How do they do that? Dig a subway tunnel

How do they do that? is a series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.

Lowering tunnel boring machine into the ground — Dec. 15, 2005 — for construction of Metro Gold Line Eastside Extension.

With the Crenshaw/LAX line, the Metro Regional Connector and the Purple Line Extension readying for construction, there will be plenty of digging going on in L.A.County. But how do you dig a subway tunnel? Dynamite? Giant corkscrew? Spoon?

In the U.S. we’ve been mining subway tunnels for more than a century. At first there were men and shovels and dynamite and excruciating physical labor. (Think Holland Tunnel under the Hudson River between New York and New Jersey. Think pressurized compartments holding workers who had to be depressurized at the end of a shift to avoid getting the “bends.”) Fortunately, we now have machines to do the heavy work.

During the past 20 years Metro has constructed three sets of tunnels: one for the Metro Red and Purple lines, another for two stations of the Metro Gold Line Eastside Extension and a third to carry the Expo Line under the busy Figueroa-Exposition Boulevard intersection.

Tunnel boring machine

Tunnel boring machine

For the Gold Line Eastside Extension, two tunnel boring machines nicknamed Lola and Vicki (see video above) were lowered into the ground in Boyle Heights to bore twin subway tunnels from First and Boyle to First and Lorena streets at a depth of 50 to 60 feet. Each TBM weighed more than two million pounds and was 344 feet long. Each built a tunnel that was 21 feet in diameter.

But before tunnel design or digging could start, massive planning took place, including a thorough site investigation that among things analyzed the type of soil (sand, clay, rock) and the presence of gas (methane, hydrogen sulfide).

Construction for the next three Metro Rail projects will include the same careful site investigation and two tunneling methods used successfully for the Gold Line Extension: 1) cut and cover for the subway stations and 2) pressurized face tunnel boring machines (TBMs) for the tunneling itself.

Cut and cover.

Cut and cover

For cut and cover, the surface street is removed, a temporary road deck is installed to maintain traffic, a trench is dug, the station box is constructed in the trench and, finally, the street restored.

For TBM tunneling, a shaft is sunk and the boring machine lowered to the depth of the tunnel. (On the Eastside Extension the Mariachi Station shaft was used as the boring machine entry point.) Then the TBM goes to work. As it did for the Gold Line Extension, Metro will again use “pressurized face” tunnel boring machines. This type of TBM maintains enough pressure on its face to prevent the surrounding ground from collapsing during construction. For obvious reasons, ground collapse is the enemy of a successful tunneling project.

Figuratively, the TBM is like a giant coffee can on its side but instead of a lid there is the rotating cutter head face with teeth that chew up the soil. Behind the rotating cutter head is a corkscrew-like device that carries the soil backward through the can and deposits it into waiting construction trains that carry the soil away so it can be lifted from the tunnel. (Yes, Metro even uses a temporary construction railroad to help build its rail transit lines.)

The TBM pushes forward, cutting and removing the soil and erecting tunnel lining segments inside the can, or shield, and then surrounding them with a cement grout, leaving behind a circular tunnel shell. The concrete tunnel lining must be built within the TBM shield so that not only is the face supported but also the surrounding ground before the tunnel lining is constructed.

Los Angeles has methane and hydrogen sulfide gas in places and this technique works well for those conditions. The closed face of the pressurized TBM prevents gas from reaching the workers during construction. A double gasket tunnel lining also resists water and gas. And all stations are lined with high density polyethylene (HDP) — a robust plastic — for protection from gas, water … well, anything we need to keep out of the tunnel. As part of the planning, tunnel paths are designed to run perpendicular to earthquake faults. The faster over a fault, the better. Running on or along a fault is to be avoided.

There are tunnels all over the planet. The Norwegian Laerdal Tunnel is the longest road tunnel (15.23 miles) in the world, linking the capitol Oslo with Bergen. The 31-mile Channel Tunnel (AKA Chunnel) links France and England under the English Channel. And the Cu Chi Tunnels near Ho Chi Minh City in Vietnam are an expansive labyrinth of tunnels — perhaps as much as 150 miles — that were used as a hiding place for the Viet Cong during the Vietnam War. They’re now a tourist attraction.


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10 replies

  1. […] How do they do that? Dig a subway tunnel – Metro Los Angeles’ The Source blog looks at how construction crews dig up subway tunnels during construction. L.A. currently has two subway lines (Red and Purple Lines) along with three of the four light rail lines (Blue, Gold and Expo Lines) that have some kind of underground segments along their routes. Construction will begin at various points in the future for several new rail lines and extensions of existing ones. […]

  2. It would be nice to know what powers the TMS’s. Also, I am not clear on the laying of the tunnel concrete: “,,,,,and erecting tunnel lining segments inside the can, or shield, and then surrounding them with a cement grout, leaving behind a circular tunnel shell. The concrete tunnel lining must be built within the TBM shield so that not only is the face supported but also the surrounding ground before the tunnel lining is constructed.”

    Is the concrete pumped inside the can, left to harden and then the shield is removed, or does it slide forward as the TMS continues boring? What is “the face?”

    • Matthew,
      Here’s more info from our expert.

      A pre-cast concrete lining typically comprising half a dozen segments is bolted together within the circular steel shell of the tunnel boring machine (TBM). The TBM moves forward by several large hydraulic jacks pushing against each completed ring of tunnel lining while a cement grout is injected behind the segments to provide stability. The circular rotating cutterhead at the front of the TBM cuts the ground, or “face”, which is supported by pressurized excavated soil. The TBM is powered by electricity using a giant power cord from an electricity substation.

      The International Tunnelling and Underground Space Association (ITA) which stated aims are “to encourage the use of the subsurface for the benefit of public, environment and sustainable development and to promote advances in planning, design, construction, maintenance and safety of tunnels and underground space, by bringing together information thereon and by studying questions related thereto” has a splendid website that provides vast amounts of information on the subject, including the use of these TBS and the link is:-

  3. In the “long tunnel” department, there is one section of the route used by the California Zephyr, namely the 6-mile Moffat Tunnel, under the Continental Divide, where passengers are asked not make any unnecessary moves from car to car, in order to keep the diesel fumes out as much as possible.

  4. Why are the machines scrapped after each project? I believe MOS 1 and 2 had different machines.

    • Mike:

      I don’t believe the entire machines are entirely “scrapped” as there are probably a lot of valuable and expensive parts in there. We’re checking.

      Steve Hymon
      Editor, The Source

    • Mike:

      More info. The tunnel boring machines are owned by the construction firms hired by Metro to build the project. Metro does not require “new” machines — Metro does require machines that meet the agency’s performance requirements whether new or refurbished. The TBMs used to build the Eastside Gold Line were new and they were not scrapped afterward — they were refurbished and are being used elsewhere.

      Steve Hymon
      Editor, The Source

  5. Just amazing how technology of creating and building a subway tunnel has advanced. I can’t wait for them to start on the Purple Line that will get me closer to La Brea Tar Pits and Farmer’s Market including a good walk northward on Fairfax.

  6. Well, the pair of moles that the SF Muni has tunneling from SoMa to North Beach are named “Mom Chung” (presumably after Margaret Chung, MD, a physician and supporter of the Allies against the Japanese invasion of China) and “Big Alma” (presumably after Alma de Bretteville Spreckels, a philanthropist known as the “Great Grandmother of San Francisco”).

    Hopefully, “Lola” and “Vicki” are named after individuals of similar distinction.

    At any rate, I’ve never understood why everybody (myself included) seems to have such a fascination with mechanical moles. In theory, they’re really not very interesting: just big boring machines.