Central Subway Program Director John Funghi was interviewed by Fox KTVU News reporter Claudine Wong for Bay Area People. Check out the segment in the video below, following the BART and CalTrain reports.
For more information on the Central Subway Project, please see our website.
Congratulations to the Central Subway Team for achieving an Award of Merit for best transportation project of 2015 from Engineering News record, a weekly magazine highlighting the construction industry. The award was given for the completion of the Central Subway Tunnel. The Central Subway Tunnel Project was cited for completing an intricate project on time and within the approved budget.
A method called deep tunneling was used to build the tunnels. Two tunnel boring machines, named for San Francisco Legends Alma Spreckels and Dr. Margaret “Mom” Chung, were used in the construction. Use of these machines enabled the work to be finished with less surface disruptions and reduced utility relocations.
The Central Subway Tunnel completion was a major milestone for the project. When the Central Subway is completed, T Third Line trains will travel mostly underground in the 2 mile tunnels from 4th Street to Chinatown, bypassing heavy traffic on congested 4th and Stockton Streets. Our team is very proud to have been recognized by ENR for this achievement.
The Central Subway Team looks forward to more success in 2016. We wish San Francisco a very happy and healthy holiday season.
For more information on this honor see http://www.enr.com/articles/38039-airporttransit-award-of-merit-central-subway-tunnel.
Below are some photos of the project.
Excavation has begun for the future site of Chinatown Station’s north access emergency stairwell. Read more about the excavation here.
Crews restore the pavement along Stockton Street between Clay and Sacramento. This follows installation of pipes and pumps for future work.
Excavation in Chinatown has reached approximately 60 feet below street level. Crews are now constructing the top of an arched structure which will lead passengers to the station platform.
If you would like to see more photos of project construction, check out our Flickr, updated weekly!
Union Square Business Improvement District (BID) Executive Director Karin Flood and Central Subway Program Director John Funghi kick-off the second annual Winter Walk. This pop-up open space, features entertainment, Off the Grid food trucks, and is located on Stockton Street between Geary and Ellis during the holiday moratorium.
The much-appreciated plaza features a nightly light show that is projected onto the Macy’s Men’s building, featuring San Francisco landmarks and holiday themed-images.
Shoppers, pedestrians, visitors and locals of all ages can enjoy the open space, festive decorations and special seating in the plaza through the first of January.
If you would like to see more photos of project construction, check out our Flickr, updated weekly!
Starting at 10pm tomorrow Friday November 6th work will begin at the intersection of 4th and King to install a new track configuration for the future surface portion of the T Third Line extension through South of Market. Starting November 6, work will last for eight days, finishing on November 14. This work will temporarily affect both pedestrians and motorists through the area, but also light rail operations. Here are some major points to be aware of:
Motorists coming down the King Street offramp from I-280 will need to take a right turn at 5th and a left on Berry to travel through the area. Once on Berry, drivers will be able to go through to 3rd Street, where they can make a right or left turn. Vehicles traveling south on 4th Street will not be able to travel past King to Berry. Both lanes on Berry will be converted to go eastbound between 5th and 3rd. Motorists traveling west on King will not be affected. Please take these temporary detours into consideration when traveling through the area.
The Central Subway project was recognized at the American Society of Civil Engineers’ (ASCE) Annual Business Meeting and Awards Ceremony on September 17 at the City Club of San Francisco.
Project Director John Funghi accepts the ASCE Transportation Project of the Year award on behalf of the Central Subway Project, a 1.7 mile-long extension of the Muni T Third Line.
The ASCE accolade was given to the Central Subway project, named the Transportation Project of the Year for 2015. When open for revenue service in 2019, the Central Subway will connect the northern and southern part of the city.
Want to see more photos of project construction? Check out our Flickr, updated weekly!
The summer of 2015 is coming to a close with a brunch of activity! The San Francisco Municipal Transportation Agency (SFMTA) is focused on projects that will make transportation safer and more reliable. In addition, BART will be shutting down the Transbay Tube for weekend track repairs. And Billy Joel is in town for a concert on Saturday, September 5 at AT&T Park.
