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.
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.
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.