Tuesday, December 29, 2009

Shotcrete Fill - Tugging the Dragon's Tail

In our last report on the Niagara Tunnel, we noted that the overbreak will be filled in by layers of shotcrete, in preparation for the main liner. I was worried about this, since it reduces stability, or the safety factor. Since we don't know anything about the tunnel, this may or may not be a problem.

I am working blind here, since all the information about the main rock burst has been kept secret. I curse a pox on the houses of OPG and the Ministry of Labour for this! This doesn't do the country any favours.

Be that as it is, I feel no shame in making wild guesses, since I am the best I have! :) If I had seen the damage, I would have noted how the 4 ft rock bolts pulled out, and whether there was horizontal or vertical force. My suspicion would be that the side of the tunnel bucked first, before the roof fall. You would tell that by seeing the side bolts thrown out, instead of merely sheared.

If the failure was due to roof pressure, then we are increasing the chance of another rock fall (another forgotten borehole? ha, ha!), as per the diagram.

The shotcrete adds extra weight to pull the roof, without the added stability of an arching action. Note that the overbreak has always had a slight offset, as if the tunnel were intersecting the principle horizontal stress at an angle. I found this to be bizarre, since we always measured nearly uniform stresses in the Paleozoic, and only strong NE polarity in the granitic basement. Perhaps the stress is much great than I imagined? I sure wish there had been pictures for the failure, since this would tell us more about the stress than anything.

So, why are we tugging the dragon's tail? Because this work will be done directly under the rock mass on a light platform. You are pulling at a giant unstable rock mass! Like digging at the toe of an unstable slope. Is it enough to pull it all down? Unfortunately, we shall see.... If it were me, I'd be layering on styrofoam.

On the positive side, once they get the liner in, they should be able to fill it up with water before they see the cracks. Then, they only notice water flow reduction in about 20 years. Of course, we'll never hear about it!

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