Rock Mechanics - 3

Rock Mechanics
Rock Mechanics - 2

Nobody had actually measured the peak stresses of a passing seismic wave, so I needed to go for another measurement. I did some computer modeling, and some research, and came to the conclusion that Peak Ground Velocity (PGV) offered a good correlation with peak stress. The only problem was, at the time, nobody gave a damn about PGV.

30 years ago, the seismic world was totally dominated by California. Strong ground motions, correlation with damage, etc, was all from the deep soil basins. The whole concept of Standard Response Spectrum and modal seismic analysis was due to the fact that basin motions were low-frequency, long rumbling things. In this world, the measurement of Peak Ground Acceleration (PGA), expressed as a percentage of g, was all the rage.

I knew there was something wrong here. We had sent a crew, measuring stresses at the recent Miramichi earthquake, and had found something unusual. The crews reported that the ground was crackling with small earthquakes after the main event. Some would really thump the drill rig! I knew that if you had measured the PGA, you would get around 1g, but clearly these things had no energy to damage. I was convinced that PGA was a useless measure for the rest of the world!

When I went back to computer modeling, I found there was no physical limit to PGA, since you could just increase the frequency, and PGA would go through the roof! It was only in California, where the frequencies were forced to be low, that it was valid.

Thus, began my great campaign to wipe PGA off the seismic map! After 30 years, there is now some glimmer of hope! But I digress, as usual. My next step was to see if there was some limit to PGV on rock, and thus a limit on peak stress. As well, I had to determine if there was some indication of how much peak stress was required to damage a tunnel or cavern.