For the Central Subway project, crews will be installing track and making infrastructure upgrades at the intersection of 4th and King Streets this Friday, Sept. 4 through Tuesday, Sept. 8. This is planned as a two phased project, with the second phase scheduled for early 2016.
Work will begin at 10p.m. Friday, September 4 and continue ‘round the clock through early morning Tuesday, September 8. Construction on Friday September 4 is anticipated to cause a significant amount of noise due to jack-hammering from 10 p.m. to 2 p.m. on Saturday as crews break into a 16-inch thick pavement at 4th and King to prepare for replacing tracks and switches.
The road work will impact Muni travel as well as people driving, walking and biking in the area. Here’s what to expect:
Pedestrians and Bicyclists
All this construction will have a great end result. When open in 2019, the Central Subway will connect neighborhoods and provide faster and safer transportation to BART and other Muni lines. To learn more about the project visit www.centralsubwaysf.com.
Heads up! Construction work at 4th & King is slated for Labor Day Weekend, September 4 through 7.
This will be a two-phase project for signal installation and track enhancement. Below is the area where work will take place.
Over the Labor Day Weekend, the T Third Line rail service will still run, but Muni Metro riders are advised that through-service will not be continuous at the intersection at 4th and King, so a transfer will be necessary. Motorists traveling on I-280 northbound are advised to take the 6th Street exit due to detour at 4th & King. Check here for future updates and details.
Want to see more photos of project construction? Check out our Flickr, updated weekly!
San Francisco Mayor Edwin M. Lee congratulates Central Subway Project Director John Funghi on completion of Tunnel Contract 1252. This contract consists of the 1.7 mile long twin tunnels connecting South of Market through Union Square and Chinatown. Also included in the contract (pictured here) is the tunnel portal located on 4th Street between Harrison and Bryant.
The twin tunnels are made up of precast concrete rings comprised of six segments, which are bolted together underground. Next steps for the Project include installation of track and systems to get revenue service ready by 2019.
Local and network media participated in a news conference earlier this week to see the completed tunnels firsthand.
Read here for more about the event.
Our recent time-lapse video highlights the break-through and removal of the tunnel boring machines (TBMs) at the retrieval shaft and gives a closer look at how work advanced at that site.
The video shows TBM Mom Chung breaking into the water-filled retrieval shaft last June. The water is drained from the shaft for the arrival of TBM Big Alma. The departure of the machines comes next as workers remove both TBMs, piece by piece, over several months. The retrieval shaft site was restored and vacated in January 2015.
On Dec. 3, San Francisco Mayor Ed Lee, the Union Square Business Improvement District, the San Francisco Municipal Transportation Agency and the Central Subway officially kicked-off “Winter Walk SF” in Union Square – an open plaza for the public to enjoy. This festive, pedestrian-friendly holiday hub runs through January 1 on Stockton Street between Ellis Street and Geary Boulevard.
More details and scheduled events can be found here.
During the next 10 days, Central Subway work continues at sites in SoMa, Union Square, Chinatown and North Beach. This section contains brief updates about what is happening in each neighborhood. For more detailed information click here or on the links below. You can also find construction, traffic detour, and Muni impact information on our project Google Map. For safety reasons, we encourage the public to follow all traffic and pedestrian notifications posted at the work sites.
Southern SoMa (Harrison to King/Berry): Potholing for utilities continues next week on 4th Street between Bryant and King. On Brannan between 4th and 5th Streets, work on utilities continues. Temporary walkways are in place around the construction zones.
Northern SoMa (Market to Harrison): At 4th and Folsom, work to build the Yerba Buena/Moscone Station continues. This includes construction of the station walls within the work site and water line replacement at the intersection of 4th and Folsom. Until Monday, July 28, 4th Street from Howard to Folsom streets will have one traffic lane. This area will continue to have a lane reduction overnight from 7 p.m. to 5 a.m.
