There is absolutely nothing boring about a tunnel boring machine (TBM). Today’s TBMs resemble massive subterranean factories, capable of cutting through harder rock and against higher water pressures than ever before. TBMs can now operate in mixed ground conditions and in a host of other environments that would have been impossible as recently as the 1970s and 1980s.
For the first episode in a special two-part Delta Conveyance Deep Dive series on tunnel construction, we invited two of the Delta Conveyance Project design team's leading consultants on tunnel design and engineering to talk about the state‑of‑the‑art technology that would be used to build the Delta Conveyance Project.
Deep Dive presenter Pat Clark spoke with Dr. Greg Korbin, a veteran of numerous big tunnel projects, including the Bay Area's BART extension to San Jose, the Port of Miami tunnel under Biscayne Bay, and the State Route 99 tunnel in Seattle; and Dan Adams, president of McMillen Jacobs Associates, a leading tunnel design and construction company based in San Francisco. Dan worked on the State Route 99 tunnel in addition to over a dozen other large‑diameter, soft‑ground tunnels that carry water, wastewater, and trains.
I have a series of short questions. First, what kind of material will the TBM be tunneling through in the Delta?
GREG KORBIN: The Delta has recent sediments up to 50 to 60 feet deep, which is comprised of peats and looser sands and clays and silts. Once you get below that depth, which is the depth for the Delta tunnel (around 100 feet to a maximum of 150 feet at the bottom of the tunnel) you’re into older alluvium, which is comprised of alluvial fans and delta deposits; denser materials, mostly. It's mostly a silty clay, silty sand with some gravels. It's actually an ideal material for an earth pressure balance‑type machine.
PC: How fast can TBMs tunnel? How long would it take to tunnel one mile, for example?
DAN ADAMS: Well, there's a lot of debate on this. A tunnel boring machine’s tunneling rate is really the meat of how a construction package is put together and whether or not a contractor is successful. So that's a long‑winded way of saying that we probably have different answers. But I was thinking ‑‑ what did we say? A typical 40 feet a day? You're better at math than me, Greg, but is that five months for a mile?
GC: Around six months, yes. A reasonably conservative number is 40 feet a day. It might go 45, but six months is probably about what it's going to take to do a mile, and that's without a major stop for maintenance.
PC: OK. Thank you. Let's talk now about surface impacts from the construction. My first question is will there be a lot of vibration and noise at the surface while the TBM is operating below?
DA: No.We wouldn't expect noise of any kind out in the Delta while the tunnel boring machine is below you in the earth. We have done multiple projects where we're driving a soft‑ground tunnel underneath houses or below buildings. On State Route 99 (SR 99) tunnel project, there were hundreds of residential towers-- condo buildings -- that the tunnel went under. There is very little noise that occurs. It's barely perceptible at the earth's surface from the tunnel boring machine running below.
PC: Would there be a risk of damage to the levees on the surface?
GC: Not really, no. We know how to operate these machines correctly. SR 99 went under roughly 300 major structures and buildings, including high‑rises, in Seattle and no damage at all was done to those structures. And that was a larger tunnel boring machine. That was 57.5 feet diameter as compared to 40 feet for this tunnel. So, we know how to operate these machines correctly. We will probably install instruments at the levees to make sure that everything is going fine, to make sure that no excessive settlements are occurring. So, the short answer is no, there really is not a problem.
PC: Turning to some challenges now. There are many underground gas wells in the Delta. Do they present a hazard?
GC: The gas wells are a hazard and they're recognized as such. There are extensive maps of the gas wells. We'll try to avoid as many as possible. Some have been abandoned. If, perchance, and there's always a chance, something isn't found or hasn’t been correctly abandoned and the machine was to hit one of these wells, there are safety devices on the machine that will immediately shut it down and lock everything off to avoid allowing the gas into the tunnel. Of course, this would lead to a significant delay because they'd have to now abandon the well correctly before advancing the machine. But, again, worker safety would be handled.
PC: Well, I'm going to end with a fun question. TBMs are usually christened with women's names. So, would you get to decide the name for the TBMs on the Delta Conveyance project?
DA: No, I don't think Greg or I would. It’s a common thing that's done for public outreach.
GC: There have been contests where local schoolchildren will submit names and vote on them, which is a nice way to get the community involved.
DA: And they're not always women, Pat.
DA: No, in Boston on the Beacon Hill project the TBM was called the Emerald Mole. Here in Seattle, we now have a project called the West Ship Canal. They recently had a naming contest for that machine, and they announced the winner just last week. The name of the tunnel boring machine is Mudhoney. That was based on 30,000 people voting, and Mudhoney is actually not a woman's name or a man's name; it's the name of a rock band, a famous rock band if you're from Ballard, which is where the job is.
PC: Very interesting. Well, I would like to thank you both so much for joining us to give us a glimpse into what it takes to build a tunnel on this scale. The animations you showed do a great job of demonstrating how sophisticated the technology is. It's simply amazing. Thank you.