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First TBM launch on LA metro extensions 31 Mar 2016

Peter Kenyon, TunnelTalk

All four machines procured for the latest metro extensions in Los Angeles are to be supplied by German manufacturer Herrenknecht.

TBM under construction at Expo/Crenshaw station
TBM under construction at Expo/Crenshaw station

Two EPBMs of 6.4m diameter are procured for the three reaches of Phase I of the all-underground Purple Line extension, for a total excavation of 5,456m of twin running alignment; a single Herrenknecht EPBM of 6.4m diameter is preparing for launch next month (April 2016) on the 3,000m underground section of the 13.5km LAX/Crenshaw transit corridor; and a refurbished contractor-owned Herrenknecht EPBM will be shipped from Seattle next month (April 2016) to begin excavation in November of the 1,768m of twin running underground alignment of the 3km Regional Connector that will link the city’s Blue and Expo Lines.

LAX/Crenshaw

Following delivery to the jobsite at the future Expo/Crenshaw station in January, TBM Harriet is the first of the four machines to be mobilised as part of the total program of extensions. The machine was lowered into the station – which is currently under construction – earlier this month, ahead of scheduled launch next month (April). The original program anticipated launch in mid-2015. “The Contractor encountered challenges in constructing the cutter soil mix-based excavation support system which pushed the box construction schedule out,” explained Jose Ubaldo for the owner, the LA Metropolitan Transportation Authority.

Lowering TBM Harriet into Expo/Crenshaw station

Design-build construction of the $2.058 billion 13.5km-long Crenshaw/LAX light rail project was awarded to the Walsh/Shea Corridor Constructors (WSCC) in May 2013 for a lowest bid price of $1.272 billion. As well as the guideway, which comprises at-grade, elevated and underground sections, the contract includes construction of eight stations, three of them underground, and all the M&E installations.

For the 3,000m-long underground section between new underground stations at Crenshaw/Expo, Crenshaw/MLK and Crenshaw/Vernon (Fig 1), anticipated geology consists of alluvial deposits of predominantly clay, silt and sands with some gravels and cobbles. Ubaldo told TunnelTalk: “The water table is anticipated to be above the TBM in the north half and transition to below the TBM on the south half. Challenges will include sandy soils below the water table, potentially gassy conditions and mining above and below the water table.

Fig 1. Alignment of the 13.5km Crenshaw/LAX corridor
Fig 1. Alignment of the 13.5km Crenshaw/LAX corridor

“A key Metro performance requirement ahead of TBM procurement was to manage risk during tunneling by utilizing a pressurized envelope to support the ground. This included pressurizing the face using either an EPB or slurry TBM, pressurizing the annulus around the TBM body with a volume stable fluid, and injecting a two-component grout under pressure through tailshield ducts as the TBM advances. To achieve these requirements WSCC selected a new Herrenknecht EPBM.”

As with all excavations under an urban environment, managing settlement is a key aspect of the drives. Ubaldo said: “The TBM has been specified with limited ground movement in mind, and contains pressure monitoring cells at the face, as well as on the shield, to identify any potential issues with pressure loss.”

He added: "The break-ins and break-outs of the stations have been grouted to limit the potential for ground loss and an extensive automated soil monitoring system has been installed above the tunnel crown to monitor any movement in real time so that any settlement exceeding specified limits can be detected quickly and corrective action implemented to limit any further movement.”

The precast concrete tunnel segment design is based on specific LA Metro performance requirements to ensure water/gas-tight running tunnels and are essentially an evolution of the lining design previously used on the Metro Gold Line Eastside Extension. Detailed segment design for WSCC is by Arup, with each 1.52m-wide universal ring consisting of six segments: 4 x 67.5° rhomboidal segments and 2 x 45° trapezoidal segments.