Union Square: Stockton Street between Ellis and Geary is closed to vehicular traffic to allow for construction of the Union Square/Market Street Station. Work is underway for pile installation to construct the station walls. For the month of July, Stockton Street between Post and Geary will be closed to vehicular traffic. Please use alternate routes and follow construction signs.
Pile installation takes place at the southwest intersection of Stockton and O’Farrell streets until August 18. One lane of travel will be open. Expect delays and please use alternative routes.
Muni service impact:
Until August 1, the bus stop at Geary and Stockton will be temporarily discontinued. Please board bus at Geary and Powell. Please check back on our weekly Construction Update for the most current information on the re-opening of this stop.
Chinatown: The contractor started station wall construction at the future station site and on the adjacent section of Washington Street. Work to replace high pressure water pipeline and install new fire hydrant on Washington is ongoing. A section of the sidewalk at the northeast corner of the Washington and Stockton intersection will be demolished in order to install a new fire hydrant.
North Beach: Work continues at 1731-1741 Powell. Sustainable Streets is constructing bulbs on Columbus Avenue between Filbert and Union streets, and widening the sidewalk on both sides of the street for this block. Street restoration work is expected to be completed by the end of August. More information is available in this notice.
We thank the businesses, residents, shoppers, and commuters of San Francisco for their continued engagement with this significant investment in the extension of our public transportation system.
Tunnel boring machine Big Alma, shown here, will cross under BART and Market Street this week.
This week the Central Subway’s second tunnel boring machine (TBM) will pass beneath Market Street and the existing Bay Area Rapid Transit (BART) and Muni Metro tunnels, crossing from SoMa and into Union Square. The tunnel, one of two being constructed as part of the Central Subway Project, will allow T Third Line trains to travel quickly beneath 4th Street and Stockton Street, cutting transit travel times by more than half along this busy corridor.
The SFMTA has worked in close coordination with BART and an independent panel of top tunneling experts to plan and carry out this key phase of tunnel construction. To pass beneath Market Street and the existing transit tunnels, TBM Big Alma will turn slightly left beneath 4th Street just south of Market Street. The machine will then veer right to head north under Market Street and then Stockton Street. The new T Third Line tunnels will be about 10 feet below the existing BART tunnels.
This graphic shows the path the Central Subway TBMs will take when they pass under Market Street. The second TBM, Big Alma, is making this crossing this week.
Starting today, Big Alma will be in operation 24 hours a day to build the approximately 425 feet of new tunnel beneath the Market Street tunnels. Late last year the first TBM, called Mom Chung, safely and successfully completed the same undercrossing, using the same techniques and interagency coordination. Mom Chung is now beginning to tunnel under Nob Hill, heading toward Chinatown. Including both tunnels, tunneling contractor Barnard Impregilo Healy (BIH) has constructed more than 6,900 feet of tunnel under 4th Street and Stockton Street so far.
In preparation for the crossing, the contractor has injected a ground-stabilizing grout underground near the BART tunnels, accessing this subterranean area via a deep shaft they constructed on Ellis Street. About 150 monitoring devices installed in the Powell Street Station and on neighboring buildings will provide live data feeds about tunneling conditions to web applications that the SFMTA, the contractor, BART and an independent panel of tunneling experts can view at all times. Similar devices are installed along the entire tunneling path, from 4th and Bryant in SoMa to Columbus and Powell in North Beach. The readings of these instruments and others on the TBMs themselves allow the tunneling contractor to respond rapidly to ground conditions around the machines as they move forward.
The crossing under BART may last up to six days. Tunneling may cause BART to run at slower speeds in the area.
More information is available in this press release from the SFMTA.
Using time-lapse cameras installed underground, we’ve condensed the first months of tunneling into this captivating two-minute video. In the video, Central Subway crews assemble tunnel boring machine (TBM) Mom Chung and drive the state-of-the-art tunnel builder north under 4th Street to build San Francisco’s first new subway line in decades. The video shows the TBM assembly process, installation of the first tunnel rings, removal of excavated ground spoils and other aspects of the complex tunneling operation.
