The tunnel-boring machines were the first of their specific kind ever used in the United States. Each weighing 93 tons, they were lowered by a giant crane into a huge shaft more than 75 feet deep, at 15th and Mission Streets. BART quickly picked up on the nickname of the machine, “the Mole”—which was how these machines were affectionately referred to in tunnel digs around the world—when it commissioned a film about the machine titled We Call It the Mole. Each machine had a tail shield, which was like a monster cylinder extending out behind the Mole. During the construction of the overall project there were thirty-six different tunnel headings, making up one of the greatest concentrations of tunneling work in history.


The shield tunnel-boring machine is 18 feet in diameter and here works almost 80 feet under Mission Street in San Francisco. Working at that depth and under high air pressure to prevent cave- ins meant construction crews were required to go through decompression when they resurfaced after their shifts. Development of this technology began in the mid-1800s with the observance of a naval shipworm.

Photo: BART

As the shield boring machine pushed forward with a system of 115 hydraulic jacks, rotating cutter arms with sharp steel teeth knifed through the soil, moving at an average rate of 4 to 4.5 feet an hour, or 35 feet a day, depending on the soil type. The cost for this form of excavation was $2,142 per foot; today that cost would be more like $50,000 to $60,000 per foot.

The shield behind the Mole provided limited protection from potential cave-ins to the “sandhogs,” tunnel workers who labored in the extremely hazardous conditions. The sandhogs also had to work in air pressure of at least 12 pounds per square inch (psi) along the Mission Street dig. The high air pressure was another safeguard against the soft earth, sand, and mud collapsing during the work. Like the Transbay Tube divers, workers leaving the bottom of the subway construction had to go through a decompression tank before emerging from the depths to prevent getting the bends.

The dirt and mud from the bore was mixed with slurry, put on a conveyor belt, dumped into a mud cart, and removed by a heavy crane from the shaft. Integral to the boring process, large robotic erector arms would then place specially forged steel rings to line and reinforce the tunnel walls and ceiling. Each ring was almost 3 feet wide and weighed 3 tons. It took six rings to line one segment of a tunnel, and a smaller ring to act like a keystone. Workers bolted the rings together, then a form of grout and gravel was forced in between the raw mined walls and the liners to minimize ground-moisture leakage. A total of 27,700 tunnel liners were produced by Kaiser Steel in Napa County specifically for the project’s tunnels at a cost of $27.5 million, or about $975 per ring. Overall they lined approximately 65,000 feet of tunneling.

The Shield's Fascinating History

A shield was employed to tunnel some of the New York City subway system. While it was much cruder than the ones used by BART almost seventy years later, it served the same basic purpose: protecting the sandhogs working behind it from cave-ins as it dug and pushed forward. The tunneling shield was first invented in London by architect Marc Isambard Brunel to excavate a tunnel under the Thames. Charles Dickens wrote about this strange contraption in his newspaper columns around 1843.

Brunel and his partner, Thomas Cochrane, patented the first such machine in January 1818 but did not begin using it until 1825, when they excavated a pedestrian tunnel under the Thames. That tunnel was 1,300 feet long, at a depth of 75 feet. At one point it was flooded, delaying the work, but it was finally completed in 1843. Although the tunnel was originally built for pedestrians, it is now part of the London Underground. Brunel’s inspiration for that early design of the shield came from a seagoing worm called Teredo navalis. The Teredo is a nightmare for owners of wooden ships because of its ability to bore through wood hulls from underwater. As this kind of saltwater termite moves through the timber, it secretes a substance that hardens and forms a shield behind it. Thousands of them working at once could destroy a ship’s hull. While the main concept developed by Brunel has remained the same over nearly two hundred years, the shield has, of course, seen major technical improvements, and it continues to be used today for major tunneling projects in the United States and abroad.

Lower Market Street Work Presents Worst Conditions

A contract for the lower Market Street tunnel segment approach-way was awarded to Perini–Brown & Root for $14.6 million. Because the San Francisco waterfront was mostly fill from Montgomery Street to the Embarcadero, the water table was high, and the deep, soft bay mud made subway excavation under Market Street particularly challenging and dangerous. The shell of the Embarcadero Station was completed in 1969, and years later, during the inauguration of the station’s finished interior, Bill Cummings, one of the architects of the Embarcadero Station, talked about the design issues he and chief architect Tallie Mahl had faced. “Hell, the shell, which was really nothing more than a concrete box, was practically floating, like a submarine,” Cummings said. He also noted they had had to squeeze the design to be slightly narrower than the other downtown stations due to adjacent structures and the high water table.

Excavating the Market Street stations was like digging holes for three-story buildings. These stations had to be constructed with three levels: a bottom twin-track level for BART trains, a middle twin-track level for eventual Muni light rail trains, and a mezzanine or con- course level that would contain entry gates, fare collection/ticketing machines, agent booths, information racks, bathrooms, and commercial vendors’ kiosks. To accommodate ten-car trains, the stations were 700 feet long—longer than the Bank of America Building is high— and 60 feet wide, except for the Embarcadero Station, which was 50 feet wide.

The Greatest Challenge of Market Street

Even more challenging was the tunneling work necessary to connect with the caisson ventilation structure 400 feet offshore. The structure contained double trackways submerged 85 feet under the bay waters, waiting to be tied in. The vent building trackways were sealed by bulk- heads on the west side. Like the Transbay Tube on the east side of the vent structure, these bulkheads would be removed once the connection with the subway approach was made. Excess water that accumulated during the process would then be pumped out.