Lowering the cutterhead of TBM Harriet
Lowering the cutterhead of TBM Harriet

To date 300 rings have been manufactured, using Cleco moulds, by CSI Hanson at its casting yard in Shafer, CA and to a 28-day concrete strength of 6,500psi. Each segment features a pair of Datwyler EPDM rubber M385-96 gaskets and an M389-32-2 cross gasket with a sliding profile on the radial joints, attached to the segments with gasket adhesive. Polymeric packing on circumferential joints is supplied by Tunnelling Accessories; Sof-Fix Anix 110 M12 dowels for circumferential joints are supplied by Sofrasar; while radial joints along convex-convex geometry will be achieved with spear bolts supplied by Turner and Townsend.

Mucking out will be via a loco system with lift off boxes that will be dumped into a surface storage muck bin. From here, excavated material will be loaded into over-the-highway dump trucks and hauled to an offsite disposal facility.

In addition to the TBM alignment and the three cut and cover stations, cut and cover excavations are under way in the Slausson–Florence West (304m) and Century LAX–Aviation LAX (914m) corridors. These are expected to be completed within the next 24 months. The TBM drive out of Crenshaw/Expo is scheduled for first breakthrough at Crenshaw/Vernon in September this year (2016), ahead of lift-out and transportion back to the jobsite for relaunch on the parallel tunnel the following month. Final TBM breakthrough is scheduled for March 2017.

Purple Line

The 5,456m underground alignment of Phase I of the Westside Purple Line Extension from the existing Wilshere/Western station via three new underground stations at Wilsher/La Brea, Wilshere/Fairfax and Wilshere/La Cienega in Beverley Hills (Fig 2), was awarded to the Skanska/Traylor/Shea (STS) joint venture in July 2014. At a contract price of US$1.626 billion, this came in at US$200 million more than the lowest of the rival bids but after a protest was lodged the LA Metro Board confirmed the award in November 2014.

Notice to Proceed was issued on January 12, 2015, and, after 14 months, overall progress is reported at 7%, with final design 75% complete and pile installation 30% complete at Wilshere/La Brea station from where the two TBMs that have been procured will launch in mid-2018.

Fig 2. Purple Line extension with shafts marked as “X”
Fig 2. Purple Line extension with shafts marked as “X”

Both EPB machines are to be manufactured by Herrenknecht of Germany at its Schwanau workshops ahead of scheduled arrival on site in late 2017. The drive strategy splits the extension into three reaches: Reach 1 of 2,926m between Wilshere/La Brea station and the retrieval shaft at Wilshere/Western station (mid-2018–early 2019); Reach 2 of 1,341m between Wilshere/La Brea and Wilshere/Fairfax stations (mid 2019–late 2019); and Reach 3 of 1,006m between Wilshere/Fairfax and Wilshere/La Cienega stations (late 2019–early 2020). Reach 3 also includes a further 183m of TBM excavation for a tail track at Wilshere/La Cienega station.

Regional Connector

The Regional Connector was awarded to the Skanska/Traylor Bros JV in June 2014 for a contract bid of $918 million. The contract comprises design and construction of a 3km underground connection of light rail transit and three new stations. Preliminary design is based on an open cut operation along Flower Street in Downtown Los Angeles with twin TBM bored running tunnels and an SEM crossover cavern along Second Street. This will connect the Blue and Expo Line terminus at the 7th/Metro Center Station to the Gold Line near its Little Tokyo/Arts District Station, which will be replaced underground (Fig 3).

Fig 3. Regional Connector alignment will utilise a refurbished Herrenknecht EPBM
Fig 3. Regional Connector alignment will utilise a refurbished Herrenknecht EPBM

A Herrenknecht EPBM – first used by Traylor Bros on the LA Metro Gold Line East Extension, and then for the U220 contract for Sound Transit in Seattle – is to be used. It will be refurbished in Tacoma, Washington State, then shipped by barge departing Tacoma later this month (April 2016).

The tunneling plan for 1,768m-long Regional Connector is to use one EPBM to mine from the east portal at the 1st and Central Station excavation, through the location of the 2nd and Broadway Station to the 2nd and Hope station. The machine will be walked across the 2nd and Hope Station invert and resume mining to the retrieval shaft at 4th and Flower Streets.  The machine will then be transported back to the east portal by truck and reassembled for the second drive.

Tunneling is expected to start in November 2016 and take approximately six months for the first drive, followed by a parallel drive of the same duration.

           

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