By the end of the video, Mom Chung has tunneled from her launch box south of 4th and Harrison to 4th between Folsom and Howard. Since we captured this footage, she has mined much farther north, crossing successfully and safely under Market Street and the existing BART and Muni tunnels last weekend.
We thank our tunneling crews and tunneling contractor Barnard Impregilo Healy for their hard work to build the Central Subway, extending the Muni Metro T Third Line through SoMa, Union Square and Chinatown. Stay tuned for more time-lapse videos as construction proceeds.
Tunnel boring machine Big Alma, shown here on the right, began tunneling under 4th Street. On the left is the entrance to the parallel tunnel being built by TBM Mom Chung.
There are now two 350-foot-long, 750-ton tunnel boring machines mining under 4th Street to construct the Central Subway’s pair of parallel tunnels. Our second TBM, named Big Alma, recently launched. She’s building the tunnel that northbound T Third Line trains will use when the Central Subway opens in 2019.
Over the coming months, Big Alma will travel north under 4th Street and Stockton Street, building tunnel at an average pace of 40 feet per day. Her tunnel will be just east of the tunnel that her twin, named Mom Chung, began building in July. Big Alma will move more slowly during the first 500 feet of tunneling, as Central Subway crews test the TBM and calibrate its many functions.
Big Alma is named for “Big Alma” de Bretteville Spreckels, a 19th century socialite and philanthropist who, among her many accomplishments, persuaded her first husband, sugar magnate Adolph B. Spreckels, to fund the design and construction of the California Palace of the Legion of Honor, at Land’s End in San Francisco. A model in her youth, Spreckels was the inspiration for the “Victory” statue atop the Dewey Monument in the center of Union Square.
The photos below show Big Alma the TBM during her assembly and launch. To learn more about Big Alma and her underground journey to build San Francisco’s first new subway line in decades, check out this press release from the SFMTA. You can also follow the machine on Twitter. She’s @BigAlmatheTBM.
This section of Big Alma’s “shield,” the portion of the machine that puts in place the tunnel segments, was among the first to be lowered into the launch box.
Known as the “cutter head,” this part is on the front of the machine, attached to the shield. It excavates the earth to make room for the new tunnel.
The cutter head is lifted by a crane and lowered into the launch box.
Underground, crews connect the cutter head to the shield.
A welder welds together two sections of Big Alma’s shield.
After the shield and cutter head were assembled, crews moved them to the northern end of the launch box, where tunneling begins.
Crews work in the launch box, preparing to move Big Alma forward.
In the launch box, Mom Chung is on the left, tunneling, and Big Alma is on the right, being assembled.
Crews assemble the trailing gear of Big Alma. This 300-foot train of tunnel-building mechanisms performs a variety of important functions.
Once assembled, the trailing gear stretches almost the full length of the launch box.
A welder works on Big Alma’s shield in the weeks before launch.
Tunnel segments are lowered into the launch box for installation by the TBMs.
After Big Alma’s launch, a tunneling crewman walks out the back of the machine. We thank our crews for their hard work as they build San Francisco a new subway line.
In the operating cabin of TBM Mom Chung, two TBM operators monitor tunneling conditions provided from a variety of state-of-the-art instruments.
Next week our first tunnel boring machine, Mom Chung, will pass beneath Market Street and the existing BART and Muni Metro tunnels, crossing from SoMa and into Union Square. The tunnel, one of two being constructed as part of the Central Subway Project, will allow T Third Line trains to travel quickly beneath 4th Street and Stockton Street when the Central Subway opens, cutting travel times by more than half along this busy corridor.
