Sunday, December 7, 2014

Seismic Hazard and Dam Safety


The stupidest thing I ever saw at the old company was their dam safety program.  Now BC Hydro is going down the same path.  The massive report that employed every seismic person in the world was a seismic hazard analysis, and what we want here is seismic risk.  One of the first things I did at Wikipedia was to nail down those definitions.

Seismic hazard is just the plotting of expected ground motions.  The big no-no is that they still work with accelerations, as though they meant something in physics.  Peak acceleration is not tied down by physical reality.  Just as bad, is the next step -- response spectra in acceleration.  Nobody wants to fix this fallacy because they all make too much money on it.

In Ontario, the acceleration zoomed up with the new calculations, mainly because accelerometers showed very high accelerations for locations on rock, and in the near-field of the epicentre.  If they were ever smart enough to put down accelerometers in Oklahoma, they would get a surprise with those shallow thrusts, right in the hammer zone.

So, at high frequencies the acceleration is very high, but there is no physics here.  In Ontario, all the dams are solid concrete on solid rock.  If you calculate the response spectrum, you will find the fundamental is at a high frequency.  Thus, combine the high response spectrum of the hazard study, and you will find that your conventional seismic analysis has the thing sliding down the river.  Millions of dollars were spent in anchoring these things down.

As well, if you measure the uplift pressure in solid rock underneath the dam, you will find it really high.  The dam is being lifted up!  Again, the wrong physics.

Believe me when I say none of this is real.  The PGV is very low and nothing will happen to those dams.  The poured concrete has a permeability orders of magnitude above the rock, and acts as a drain -- no uplift.

BC Hydro will now have their fun.

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