Q/A -- Building the Tunnel, Part 2: At Home Inside a 20,000 Horsepower Tunneling Machine
The Delta Conveyance Deep Dive video series continues with “Building the Tunnel Part Two”. In Part One we heard about the state of the art technology behind tunnel boring machines. In the second episode, Delta Conveyance Design and Construction Authority (DCA)Executive Director Graham Bradner and the project’s chief engineer, Steve Minassian, discuss tunneling in the specific context of the Sacramento-San Joaquin Delta and describe what to expect if the project moves forward.
Graham Bradner has worked as an engineering geologist on major resources projects in Northern California for over 20 years. He was an integral part of the team that developed the conceptual designs for Delta Conveyance and became executive director earlier this year.
Steve Minassian has over 30 years of national and international experience with mega infrastructure projects, including the Lake Mead Tunnel in Nevada and the Port of Miami Tunnel.
DWR’s Pat Clark hosted the interview.
PAT CLARK: In the first episode we learned that for a project of this size, there would be multiple contractors involved. How many tunnel boring machines (TBMs) would you expect to be operating at the same time in the Delta, and what's the estimated length of each section, or “reach”, of tunnel?
STEVE MINASSIAN: The minimum number of TBMs we would have on the project will be four. That will be for the Bethany alternative [corridor]. The maximum we would have would be six, which will be for the eastern or central corridors and using a 7,500 cubic feet per second (cfs) flow rate tunnel. More than likely we'll have four or five because we will more likely have the 6,000 cfs rate tunnel. Each TBM will be launched at a different launch site and will be launched at different times. So, depending on the length of the reach(for example, some of the reaches are 12 miles, some of them are 14 miles) these are on the critical path. We would launch these first to make sure we don't delay the project. Then the other TBM reaches, which are eight miles, will be launched later.
So, there will be simultaneous excavation of these TBMs, but at the same time they will be completely independent projects and most likely different contractors. It’s all planned out, as Graham mentioned earlier, according to how to handle the reusable tunnel material, the length of the reaches, and the related logistics. Graham, anything else you'd like to add?
GRAHAM BRADNER: Well, I think it's worth emphasizing, we've talked about the construction activity that would be taking place at these launch sites and it’s significant; so much so that we've looked at the various combinations that Steve was describing, and we’ve determined that it's definitely beneficial to have as few launch sites as possible and have some of these longer tunnel reaches.
SM: You know, one thing I want to mention; something like this is not unprecedented. For example, years ago I worked in the Middle East, and we were working on the Doha Metro in Doha, Qatar. They had a very aggressive schedule to build the metro and at one point in time, I think, we had approximately eight or nine tunnel boring machines excavating at the same time. So, having four, five, or six is not unusual. What is unusual here is the size of these machines because these would be more like 40-foot diameter tunnels versus a typical metro tunnel which is closer to 22 or 23 feet in diameter.
PC: Steve, since you do have so much personal experience working with TBMs, I wanted to ask you what's it like inside one of these machines? And how are they kept watertight?
SM: That's a very good question, Pat. I've spent a lot of time in my 35-plus year career in the tunnels, and when I go in the tunnel, near the machine, I actually feel like I'm at home. More at home than I am at home. When you get to the heading, which is where the work is ongoing, where the tunnel boring machine is, it's very exciting. There's a lot of activity. On one hand, the machine is excavating the material. You see, you feel the machine. You hear it underground and these machines are very powerful. Something this size is probably close to 20,000 horsepower. So, you get a lot of power and it's doing the physical work of excavating the material. You see the material coming out of the conveyors, then once the machine push makes its stroke, which is usually five or six feet, then it stops. Then you build the next ring, which is what the precast concrete segments are.
The machine itself is watertight against the ground and the soil, and as you're installing the segments, right behind the tail shield you have three and sometimes four sets of tail seal brushes, and in these brushes there is a biodegradable material pumped in so the water and the [soil] fines don't come into the tunnel. There’s usually seven or eight pieces of the ring for something this size. They have bolted, gasketed segments and the gaskets are designed to keep the water and the fines out. So, the tunnel that's behind us is dry. You don't see any water coming in. You may see occasional seeps or maybe just a drip here and there, but for the most part, 99.9 percent of it is dry. We've done things like this under large bodies of water, underneath the oceans, rivers, and lakes. For example, Lake Mead and the Port of Miami tunnels were constructed under very similar conditions where we cross levees and so forth. It's not a problem. It's not an issue at all.
PAT: Well, I'd like to thank you both very much for taking time out of your busy schedules today to talk to us about what construction of the Delta Conveyance Project would look like. It's been such an interesting discussion.