\ Twin EPBs and Compatible Conveyors Move Muck Below San Francisco

Twin EPBs and Compatible Conveyors Move Muck Below San Francisco

Twin EPBs and Compatible Conveyors Move Muck Below San Francisco

The TBMs were designed with a number of special features to efficiently manage the varied geology, navigate the steep and turning alignment. Photo: Courtesy SFMTA

In the “City by the Bay,” a new subway line will restore light rail service to a high transit ridership area for the first time in 50 years. The Central Subway is part of the San Francisco Municipal Transportation Agency’s (SFMTA) Third Street Light Rail Transit Project. Phase 1 of the 10.9-km (6.8-mile) long, two-phase project began revenue service along the Third Street corridor in April 2007. When Phase 2 is complete, the 2.7-km (1.7-mile) Central Subway line, excavated at depths ranging from 15 to 45 m (50 to 148 ft) 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. Scheduled to open to the public in 2018, the rail route is predicted to serve about 65,000 passengers daily by 2030.

Completing the Central Subway tunnels, however, requires surmounting considerable obstacles. Two Robbins EPBs with integrated continuous conveyor systems are excavating mixed face conditions below sensitive building structures with settlement tolerance as low as 13 mm (0.5 in.). One crossing in particular has put the TBMs to the test: A section just 3.4 m (11 ft) below the active Bay Area Rapid Transit (BART) line, where settlement had to be minimized.

Curveball Logistics
In a city home to the curviest street in the world and a rolling, hilly topography, the alignment of the Central Subway tunnels is anything but straight. Fairly tight curves down to a 137-m (450-ft) radius were designed as part of the alignment, as the result of right-of-way considerations and the desire to keep two station sites as close together as possible. The Union Square/Market Street station platform was placed as close to the Powell Street Station as possible in order to minimize patron transfer times from the Central Subway to the BART and Muni Metro lines.

Limited space was available for the station sites and the launch and service portal. Most of the construction yard is located within the busy on-ramp/off-ramp interchange of Interstate 80 and Fourth Street. The launch box (137 m x 12 m/450 ft x 39 ft) for both machines was excavated directly below Fourth Street with a small access window (11 m x 11 m / 36 ft x 36 ft) 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.

Preparing for Mixed Ground
Geological investigations revealed layers of mixed ground. Along the southern third of the alignment, approximately 4.6 m (15 ft) of loose to medium dense sand fill over¬lies 7.6 m (25 ft) of medium dense sand with clay and clayey sand. Underlying the sand and clayey sand is the Colma Formation, a dense to very dense sand with silt to silty sand and beds of clay. The Colma stratum, approximately 14 m (45ft) thick, is underlain by very stiff clay and dense to very dense silty sand or clayey sand referred to as Undifferentiated Old Bay deposits. Below that is the highly variable Franciscan Formation, a bedrock comprising weathered to fresh beds of fractured sandstones, shale and a mélange unit made up of the same rock types in a weak, crushed matrix. The middle third of the alignment of each tunnel through Nob Hill traverses the Franciscan Formation. Groundwater levels vary from 2 m (6 ft) to 9.8 m (32 ft) below ground surface in the soft-ground portions of the alignment south of Nob Hill.

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

Robbins provided two 6.3-m (20.7-ft) diameter EPBs for the BIH JV, nicknamed “Mom Chung” and “Big Alma” in honor of local historic figures. “The unique challenges of this project include the dense urban environment and curved tunnel alignment. Use of TBMs minimizes 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 JV, of the sharp curves along the rail route.

The specialized TBMs were also designed with a number of features to efficiently navigate the varied geology, steep grades and turns along the alignment, and to 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-in., pressure compensated disc cutters when encountering rock or concrete. The ample opening ratio is 31 percent to allow efficient and controlled muck flow through the head.

The cutterhead is driven by five, 210-kW, 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, replaceable wear protection on both flights of the screw was provided and the screw casing.

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 lessens the risks of segment damage, ring deformation and settlement during boring.

To help minimize ground loss and settlement, the machine is fitted with multiple monitoring systems – two electronic scales and a radar system constantly measure the weight and volume of the tunnel muck traveling 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.

To get muck out of the tunnels, 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 500-m (1,640-ft) capacity belt storage cassette and splicing stand to allow the TBMs to bore approximately 250 m (820 ft) 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.

Boring Below BART
After the TBMs were launched in 2013, extensive settlement monitoring began in preparation for the complex crossing below the live BART and Muni Metro tracks under Market Street. The Parsons Brinckerhoff/Telamon JV, responsible for utility relocation and tunnel design, performed extensive numerical 3D soil-structure-interaction modeling to estimate the ground and structural behavior of the twin BART tunnels. Results indicated that properly operated EPB machines 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 lower usage weekend time, preferably after 9 p.m. on a Friday. But, 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 Matthew Fowler, Project Manager for the Parsons Brinckerhoff/Telamon JV design team. With built-in TBM ground control systems and careful excavation, minimal settlements were measured and no compensation grouting was needed.

The second machine excavating the northbound tunnel completed its crossing in February 2014, with equally successful results. “A lot of credit needs to be given to Barnard/Impregilo for running the machines so well, and maintaining excellent face control. We were just over 11 ft at the closest point to the BART tunnels. It was a major technical challenge, and now that we are past that the worst is behind us,” said Fowler. The contractor kept a close watch on face pressure, using a mix of foam for conditioning with some bentonite. Compensation grout injection was again not needed for the second and final BART crossing.

Smooth Operators
Since their launch, the EPBs at the Central Subway have excavated incredibly well — as much as 39.7 m (130 ft) in 24 hours. The TBM “Mom Chung” has excavated up to 378 m (1,243 ft) in one month at an average of 18 m (59 ft) per day. TBM “Big Alma” has fared even better, turning in a best month of 513 m (1,683 ft), with a 20.5 m (67.3 ft) daily average.

With tunneling nearly completed and the Retrieval Shaft in the North Beach area 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 allowed at the launch box 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 far more stringent noise restrictions requiring noise mitigation structures and limited work hours. At the retrieval shaft, there will also be noise and work hour restrictions,” said Fowler. The TBMs will break through one by one into the 18 m (60 ft) deep shaft, and will be disassembled in the shaft bottom for removal.

BIH JV has developed the TBM breakthrough plan at the retrieval shaft. As a precaution, the retrieval shaft will be partially filled with water to balance groundwater pressures behind the cutter-soil mix headwall and prevent soil losses through the shield gap. The first machine is scheduled to break through in late May, and the second soon thereafter. Once the water is drained, onlookers will be able to get a view of the two cutterheads before removal from the shaft.

This article was submitted by The Robbins Co.

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