One of the perks of the State Rep gig is that I get some cool tours. I’ve been inside pontoons on the 520 bridge, an aircraft carrier, Sound Transit tunnel construction, and last week the orifice the Bertha is creating. I took some photos that illustrate some aspects of the visit, though it’s hard to capture the sense of the project from a single walk-through.
The entrance to the actual tunnel is a large pit that I believe will eventually form the floor of the lower level of the tunnel. Right where the red erector set-like construction is in the back the level drops down and you’re below the level of the driving surface.
The tunnel is made by a huge drilling machine, the Tunnel Boring Machine (TBM) or “Bertha” to its friends. Trailing along behind the TBM is a vast collection of machinery that installs concrete rings, one after another. The TBM pushes itself forward by applying immense pressure to the existing set of rings with huge hydraulic jacks.
The picture here is of a stack of segments that haven’t been installed yet. Each segment has a particular order in the installation. You can see the bolt holes in the sides and end of the ring in the foreground. I do not have wrenches large enough to move these nuts and I thought I had every significant wrench available at Sears.
Once you enter the tunnel you start to get a sense of the scale of this thing. “Large” doesn’t really cover the concept.
The black strip to the upper left in this photo is a conveyor belt that is used to extract the dirt and rocks when Bertha is actually digging, which she is not doing today. Or tomorrow. Or even next week, but perhaps sometime in the first quarter once they get the bearings replaced… The conveyor belt will eventually become very long.
The orange tube moves air into the other end of the tunnel. The goal being to keep the workers alive and breathing, which seems like a reasonable goal. If there is a fire or some other problem at the head end they can reverse the flow and suck the air out instead of pushing all the smoke out through the tunnel.
More photos that give you a sense of scale, both looking in and out of the tunnel.
The TBM drags along a vast collection of machinery and scaffolding. Most of it supports a set of vacuum devices that pick up segments, rotate them into the correct orientation, and then put them into place. People then crawl all over them doing up the bolts, or so I surmise because no segments are being installed today, or tomorrow…
There’s probably a couple hundred feet of machinery trailing behind the TBM. Sen. Jamie Pedersen is pointing at something, probably one of the rollers moving the machine forward. You can see he is standing on some of the already installed segments, with the trapezoidal bolt holes where they are bolted together.
There’s a lot of water in the tunnel today. I’m assured that as they go through the process of grouting all the holes they take care of this. Puget Sound is a hundred feet away, and the tunnel is below the water table, but I suppose many tunnels are, so this is not a significant problem, one would hope.
My favorite part of the ginormous machine was the collection of jacks they use to push the machine forward and ensure that the segments are locked into place.
Each one of these jacks is at least a foot in diameter and there are a lot of them. It’s hard to get a good photo as you can’t really see all of them at once – there’s a lot of machine in the way and I don’t have a fisheye lens, nor could I have brought a tripod into this thing.
They warned us that if we didn’t like small spaces we shouldn’t go on the tour. This was excellent advice. It reminded me of the time I took my Cub Scout group to OMSI in Portland and slept over inside an old attack submarine. (Best field trip ever – worth doing.)
I think it would be pretty impressive to see this thing moving forward, but I probably wouldn’t have been invited to the tour if it was.
As you can imagine, there’s a lot of dirt that comes out the back end of this thing. It has to get from the front end to the back, and it does so inside a big tube with a screw in it. because the head is underground, and underwater, it’s at several atmospheres of pressure.
If they just had the dirt come out of the back of the cutterhead it would spray everywhere. The screw inside the tube equalizes this pressure, then dumps the stuff gently onto the conveyor belt.
The whole pressure thing is pretty intense as well. When they have to do something up at the cutterhead they have to use professional divers who know how to deal with pressure. They have a decompression chamber they enter the machine through, and then sit in after they have been working inside for a while. They can only work for a short time or they have to spend forever decompressing. If they are going to do a lot of work like that they bring in a crew that stays in a compressed state inside a couple of stacked shipping containers up on the surface. No windows, inside for several days at a time. I’ll bet those guys get paid a lot for this.
The decompression chamber is even more of the “if you don’t like small spaces don’t sign up for this” meme.
To get the cutterhead out they are digging a 120′ hole from the surface. In order to keep the dirt and water from filling up the hole prematurely they are lining it with concrete. A lot of concrete. They drill a 10′ diameter hole and push metal casings down as they drill. Once they get deep enough they start filling it with concrete and pulling the casing out as they go. This is what is taking a long time.
You can also see that this project is happening right on the waterfront in the middle of a lot of buildings, and the seawall project is about 100′ away. It’s a tight space.
I am not a construction guy, but the project manager for the tunnel project seemed to know what he was doing. I’m irritated that Bertha isn’t moving forward, but more encouraged than I expected to be before I did the tour.