Saturday, February 2, 2008

Rock Mechanics - 4

Rock Mechanics
Rock Mechanics - 2
Rock Mechanics - 3

At the time, 30 years ago, PGV was never recorded, let alone discussed. So I made a study of the Modified Mercalli Index, which had recently been tied to PGV, and was the best surrogate. The PGV approximately doubled for every jump in the MMI, and I began to collect stories about the several point difference between rock and soil.

Thus began my classic report Seismic Ground Motion on Rock and Soil (which I loved!). In that report, I looked very hard for situations where there were both rock and soil reports of ground shaking. It was difficult, since nobody ever reported where there wasn't any damage!

I was lucky to find this absolutely inspirational book in the old library at Hydro (since kaput!): Freeman,J.R., 1932. Earthquake Damage and Earthquake Insurance. He was a total rationalist, and looked at actually earthquake damage and non-damage. (I should have stolen that damn book!).
He noted that there was no damage on Telegraph Hill (solid rock), and buildings designed with a flat 10% lateral load did quite well. The difference in intensity between the hill, and the landfill harbour was about 5 points! Later, I had the opportunity to walk the Hill, and view the harbour, but I digress.

I had lots of stories, for example the 1944 Cornwall earthquake. On the Leda clay, people couldn't stand up, and there was extensive damage. But according to my old grandfather-in-law, there was nothing on the highlands (hard till). The 1925 St. Lawrence earthquake devastated the river lowlands, but didn't wake the guests at the Chateau, nor plink off any icicles!

After this, I concluded that PGV was exceedingly low on solid ground (rock or hard till), and soft sludge could amplify by a factor of 10 to 100 times!

I, therefore, went with a max of about 5 cm/s on rock for Eastern North America (ENA), and set about to check the stress levels for this PGV. As well, I had to determine the 'sensitivity' of the rock to stress changes.


monado said...

Interesting! This seems to be more recognized now, e.g. that Tokyo's soft soil magnifies earthquake damage.

Harold Asmis said...

It's only very recently that the true amplification factors of 10-100 are being recognized. Before that, it was only about 2. The Canadian building code is probably still behind on this.

Harbles said...

San Francisco, liquifaction ...

But what of soil deposited on top of hard rock like say a river valley?

Harold Asmis said...

Deep soil basins usually get a factor of 10 from the basin geometry, and another factor of 10 from the soil. This is what happened in Mexico City.

Our Canadian soft soil pockets on rock are extremely bad, since they trap all the seismic energy, due to a very sharp, reflective interface between the soil and the rock.