Sunday, February 17, 2008

Rock Mechanics - 7

Rock Mechanics
Rock Mechanics - 2
Rock Mechanics - 3
Rock Mechanics - 4
Rock Mechanics - 5
Rock Mechanics - 6

I've always tended to approach analysis slightly differently than most people at the old company. They always wanted to go for the 'Big One' that proved a political point, such as 'the reactor is safe'. I always called that 'Snow, by analysis'. I used analysis to improve the fundamental science, since I knew what uncertainties were (as opposed to them!), and I wanted to see the effects of 'what if's'.

I love working with wave propagation model (no, not that kind!) This is the most fun a dynamics person can have cheaply! My first work was with code I had written myself, and then I eventually went to commercial code. But for that, you needed powerful Linux computers, and not the namby-pamby Windows crap that bureaucracies like to buy! Needless to say, in modern times, I overran the computing power I had, and became bored. Still, I learned something.

With my simple wave modeling, I showed that the tunnel was a small fraction of the seismic wavelength, and did no 'funny stuff'. To show funny stuff, I propagated into beds of soft soil, and saw tremendous amplifications!

I concluded, that under normal, well-designed conditions, a tunnel had nothing to fear from earthquakes, and this has been proven out in the field. But what of poorly designed underground structures? They had a serious problem...

Thus I created (just now!), Harold list of tunneling bo-bo's:

-Don't rely on grout! Any water channels get reactivated by seismic motions. The tiniest amount of differential movement can shatter grout. Many tunnels have been instantly flooded after an earthquake.

-Don't have open zones, or major fractures near your cavern. Earthquakes can suddenly change the regional groundwater flow. Seismic ground motion from very far away can dramatically affect well levels, and even blast natural gas into the air.

-Build in a lot of margin. The stress impact of an earthquake is less than the stress changes you would expect over the life of the cavern. It has to be designed so that there is no chance of spontaneous failure, or rockbursts.

This is the end of this series.

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