Grinding two metro-sized EPBMs through urban tunnels in dense silty sand just 9.7m apart is dif_ cult enough. Add into the mix a pair of 137m radius horizontal curves, a 13mm settlement tolerance, sensitive building structures, and live rail tracks just 3.4m overhead, and you have a challenge. San Francisco’s Central Subway rail tunnels, snaking through downtown areas, were required to be driven below the existing Bay Area Rapid Transit (BART) line. "It was a major technical undertaking, and many were skeptical that it could be done. The BART System couldn’t afford to be taken offline," said Matthew Fowler, project manager for the Parsons Brinckerhoff/Telamon JV that is responsible for the utility relocation and tunnel designs.

Building connections
The Central Subway is part of the San Francisco Municipal Transportation Agency’s (SFMTA) Third Street Light Rail Transit Project. Phase 1 of the 10.9km, two-phase project began revenue service along the Third Street corridor in April 2007.

When Phase 2 is complete, the 2.7km Central Subway line, excavated at depths ranging from 15m to 45m below the city, will extend light rail service through the South of Market and Union Square Districts to Chinatown, as well as provide direct connections to BART and Caltrain, two of the Bay Area’s largest regional commuter rail services. The Central Subway is scheduled to open to the public in 2018.

The project has been more than 10 years in the making: "The earliest incarnations involved excavation under different streets and the use of cut and cover — it was driven by the BART tunnels under Market Street acting as a barrier to passage of the Central Subway tunnels. Eventually, we looked for ways to use mechanised tunnelling with EPBMs as an alternative. We moved the alignment from a less direct route along Third Street to Fourth Street, which went under BART," says Fowler, who has been involved in the Central Subway since 2003 when he served as the tunnel and station design lead during the preliminary design phase.

At other locations, including the area near the Chinatown station, the profile was lowered to 23m in order to take better advantage of the bedrock at the location. An SEM-excavated platform cavern and cross-over cavern are planned at this station site.

Fairly tight curves down to a 137m radius were always part of the alignment, as the result of right-of-way considerations and the desire to keep the Union Square/ Market Street station platform as close to the Powell Street Station as possible to shorten transfer times to BART and the Muni Metro lines.

Construction & testing
Contracts were awarded for tunneling in 2011 to the Barnard/Impregilo/Healy JV (BIH), while station contracts were awarded in 2013 to Tutor Perini. The plan is to build the three underground stations after tunnelling is completed.

Geological testing revealed layers of mixed ground. Approximately 4.6m of loose to medium dense sand fill overlies 7.6m of medium dense sand with clay and clayey sand. Underlying the sand and clayey sand units is the 14m thick Colma Formation, a dense to very dense sand with silt to silty sand and beds of clay. The Colma sand is underlain by very stiff clay and dense to very dense silty sand or clayey sand referred to as Undifferentiated Old Bay deposits. Groundwater levels were measured at 9.8m below ground surface.

Customised machines
Robbins provided two 6.3m diameter EPBMs for the city’s newest rail route. The machines, for the Barnard/Impregilo/Healy JV, were nicknamed "Mom Chung" and "Big Alma", after local historical figures.

"The unique challenges of this project include the dense urban environment and curved tunnel alignment. Use of TBMs minimises impact to the community, and the design of the TBMs allows us to negotiate curves using active articulation," said Alessandro Tricamo, chief engineer for BIH. Both machines were designed to enable smooth excavation around tight turns down to 140m in radius.

The TBMs were designed with a number of special features to efficiently manage the varied geology, navigate the steep and turning alignment, and bore in what has been rated as "Potentially Gassy with Special Conditions" by Cal/OSHA.

A mixed face cutterhead was selected and designed to excavate the anticipated wide variety of ground ranging from soft soils to thinly bedded siltstone, shale and sandstone bedrock, as well as concrete diaphragm walls. The wear surfaces of the cutterhead are clad in a combination of chromium carbide plating, hard facing, and tungsten carbide bits to ensure the life of the head in the abrasive environment.

