Friday, October 24, 2008

Rock Mechanics 101 - Tunnelling in High Horizontal Stresses

I don't think anybody understands the true horror of what is going on with the Beck tunnel. In order to bump up the speed of the TBM, they are leaving behind a trail of destruction.

If you tunnel a circular hole in high horizontal stresses, the natural inclination of the rock is to try and squoosh your tunnel flat.

This can't be done easily in brittle rock, so the rock at the crown and floor begin to fail, in a progressive manner. There is nothing holding the roof, so the rock falls. But the floor, as well, is turning into rock pudding!

This type of failure can be non-stable, in other words it never settles down. Most tunnel excavations have some rock that fails, but the final cross-section is stable.

You can produce a very stable cross-section in high horizontal stresses, and is done all the time with the big uranium mines up north.

In this case the shape is horizontal-elliptical, and the stresses become even (and below failure!) all around the opening. Leaving it open for 6 months should totally stabilize it, and it would easily last 100 years, after concrete lining. The old Beck tunnels effectively did this by having a strong, flat roof.

1 comment:

Mona Albano said...

That's why eggs are strong: a dome either way