Thursday, February 14, 2008

Rock Mechanics - 6

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

I was working on the sub-theory that propagating seismic waves could only carry a little bit of stress, due to the general weakness of the rock. For that to hold, the basement of ENA would have to be at its Limit State, which means it is as close to failure as you can get, and still be relatively stable.

At the time there was a lot of evidence in favour, and subsequent papers continue to support this. Essentially, we have an ancient craton, that has been pushed and pulled to a great extent over the last billion years. It would get greatly pushed (compressed) in between expansion cycles, when it was over cold mantle. It would be pulled (tension) when the heat built up underneath, and the continents were splitting up again.

Currently, we are in a big cold trough, and the craton has settled in. This puts it a generally high compressive state. You can see that whenever the water head is increased by about 10 m (induced earthquakes), you get earthquakes, or when a large extent of rock is removed (as in quarrying), of about 3 m.

Into this mix came glaciation, which had a tremendous effect on the rock. Although a uniform ice load would merely act as a big wet blanket, the ice loading is far from uniform. During surges, and retreats, it builds up a very high shear stress, combined with high water pressures being injected into the rock. I don't think that anyone has appreciated this, except moi!

Although the glaciation relieved some stress, it also shattered the rock, through hydrofracturing (which is the splitting of rock through injected fluid pressure). My prediction from this, was that we would see extensively fractured rock down to about 1 km, at relatively low stress, then we would see unfractured rock at very high compressive stress. In other words, the measured stress increased with the rock's ability to hold it.

I was lucky to have this sub-theory tested with the construction of the mostly useless URL (Underground Research Laboratory), which was a great "FEED ME!" AECL gift. This mine went about 1 km down, where they encountered great big 'sub-horizontal' features, which were essentially underground rivers hooked to the surface. Seeing that this was probably not a good thing (except for Bruce!), they proceeded to go under these rivers.

What they encountered was extremely high stresses. So much so that the rock virtually exploded when touched! We really couldn't put used fuel here because of all the heat produced, and the rock would be shattered within a decade! Thus, URL ended both with a bang, and a whimper!

So, in the end I had good justification that the maximum stress from a seismic wave was only about an atmosphere, and the PGV was limited to about 5 cm/sec. This was scarcely enough to spill a coffee in a mine, and shows how even large earthquakes mostly do nothing to mines. Still, I was determined to model it in a computer, because that was fun!

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