Sunday, January 25, 2009

The Disaster of the Niagara Tunnel - Part 5

Part 1, Part 2 Part 3
Part 4

Ok, let's put this one to bed, and let the pennies fall as they may!

The poor little Austrian tunnelers proceeded as they always had before: bid low, and pile on the extras! But this time would be extra worse!

They were used to tunneling in uniform rock with the major stress aligned with gravity.

If you bore a hole in such a rock, you will get a very benevolent stress distribution. Low stress in the roof, which has to be tacked in with a few bolts, and low stress in the floor, which is settled by gravity. The walls nearly always take the higher stress because of the increased confinement of the circular geometry. This is extremely stable.

On the other hand, there is a problem if you dare punch a circular hole through rock with high horizontal stresses.

This is actually quite a bad situation. Gravity is no longer your friend! At the roof, the rock starts to go back in time, like the Button Guy, and all the formational micro-shears are activated. The shale interbeds start to crumble, leaving the limestone all alone. The intense stress activates the latent jointing, and big hunks of rock start to fall. The exact same thing is happening at the floor, but nobody ever notices that!

As the failure progresses, weird and wonderful things happen. The stress continues to concentrate at the tips, causing more failure, and the stress lines shift. That means more and more of the side rock mass is becoming stress relieved. This is just like our laboratory experiment! The rock slowly expands into the tunnel with a tremendous force.

What is to be done? In a normal overbreak situation, the loose rock is chewed out (scaled), and filled with concrete. The liner has to be a lot thicker to accommodate the extra weight. This, of course, drifts us farther away from 'fantasy'. A thicker liner has a tremendous effect on flow, thanks to the cube law.

This is not a normal situation! I frankly don't have a clue what to do! As the sides continue to expand, they will crush any liner. Nearly all corrective work is done on the roof, and never on the floor. I fear the whole tunnel is lost.



1 comment:

Silver Fox said...

A nice series. I like your diagrams! Send me a Star Trek teleporter when you get an extra one.