Monday, January 28, 2008

Rock Mechanics - 2

I don't even think these stories link together, they are the ramblings of an old guy who is finally healing from his stint in a dysfunctional techno-bureaucracy.

Rock mechanics has been mostly concerned with rock failure under the following-load of gravity. If your tunnel doesn't fall down on your head, you're happy! What I was studying 30 years ago, was the impact of earthquakes on caverns and tunnels.

Underground workings have generally done very well with seismic shaking. They do not resonate, and thus, are not very amenable to conventional seismic analysis, which I think sucks, anyway! For underground structures, I had to go into a totally different type of analysis, which is just now being applied to buildings, thanks to cheap computing!

Probably the best way to look at the seismic performance of underground things, is to use the methodology of 'Experience Data', which I used extensively for nuclear plants. Around the world, there have been many tunnels and mines exposed to strong ground motion. The performance has varied from very good, to very bad. "It never rains, but it pours!". If a tunnel fails during an earthquake, it can flood within seconds!

I went into the analysis of what actually happens to a tunnel during an earthquake. The earthquake source radiates seismic waves, which is easy to model with a wave propagation program. What I discovered 30 years ago, was the variation of peak seismic stress with distance. Although there is virtually no limit to peak stress in the near-field, seismic waves that propagate a decent distance, cannot be non-linear. In other words, the peak stress disturbance must go down to the level where it does not interact with the rock! This is extremely important, and saves us a lot during earthquakes.

What is the maximum stress for a propagating seismic wave? The search for that took me on another tangent....

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