The TBMs can be equipped with either a full dress of soft ground tools (picks, rippers, scrapers, etc.) or a mixed dress that incorporates 17-inch, pressure compensated disc cutters when encountering rock or concrete. Forty specially designed housings have the ability to mount either disc cutter or soft ground tools. The ample opening ratio is 31 per cent to allow efficient and controlled muck flow through the head. Grizzly bars are also incorporated to prevent boulders that are too large to pass through the screw conveyor from entering the mixing chamber. The cutterhead features five foam and two water injection ports for soil conditioning as well as a programmable copy cutter to create additional overcut in order to negotiate tight turns. To alert the operator of the possible need for an intervention, there are also three sets of wear detection bits to automatically detect when the wear of the cutters gets down to unacceptable levels.

The cutterhead is driven by five, 210kW VFD-controlled electric motors and during excavation the machines remove muck using a single-stage shaft-type screw conveyor. Due to the abrasive quality of the muck, the owner has specified replaceable wear protection on both the flights of the screw as well as the screw casing. To accommodate this, the shell of the screw conveyor is built of multiple replaceable sections.

Steering the TBMs accurately through tight curves is one of the key challenges of the project. An active articulation system was integrated between the TBM shields as it allows the thrust cylinders to remain parallel to the tail skin and react evenly with the segments. This feature mitigates the risk of segment damage, ring deformation, or settlement during boring.

Like all highly urban tunnel projects, another key challenge is ground loss or settlement, especially where the alignment crosses under live metro rail tunnels. As noted above the owner and contractor have highly instrumented, key areas where settlement could pose a serious risk to existing property or infrastructure. To prevent ground loss a precise system of control and measurement of the excavated material must also be implemented to eliminate over-excavation. The machine is fitted with two electronic scales and a radar system that constantly monitors the weight and volume of the tunnel muck travelling down the backup conveyor. These measurements are then compared to theoretical values to determine if over-excavation is occurring. The machines also have an active face support system that can detect if rapid pressure loss is taking place at the excavation face. The system will automatically inject pressurized bentonite slurry into the mixing chamber to restore the lost pressure. The operator will then close the guillotine gates on the screw until face pressure is restored and it is safe to resume operation.

Occasional instances of methane were detected during the geologic investigations, resulting in Cal/OSHA classifying the project as potentially gassy. For this reason, a requirement of the electrical design for both machines is to be ANSI/ NFPA Class 1 Division 2 compliant. This was achieved using a combination of both intrinsically safe electrical designs and sealed or purged cabinets in all areas of the machines. The machines are also equipped with a gas detection system in order to identify the presence of multiple gasses in the tunnel.

Specialised supporting equipment
To get the muck out of the tunnels the BIH JV sourced Robbins continuous conveyors designed and manufactured per the specific requirements of the site. Both of the extensible conveyor systems are equipped with a 500m capacity belt storage cassette and splicing stand to allow the TBMs to bore approximately 250m before more of the fabric-reinforced belt needs to be added. The continuous tunnel conveyors deposit onto a single stationary overland conveyor located in the aft end of the launch box that feeds a nearby radial stacker conveyor.

Supply of tunnel lining segments, utility extension supplies, other materials and personnel is handled using specially designed rubber tired vehicles (RTVs).

The RTVs are engineered to drive on the curved tunnel invert with automated self-centering, as well as flat ground with seamless transition from one to the other without operator input. To negotiate the tight curves and fit through the trailing gear of the TBM, the RTVs are articulated. The unique RTVs have operator’s cabins on each end, allowing them to be driven in and out of the tunnel without being turned around or needing to be backed out.

Limited space jobsites
The launching and service portal presented a unique challenge to the project. Most of the construction yard is located within the busy on-ramp/ off-ramp interchange of Interstate 80 and Fourth Street. The launch box (137m by 11m) for both machines was excavated directly below Fourth street with a small access window (11mby 11m) and ramp to the box at the south end. All the tunnel services and operations are squeezed into the available space between and below the highway and ramps. When both machines were launched and boring became fully operational in 2013, the tunnel service vehicles had to navigate a tight course through the jobsite and down to the tunnel portal. This included a sharp 90 degree turn down the ramp to the launching box.

Live bart tracks
Once the TBMs were launched, preparations began for the complex crossing below live BART tracks. The lowest elevation of the Central Subway profile is controlled by the assumed vertical clearance to the 19.8m deep invert of the lower pair of the Market Street tunnels. Constructed between 1967 and 1970 as part of the Bay Area Rapid Transit project, the four 5.3m diameter steel-plate lined tunnels are used by BART and SFMTA’s Municipal Metro subway systems and together serve as the main underground transit backbone for San Francisco.

