On July 12th, @metrolosangeles used Twitter to crowdsource questions about the two tunnel boring machines (TBMs) that are currently being manufactured and will dig the tunnels for the Purple Line Extension’s first section.
Below are the answers to your questions:
Reed Alvarado @Reed_Alvarado: Do they have a start date for boring to begin?
The TBMs are currently being manufactured by Herrenknecht in Schwanau, Germany — the firm is one of the world’s leaders in tunneling technology. The TBMs are anticipated to arrive in Los Angeles toward the end of 2017 and are expected to start work next year after they have been assembled and tested.
0ntir @0ntir: Do they, unlike those which drilled the UK’s Chunnel, have a “reverse” drive?
The TBMs that will bore the tunnels for the Purple Line Extension do not have a reverse gear. Even if they did, the machines could not back up. This is because they place concrete rings behind them. The circle formed by these rings is smaller in diameter than the TBM itself.
LA Denizen @LA_Denizen: What happens if it hits a methane pocket?
Fortunately, methane needs precise conditions to explode. The physics of an explosion requires that the methane/oxygen ratio be in a specific range and a source of ignition (a spark of some kind). If the mix is too lean or too rich or there is no spark, the gas will not explode. Sixty to ninety feet under the surface there is no air in the soil, other than the small amount in the TBM’s mixing chamber, and there are no ignition sources in the chamber, therefore the potential for an explosion in front of the TBM is prevented through engineering controls.
Colin Parent @ColinParent: Do they have fun names yet?
The TBMs are currently referred to as S986 and S987. An art and naming contest will be launched in early fall 2017 to give the Purple Line TBMs proper names before they begin their journey through the earth.
The truth is, you never know what you will find under there. However, since the TBMs will always bore at least 20 feet under the subsurface water table they’re expected to run into two major types of geological formations, which are known as San Pedro and Fernando. The excavated material will be trucked off-site for recycling if possible.
We’re always looking forward to satisfying our inquisitive audience, so here are some more interesting facts about the Purple Line TBMs.
Size: the TBMs are about 400 feet long — or the length of 10 school buses for perspective. The cutter head measures 21 ½ feet in diameter. This is big but not as big as Seattle’s Bertha which is a gigantic 57 feet in diameter (and is digging for a road project). Each TBM will weigh about 1,000 tons. They’re as heavy as 167 wooly mammoths, a typical male woolly mammoth weighs approximately six tons!
The path: The TBMs will dig side by side starting at Wilshire/La Brea in the Miracle Mile section of Los Angeles and continue toward Wilshire/Western in Koreatown. Then they will be disassembled for transporting back to Wilshire/La Brea and then reassembled for the last leg of their journey through Wilshire/Fairfax and finishing at Wilshire/La Cienega in Beverly Hills. The TBMs will be digging up to 60 feet per day to create the 3.9 miles of new subway in Los Angeles and Beverly Hills.
What’s that?: Noise and vibration levels are not expected to be perceptible. Even imperceptible levels of noise or vibration will be captured and evaluated by engineers through an array of monitors placed along the project alignment. From time to time, the public near the tunnel entrance may hear the sound of a horn — similar to that of a car — that is used as a safety signal and cautionary measure to alert construction crews of a service train entering or exiting the tunnel. This can happen as frequently as every hour.