New pics: with digging done on first tunnel, TBM parts moved back to Little Tokyo

Regional Connector crews last weekend extracted the tunnel boring machine (TBM) cutterhead and one shield segment from underground — at 4th and Flower streets in downtown Los Angeles. The TBM recently completed digging the first of two tunnels for the project and must now be disassembled and transported back to Little Tokyo, where it will begin the second tunnel later this year. 

In order to lift such heavy machinery, a specialized crane was assembled over Flower Street and used to gradually lift each segment. The cutterhead alone weighs 144,000 pounds. This same type of crane will be used to lower and piece the machine back together in Little Tokyo. 

To complete their work, crews transported the cutterhead and shield segment to Little Tokyo using a heavy-duty dolly. Crews waited until the middle of the night to close down 7th Street to ‘walk’ the machine across DTLA. The remainder of the TBM shield will be retrieved this weekend and the crane will be demobilized the following weekend.

The Regional Connector will connect the Blue, Expo and Gold Lines via twin 1.9-mile light rail tunnels in DTLA. When complete, Metro will operate two light rail lines. One will run from East Los Angeles to Santa Monica and the other between Azusa and Long Beach. The two lines will share five stations in DTLA and the project will make light rail trips to and through DTLA faster with fewer transfers.

The project is currently forecast to be completed in late 2021. It is funded by Measure R, a federal New Starts grant and other state and local sources.

8 replies

    • Because 7th/Metro is an active station, the TBM isn’t used to tunnel all the way through. The remaining distance will be created using a cut and cover mining.

      Anna Chen
      Writer, The Source

    • This document describes local conditions, if you want the full version:

      The short version: the ground between 4th street and 6th street is a nightmare for tunneling. It’s soft silt, making a street collapse possible. The ground is full of old cables and anchors that were placed to build the surrounding buildings and then abandoned, with very poor records of how they were left. Lots of similar issues.

      Cut-and-cover is a big pain for the street. But it’s a whole lot safer than tunneling. The good news is that it only needs to go 100 feet south of 6th street. That’s where the existing tail tracks end.

    • I believe not much but out of an abundance of caution and pressure they are doing a cut and cover method. If you google “threading the needle London tunnel” the tunnel diggers in London were able to build a tunnel in between an active tube station just in between the tunnel and an escalator. Where it says right here is where the tunnel was placed. there is a video on YouTube documenting just how they did it.

    • Hi,

      As I understand it, the TBM cannot be turned around within the pit, so it must be disassembled and moved back to the original starting point to work on the next tunnel.

      Anna Chen
      Writer, The Source

    • Because of the length of the TBM (~400′) it needs to launch from a pit that can handle most of the length in a single go. The station is not long enough. The cutter head and shield are small section of the length. They are also the parts that are larger in diameter than the partially finished tunnel.

      • Right, I wasn’t expecting it to travel back to Little Tokyo via tunnel; I was sort of visualizing it turning around, moving 10 feet (or whatever) to the left, and then digging the second tunnel back towards Little Tokyo. That seems simpler than moving the whole thing above ground back to Little Tokyo, but I guess the pit at the Bunker Hill end doesn’t have enough room to maneuver.