During the excavation work the contractor encountered a jungle of old, deeply embedded timber piles that had to be removed. Some of the pilings, about nine hundred in all, had supported the original ferry building wharf and slips that had been used back in the early 1900s, when ferry service was the only cross-bay mode of travel. Also discovered were the carcasses of three old sailing ships from the gold rush days, as well the bones of early Indian inhabitants. One rumor had it that the partial remains of a Barbary Coast speakeasy, including a bar, had been found during the dig, but this story has never been verified.

As the early stages of the dig progressed, the contractor, Perini–Brown & Root, encountered bay mud 100 feet below the water table in some places. While this was expected based on the deep soil samples taken at the foot of Market Street in 1963, such excavation was considered unprecedented. Another major challenge during the digging of the lower Market Street subway was the discovery of yet another network of utilities and sewer lines below the surface, which in some places looked like a pile of spaghetti that had to be untangled. The relocation work was costly and time consuming but ultimately successful. That effort alone was considered an amazing feat.


Civic Center BART station under construction, c. 1970.

Photo: BART


Civic Center BART station on opening in 1971.

Photo: BART

Sexist Superstition

At one point the subway construction was made accessible for special tours and visitors, both domestic and foreign, who had heard about this futuristic new system being built. Descending into the catacombs of the deep subway construction held a certain fascination that made it a popular venture. Kay Springer, an early employee of BARTD who held a number of jobs during her long career at the transit district, arrived in 1966, when the staff was still relatively small and employ- ees were often called upon to perform a number of tasks outside of their official job descriptions, such as giving slide show presentations to community organizations (mostly in the evenings, on their own time) or taking visiting public officials to outlying construction sites. On one particular day, Springer was assigned to give a tour for a group of male visitors interested in the subway construction techniques being employed by the BARTD project. When she reached the access entrance, the foreman on the job told her that she herself could not go down into the construction area. The sandhogs were superstitious about women being in the tunneling area during construction.

“Why’s that?” Springer asked.

The foreman shook his head. “Because it’s considered bad luck,” he said. “There’s no way you’re going down there with your group.”

Springer asked her group to wait while she went to the nearest pay phone and called headquarters to report the problem. The issue reached general manager Stokes’s desk and, through PBTB, he ordered that Springer and her group be allowed access, thus breaking a long-standing taboo.

Special Precautions Taken

Because of the tremendous hydrostatic pressures from the watery bottom and deep mud of the subway excavation, extraordinary measures had to be taken between Montgomery Street and the vent building. The contractor was concerned that the excavations in the alluvial soils along the lower end of Market Street could endanger some of the long-standing buildings. Slurry walls had to be built, and heavy- duty soldier pilings were sunk down more than 100 feet on each side of the open cuts to shore up structures along both sides of the street.

Here again, the contractor employed hydraulic-driven shields to bore through to the caisson at a very slow rate of about 2 to 3 feet per hour. The sandhogs had to work in air pressure of 36 psi, which slowed the operation considerably. For every hour of actual work, the sandhogs had to spend three hours in a compression chamber and three more hours in a decompression chamber to avoid getting the bends. In addition, 7-foot-thick reinforced concrete bedding for the trackways 80 feet down had to be poured to accommodate the weight of the trains. This bedding was almost three times the thickness of the typical track beds being constructed elsewhere on the system. All of the work along Market Street had to be conducted while streetcars, buses, and auto- mobiles continued to roll along overhead, often making officials nervous that some disaster might befall the traffic flow.

A Theater Lawsuit with Far-Reaching Implications

During the digging and construction of the Civic Center Station, as with the other stations, temporary pilings were driven to shore up adjacent buildings. One of those buildings was the Orpheum Theatre. Its owners later sued BART for damages in the amount of $900,000, a key part of the lawsuit claiming that the proximity of the BART station had impaired pedestrian access to the theater. After six weeks of trial in San Francisco Superior Court, a jury determined that not only were the theater owners not damaged by BART’s construction, but the value of their property had actually increased from between $5 and $10 per square foot, based on appraisals that had been testified to by real estate experts. The jury’s decision carried significance beyond this particular lawsuit; it established a precedent that proximity to a rapid transit station has a beneficial rather than negative impact on real estate values, a contention that the District had always promoted.

Shifting Gears

The basic Phase One work was completed along Market Street without a major disaster, and one can imagine a great sigh of relief coming from all concerned. This might have been particularly the case for the officials of San Francisco, who were vulnerable to the least perturbation that might have taken place. Their signatures were on the many agreements that had to be worked out between BARTD and the city for the project to move forward.

Following closely on the heels of Phase One—the shell construction of the San Francisco BART line—was the enormous amount of detail work to be completed. Numerous contracts were let for such things as station finish (using marble from North and South Ameri- can quarries), electrification, plumbing, track laying, fare equipment installation, and, finally, the installation of the automatic train control equipment.

Two separate systems were set up for electrification. One system was dedicated solely to the 160 miles of the third rail throughout the system, including storage yards. That system used strategically located substations to energize the third rail with 1,000 volts of direct current to power the trains. The early testing program had shown that using 1,000 volts instead of the traditional 650 volts would be essential both for the high speeds called for and for negotiating inclines of up to 4 percent (that is, a rise of 4 feet for every 100 feet traveled). The second electrical system provided general power to support everything else, from station lighting and equipment to three maintenance shops and storage yards (the Daly City shop and yard came much later), the control center, and the newly constructed headquarters building in Oakland, over the Lake Merritt Station.


The 24th Street BART station open house prior to its opening, 1971.

Photo: BART

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Originally published in BART: The Dramatic History of the Bay Area Rapid Transit System Heyday Books: Berkeley CA, 2016