The Central Subway tunnels are being constructed perpendicular beneath the existing BART tunnels. Permanent support of the 5.44m inside diameter Central Subway tunnels is being provided by a single-pass, 280mm thick precast concrete segmental lining system composed of six tapered, 1.5m nominal width segments. Narrower 1.22m segments were cast for the curve sections. The circumferential joints are joined with 16 dowels per ring and the radial joints with two bolts per joint. Embedded-type gaskets supplied by Datwyler-Phoenix and designed for 6 bars of pressure seal each of the segments. The PB/Telamon JV performed extensive numerical and 3D modelling to estimate the ground and structural behavior of the BART crossing. Results indicated that properly operated EPBMs could effectively excavate the crossing without disturbing the rail lines. As an added measure of protection against settlements the design included a radial array of compensation grout pipes placed between the Central Subway and BART tunnels from a shaft adjacent to Market Street. Compensation grouting pressure and volume criteria, as well as pipe layout details, were designed by Arup for BIH JV, and then installed and preconditioned by subcontractor Condon Johnson-Nicholson Construction JV.

"Our hope was to cross under the BART tunnels at night during a low usage time, preferably on a weekend or after 9pm on a Friday. But, as with so many things in construction, the first TBM driving the southbound tunnel arrived at a different time than planned- -the day before Thanksgiving. This meant we had to cross under BART on Black Friday, one of the busiest shopping days of the year," said Fowler. With careful excavation, minimal settlements were measured, and no compensation grouting was needed. The second machine excavating the northbound tunnel completed its crossing in February, with equally successful results. The contractor kept a close watch on face pressure, using a mix of foam for conditioning with some bentonite. Backfill and muck volume were closely monitored in a number of ways. Belt scales measured muck weight, which was compared with radar readings of the volume, face pressure and annular grouting volumes. "With radar plus belt scales, we can eliminate variation. For the most part the two methods closely tracked one another. The proof is in the pudding — we’ve had minimal ground movements so far during tunneling," said Fowler. Compensation grout injection was again not needed for the second and final BART crossing.

Efficient excavation
Since their launch, the EPBMs at the Central Subway have excavated as much as 27m in one 12-hour shift. The TBM "Mom Chung" has excavated up to 378m in one month at an average of 18m per day. The machine is on track to hole through at the Retrieval Shaft in North Beach in May.

TBM "Big Alma" has fared even better, turning in a best month of 513m, with a 20.5m daily average. The second machine is expected to break through in late June or early July. "One of our biggest challenges was when the screw jammed in the Franciscan Deposit due to sharp shards of dense rock that packed the screw," explained Leonard Markley, Robbins field service manager. "The operator could not get the screw to budge, and we identified the problem by removing the inspection covers, pumping Bentonite into the screw to get lubrication, and finally retracting the screw while turning it, inch by inch until it broke free. This took about 12 hours to get back into production and was very successful."

Work to be done
With tunnelling nearly completed and the Retrieval Shaft ready to receive the machines, work on the station sites designed by the Central Subway Design Group JV is commencing. Noise mitigation at the station sites and retrieval shaft will be quite different than that required for tunneling: "The TBM has been allowed to go 24 hours a day. Our launch box site was very noise tolerant, as it was underneath the Interstate 80 viaduct that leads to the San Francisco Bay Bridge. At the station sites, there will be noise mitigation structures and limited work hours. At the retrieval shaft, there will also be noise restrictions," said Fowler. The TBMs will break through one by one into the 18m deep shaft, and will be disassembled in the shaft bottom for removal.

The three station sites, under construction by Tutor Perini, have their own challenges. "What immediately lies ahead is construction of diaphragm walls that parallel the tunnels on each side. There is little clearance, so the operator must be very accurate in placement of the secant pile walls and slurry walls — there is only about a foot of clearance."

But for Fowler, who has been involved in the project for over a decade, the hardest part is over: "My biggest challenge was the BART crossing, and making sure everything was in order for that to happen. But, now I can look back on it and we took all the right precautions; all the tunneling went really well. The Barnard/Impregilo/Healy JV gets a lot of the credit in making this work, and the machines do too"