The SFMTA has worked in close coordination with BART and an independent panel of top tunneling experts to plan and carry out this key phase of tunnel construction. To pass beneath Market Street and the existing transit tunnels, TBM Mom Chung will turn slightly left beneath 4th Street just south of Market Street and travel partially under 801 Market Street, home to Old Navy. The machine will then veer right to head north under Market Street and then Stockton Street. The new T Third Line tunnels will be about 10 feet below the existing BART tunnels.
This graphic shows the path the Central Subway TBMs will take when they pass under Market Street. The first TBM, Mom Chung, is expected to begin this crossing on Monday.
Starting yesterday, Mom Chung is in operation 24 hours a day to build the approximately 425 feet of new tunnel beneath Old Navy and the Market Street tunnels. Mom Chung is expected to begin crossing beneath the Market Street tunnels on Monday, Nov. 25. The tunneling methods used during this key crossing will be the same that are in use along the rest of the tunneling route. As the TBM moves forward, it installs tunnel segments within the section known as the “shield,” a 20-foot-diameter cylinder at the front of the machine. The shield and the newly installed tunnel lining create a watertight barrier that holds the ground outside in place. Using these methods, tunneling contractor Barnard Impregilo Healy (BIH) has constructed more than 2,100 feet of tunnel under 4th Street so far.
“We’re thrilled at the progress our tunneling contractor and tunnel boring machines are making to build this essential new subway line for San Francisco,” said SFMTA Director of Transportation Edward D. Reiskin. “We are using state-of-the-art technology and the country’s top expertise to ensure the entire tunneling process safeguards the city as well as the region’s critically important transportation infrastructure. We thank BART for the excellent partnership in this process.”
With the help of TBM Mom Chung, Central Subway crews have now built a new subway tunnel under 4th Street from Harrison and Stevenson.
In preparation for the crossing, the contractor has injected a ground-stabilizing grout underground near the BART tunnels, accessing this subterranean area via a deep shaft they constructed on Ellis Street. They will inject additional grout as needed while Mom Chung is mining below BART.
About 150 monitoring devices installed in the Powell Street Station and on neighboring buildings will provide live data feeds about tunneling conditions to web applications that the SFMTA, the contractor, BART and an independent panel of tunneling experts can view at all times. Similar devices are installed along the entire tunneling path, from 4th and Bryant in SoMa to Columbus and Powell in North Beach. The readings of these instruments and others on the TBM itself allow the tunneling contractor to respond rapidly to ground conditions around the machine as it moves forward.
BART may run at lower speeds between Powell and Montgomery stations while tunneling is in progress under BART. Please visit BART on the web at www.bart.gov/alerts or call 511 to get up-to-date service information.
For more about this major tunneling milestone, check out this press release from the SFMTA.
This building in Chinatown will be demolished soon to make way for the Central Subway Chinatown Station.
Last month, we started the first construction activities to build the Central Subway’s stations, tracks and operating systems. This work will extend the Muni Metro T Third Line underground to improve public transit in some of San Francisco’s busiest neighborhoods. Once in operation, the Central Subway will cut travel times in half along congested Stockton Street and 4th Street while enhancing connections to BART, Muni Metro and Caltrain.
The first construction activities are preparatory in nature and include installing monitoring devices, putting up fences and removing hazardous materials at future subway station sites in Chinatown and SoMa. After completing this work, contractor Tutor Perini will demolish the existing structures at the sites of the future Chinatown Station (933-949 Stockton Street) and Yerba Buena/Moscone Station (260-266 4th Street). Station construction will follow.
At other locations, construction will commence later this year or in 2014. Construction timelines and impacts will vary significantly at the various project sites (more details below). Construction is expected to finish in 2018.
Over the next few years, we will build the Union Square/Market Street Station on Stockton Street. This rendering shows the station entrance that will be built in Union Square Plaza, at the corner of Stockton and Geary.
The construction is part of SFMTA Contract 1300 (Stations, Track and Systems), awarded to Tutor Perini, a leading California-based construction firm, earlier this year. As part of this contract, Tutor Perini will construct three subway stations, one surface-level station, 1.7 miles of train tracks and the operating systems for the T Third Line extension.
