Peter Kenyon TunnelTalk
- Nine months after launch of the first TBM, the Barnard/Impregilo/SA Healy tunnel construction joint venture contractor is closing in on first breakthrough of the 2.7km twin running underground section of the Central Subway in San Francisco.
Fig 1. Third Phase 2 of Central Subway
- “TBM Mom Chung, the 6.27m diameter Robbins EPBM procured to excavate the southbound tunnel, has advanced approximately 6,800ft (2,073m),” said Betty Chau for the owner, San Francisco Metropolitan Transportation Authority (SFMTA). “Her best tunnel rate so far is 115ft/day, at an average of about 50–80ft of new tunnel each day since launch in July 2013.
- “Operation of both machines, which are designed for a maximum operational pressure of 4 bar, has been mainly in closed mode, and we have been mining mainly in the range of 1.5-2.5 bar. Water ingress during TBM excavation has been much less than had been anticipated.”
- All but approximately 200 of the precast concrete tunnel lining segments have been manufactured, with the last remaining rings expected to roll out of the Las Vegas casting yard of the Precast Management Company (PMC) in the next few weeks. Segments are supplied in 4ft or 5ft (1.2m or 1.5m) widths and 11in (280mm) thick, for an i.d. of 17ft 10in (5.44m). A single pass system is being used.
- Mom Chung and her sister machine Big Alma (which launched in November 2013) are now both safely under the BART and Muni Metro tunnels – a landmark achievement on the US$1.6 billion project whose scope also comprises a short section of surface level trackwork, three underground stations (Chinatown, Union Square/Market Square, and Yerba Buena/Moscone), and a further surface level station (4th Street and Brannan, Fig 1). A separate single construction package for these was awarded to Tutor Perini in April 2013 for a low bid contract sum of US$852 million.
TBM Big Alma crew monitors tunneling conditions
- “The southbound tunnel is currently mining in the Colma formation, which is made up of dense sand to very stiff clay and sandy clay formation. The northbound tunnel, where 5,585ft of TBM excavation is complete, is currently mining within the Franciscan complex, a bedrock formation consisting of sandstone, shale and mélange,” Chau told TunnelTalk from San Francisco. “Mining in these formations has been good for both TBMs.”
- TBM Mom Chung made the crossing underneath the BART and Muni Metro tunnels in November 2013; Big Alma, the second Robbins machine on the project, crossed under in January this year (2014). This required moving to a 24/7 operation, with tunneling crews permanently stationed onsite to prepare for grouting if necessary.
- “The clearance between the Central Subway tunnel and BART was around 10-14ft,” explained Chau. “The proximity of the two tunnels required additional monitoring support and installation of a compensation grouting program to address concerns arising from settlement. Tunneling was also required to be performed continuously under the BART tunnels. Additional modeling of the tunneling effects to the BART tunnel was also performed during design. Additionally, daily coordination between BART, SFMTA, and an expert panel of experts was conducted before and during the BART crossing. In the event it was completed without incident and minimal settlement was observed.”
Southbound tunnel, now less than 700m from completion
- In fact, minimal ground movement has been a feature of all underground excavation of the running tunnels under what is a highly urbanized environment above ground level.
- “Ground movement observed has been around 1/8in (3.2mm), with a maximum recorded movement of a little over ¼in (6.4mm), well within the trigger level for ground movement of ¾in (9.6mm),” said Chau. “Surrounding buildings and structures have instrumentation installed that monitors ground movement automatically, and the mining crews closely observe the excavated and backfill volumes of the TBM. The TBM crews also check grouts behind the TBMs to mitigate any voids during mining.”
- Cutter wear is described as “excellent.” Chau said: “To date, only 1-2 cutting tool changes have been performed on each machine, and few modifications have been performed on the cutting tools during mining.”
Construction of reception shaft
- As work advances towards the completion of TBM excavation, attention now focuses on construction of the underground stations. Utility relocations started at the beginning of this year (2014), and progress is being made on installation of the guide walls and excavations necessary for construction of the slurry walls.
- Meanwhile, at North Beach, the concrete pour for the floor of the TBM reception shaft is completed and work continues ahead of arrival of the first machine in the coming weeks.
Central Subway bids open as first TBM arrives - TunnelTalk, April 2013
Central Subway tunnels contract awarded - TunnelTalk, June 2011
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