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re: Over-Engineering

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http://news.discovery.com/earth/balanced-rocks-earthquak=e-hazard.html

I have had some visits with Jim Brune about 13 years ago. His precarious rock studies are not new. The commercial interest is to find storage sites for nuclear waste. There might be a limit to how high seismic accelerations can reach in a 10,000 year event.

Dr. Brune has a personal interest in my Sierra Mountains cabin. There is a standing precarious rock on the cliffs behind. That rock's position might be about 10,000 years old. Storing nuclear wast, with 10,000 year half life, in my back yard might make me some money and preserve the wilderness like setting! My real interest is in the cabin. It is a 150 year old two story wood house with stud-less interior walls of 1x24 vertical planks and news paper.

A precarious rock founded on hard rock soil types and with a low natural rocking period, might be quite safe from tipping over. That is not the issue. When projecting our few hundred hundred years of estimated earthquakes into the future of about 10,000 years, the resulting design loads are suspiciously high. There might be a limit to how high seismic accelerations can reach. Energy dissipation of the soils may increase more than the increase of energy for a seismic event that is approaching a 10,000 year return period. Say, in a 10,000 year event, the landscape might move over 20 feet. A balanced precarious rock may prove that it did it more gently than expected.

The rock, pictured in the article referenced, is unusual. Dr. Brune's studies are of more stable rocks that are of harder composition and not as susceptible to wind erosion. I found his test methods interesting. I like that he hauled his electronics with horses or mules and camped on the way. They attached motion sensors and pushed over large precarious rocks a slight amount and released them to create motion. His first estimates were mathematically modeled by hand. The equations are too long, increasing the chance of error. I hope he has switch to computer modeling of a solid mass with non-linear links to the base. I was interested because of my experience with some large concrete blocks used for a Livermore lab building, rocking but with vertical post-tensioned tendons.

I heard President Obama has canceled the nuclear storage project in the Yucca Mountain, Eureka County, Nevada. I have not heard what issues have not been resolved. Is a 10,000 year seismic event part of it?

David B. Merrick, SE
Sacramento, CA

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