Tutor Perini has extensive experience in building public infrastructure in the Bay Area and around the country. Tutor Perini improved the seismic reliability of the Richmond Bridge and is currently building the expansion of the Caldecott tunnel in the East Bay, among numerous other major projects.
To inform local businesses, residents, property owners and community groups about construction timelines and impacts, the SFMTA and Tutor Perini will work in partnership to disseminate information and host community meetings before major work begins. The public may learn about Central Subway community meetings and construction by signing up for the project’s weekly construction emails. An online signup form is available at http://eepurl.com/oOs-b.
Construction will take place at the sites of the four new stations, an on 4th Street south of the 1-80 overpass.
The following provides an overview of the work included in this major construction undertaking. More information is available in this press release from the SFMTA.Timeline
Construction will be in progress at various sites from September 2013 to February 2018. Timelines will vary significantly at each site. Additional information will be provided in the coming months.Locations and Scope of Work
Father John Takahashi and Central Subway Program Director John Funghi stand beside concrete blocks that once formed the foundation of a historic North Beach church.
Earlier this month, construction crews demolishing the Pagoda Palace unearthed a long-lost piece of San Francisco history: the foundations of a church that stood on these grounds more than a century ago.
The Holy Trinity Orthodox Cathedral, now located at 1520 Green Street, once stood in North Beach at the corner of Powell Street and Columbus Avenue. Built in 1888, the dramatic onion-domed structure was completely destroyed by the great San Francisco earthquake and fire of 1906. All that remained were five bronze bells – and they survived because they were off site at the time, being repaired. The church relocated to the Green Street location soon after, opening in 1909.
Last week a concrete block from the old church’s foundations joined the bells at the Green Street cathedral. After construction crews unearthed the foundations, we contacted Father John Takahashi, the senior priest of the Holy Trinity Orthodox Cathedral, to inform him of our discovery. We also invited him to visit the construction site and see the newly uncovered relics of his church’s past.
The Holy Trinity Cathedral on Powell Street, circa 1890.
Once on site, Father John spoke with Central Subway Program Director John Funghi, and the Central Subway team gave Father John a block of concrete from the old foundations. The block will now live in the garden of the Green Street cathedral.
Central Subway crews discovered the historic foundations while dismantling the thick concrete slab that formed the base of the Pagoda Palace Theatre. Once the slab was removed, the outlines of the foundation revealed themselves amidst dirt and rubble.
Archeologists and construction personnel examined the unearthed foundations and concluded that they were consistent with building practices in the late 19th century. The absence of rebar, the consistency of the concrete and the size of the aggregate in it helped support this conclusion.
With a piece of the old church now safely preserved, crews then broke apart what remained of the foundations to make room for the next phase of Central Subway construction, and the next chapter in the life of this storied North Beach site.
Late in the night last month, crews installed the cutter head of tunnel boring machine (TBM) Big Alma. The massive machine’s cylindrical shield was already in place below.
Big Alma has arrived. The massive tunneling machine — the identical twin of recently launched Mom Chung — is now under assembly underground. Last month, tunneling crews lowered large sections of Big Alma into the tunnel launch box, the major excavation under 4th Street between Harrison and Bryant where tunneling begins. After about two months of assembly, Big Alma will begin building a tunnel parallel to Mom Chung’s, extending the Muni Metro T Third Line through SoMa, Union Square and Chinatown.
The photos in this post show the installation of Big Alma’s cutter head. The cutter head, a spinning excavator at the front of the machine, will dig through bedrock, clay and sand as Big Alma travels north beneath 4th Street, Stockton Street and Columbus Avenue. This major component of the TBM is about 20 feet in diameter and weighs about 143,000 pounds. While tunneling is underway, the cutter head will pump out an environmentally safe, soap-like foam to condition the ground as it cuts through the earth like a cheese grater. Once loosened, spoils pass through holes in the cutter head for transport out of the tunnel.
