Tuesday, January 12, 2010

Niagara Tunnel Rolls Again - Overbreak Worse Than Ever

December 31st 2009
At the end of 2009, the tunnel is 5480.7 meters (17,981 feet) long at an elevation below the surface of 88 meters (289 feet).
The tunnel over-break is 3.4 meters (11.15 feet).

Yeah, the TBM has started to grind away. If you read through their logs, it's a bit sad on how they always hoped to get into better rock. I think their cost estimates were based on this, so the real final cost has probably gone through the roof! A realistic cost would have overbreak all the way, and at least one more major tunnel failure. Don't hold your breath waiting for them to announce this extra 2 billion!

And now for some rock mechanics fun! This overbreak takes the classic shape of a circular opening in high horizontal stress. They are still pinning weebly steel sets with short 4 foot bolts. But a classic failure would be symmetrical! We are looking at serious rock mechanics here! What a pity we won't learn anything.

For a minute I was thinking that the tbm can actually exert a shear force on the face to cause this, but the use of rock disks should make this impossible. The only explanation is an extremely strong NE direction to the maximum horizontal stress. As I've said before, our measurements showed this in the granitic basement, but never in the upper Paleozoics. But here it is!

This would be a very interesting scientific result, if anybody had a peabrain's hoot about this. The unfailed wing of the asymmetric failure must be marginally stable, and the bolts don't reach through the damaged zone. When they hang the extra 1 ton per metre shotcrete load on this, I will close my eyes....


Jacek said...

do you know if anybody modeled it numerically?

Harold Asmis said...

No, it would be an interesting project. You would need full 3-d modeling.

Anonymous said...

Perras 2009 looked at horizontally laminated ground response to stress. Check out his thesis here. Lots of great models that explain what happened at Niagara.