Big Bertha Finishes a Giant Hole

Cool. As a former Seattlite, I've been watching the progress. What the article doesn't mention is that the thing we stuck for a couple of years. IIRC, something broke, and it took a long time to dig a hole to access it and fix it. For a time, the whole project was in doubt.

Being scrapped and reused in the finished tunnel is a noble end for this machine's steel structural components. Outside of the systems components there is little resale for the machine as a whole given its now-questionable mechanics and incredible size. The hydraulic, electrical, tunneling-specific mechanical items will likely be reused in future, smaller machines. There isn't as much of a market for the super-sized tunnel boring machines (TBMs) as there is for smaller metro- (6m-ish), sewer/water-sized (3m-ish) TBMs.

The problems encountered on that project were not extraordinary in the tunneling industry, but were significantly magnified by the size of the TBM, the density of nearby structures, and the costs associated with those items. Not to mention the wide publicity the project received and the political football it became.

Unfortunately, for all of our geotechnical exploration and investigations, much of what's beneath us is still a big question mark. Hence much of what lays ahead of a TBM is the same. While the machines have become incredibly advanced, tough ground conditions (flowing sand, highly-stressed rock, highly-charged granular soil, mixed-face) and underground obstructions (timber, steel, sometimes boulders) happen and can/will stop a TBM in its tracks. In the case of Bertha, she found an old steel pipe and did not like it one bit. It took months to remove the pipe and repair the TBM's cutting tools, but that gave way to a much greater problem.

A TBM's rotating cutting head is supported on a massive bearing (super-massive in this case). This bearing has to be sealed against the harsh conditions outside the machine, generally employing a series of sophisticated seals in addition to redundant seals and positive internal pressure. If the seals fail, as they did on Bertha, soil can flow into the bearing and wreak havoc on it. If not caught immediately (often via thermal sensors), the deteriorating bearing can increase the torque required to turn the cutting head against the ground/thrust pressure. Eventually, this will either over-stress the drive gears or over-work the drive motors. With motors and gears the size of those on Bertha, it becomes a massive undertaking. Especially considering only one of those, perhaps neither, are off-the-shelf items.

That's just the TBM issues. They also had trouble dewatering the area in order to safely access the exterior of the machine. The dewatering operation caused ground consolidation and damage to surface structures (differential settlement).

If I understand it, a lot of arguing has been done (and may continue) over who's at fault for that particular steel pipe.

I'm an engineer with a tunnel construction company up here and so you've tapped into my professional life. I could speak at (long) length about it, but most people aren't too interested. It's not too flashy, you might even say it's boring. :-D

The North American tunneling industry is a relatively small and reasonably well-connected industry. Many of the names have worked for or with most of the major players, both on the client and constructor sides. So we all get to hear about each project's "challenges" (often a little more candidly than a news article) and, more importantly, their solutions. As I mentioned, a lot of the problems experienced in a tunnel drive stem from the same root causes. So it's interesting to see how other's have handled their problems and how what type of equipment they were using at the time influenced the outcome.

It's never a dull day when you're blindly driving an armored submarine (in the case of a sealed soft-ground machine).

Mechanical explanations are never boring.

Would love to be a fly on the wall in some of these conversations about boring (as in drilling) challenges!

Actually, I think it's pretty exciting.

I can believe that.

I do look forward to seeing Seattle without the Viaduct in front of the waterfront. That city has changed so much in the last 25 years that I hardly recognize it--and I knew it by heart in the late 80s.