A large red crane, called a gantry crane, lifts the cutter head from the ground.
When Big Alma tunnels, these wheels will spin to loosen the ground, aided by an environmentally safe, soap-like foam that conditions it.
Operated by our tunneling crews, the crane lifts the cutter head into a vertical position to allow for installation.
Crews help stabilize the cutter head before lowering it underground.
The crane moves the cutter head into position over the tunnel launch box, where Big Alma’s cylindrical shield awaits.
The shield, already underground, will stabilize the tunnel and form a watertight barrier while mining is in progress. Concrete tunnel segments are installed within the back of the shield and bolted together by tunneling crews.
Above the shield, the cutter head descends into the excavation.
The front of the shield, before the cutter head is installed. During tunneling, excavated ground spoils will pass through the hole at the bottom for transport out of the tunnel.
The cutter head continues its descent into the excavation.
Once the cutter head is lowered, crews work to connect it to the shield.
To connect the TBM segments, crews bolt them together. Later, they will reinforce the connections with welding.
The TBM assembly team poses in front of Big Alma. We thank our dedicated crews for all their hard work.
This video shows some of the first ground spoils excavated after our first TBM, Mom Chung, launched this week.
This week tunnel boring machine (TBM) Mom Chung started digging, kicking off construction of San Francisco’s first new subway tunnel in decades. Over the next 10 months, the 350-foot-long, 750-ton machine will excavate and construct the tunnel that southbound T Third Line trains will use when the Central Subway opens in 2019.
As Mom Chung travels, you can follow her on Twitter — she’s @MomChungtheTBM. Her twin sister, Big Alma, recently arrived in San Francisco. After about six weeks of assembly underground, she will begin constructing a tunnel parallel to Mom Chung’s. (You can follow Big Alma at @BigAlmatheTBM.)
The tunnels are a key component in extending the Muni Metro T Third Line through SoMa, Union Square and Chinatown, vastly improving transit in these neighborhoods.The Tunneling Journey
Mom Chung and Big Alma will excavate and construct the 1.5-mile-long tunnels at a pace of approximately 40 feet per day, though their pace will vary based on ground conditions and other factors. Most of their journey will be through two major ground formations: the Franciscan complex, a bedrock formation that forms Nob Hill; and the Colma formation, a dense mixture of sand and clay.
The TBMs will be so far beneath the surface – between 40 and 120 feet underground – that no vibration or noise will be felt above ground when they pass below.
In this video, a welder works on the TBM’s trailing gear.How TBMs Work
The machines consist of three main sections: a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-foot train of tunnel-building mechanisms (the trailing gear).
This conveyor belt on top of Mom Chung’s trailing gear will help transport ground spoils out of the tunnel. In the background you can see the cutter head of our second TBM, Big Alma.
The cutter head, a spinning excavator at the front of the machine, pumps out an environmentally safe, soap-like foam to condition the ground as it cuts through the earth like a cheese grater. Once loosened, spoils pass through holes in the cutter head and onto a large screw. The screw carries the spoils onto a series of conveyors for transport out of the tunnel.
To launch, Mom Chung pushed off of a steel frame as her cutter head began to spin.
The large concrete segments shown here are what will form the Central Subway tunnels. The TBMs will install them, and crews will bolt them together, as the machines move forward.
As she tunnels, Mom Chung will stop every five feet to install the concrete segments that make up the tunnel’s lining. The concrete segments are installed within the back of the TBM’s cylindrical shield. The machine lifts the segments into place, and then crews bolt them together. Hydraulic jacks within the shield then push off of the newly installed tunnel lining, propelling the massive machine forward.
These concrete rings, already installed, are held in place by hydraulic arms (seen on the left) as the machine moves forward.
A crew of about 10 people operates the machine and bolts the tunnel segments together. Crews will be at work 24 hours a day, six days a week to build the Central Subway’s tunnels.Learn More
Want to find out more about the Central Subway’s complex, high-tech tunneling machines? Check out the following documents: