Liquefaction in Japan

The derivative Fukushima disaster has obscured the fullextent of the earthquake disaster itself and the all important lessonslearned.  The big lesson is that bigquakes release their energy over time as happened in the big Mexico City quake.  Thus movement will shake structure over anextended period which is rarely engineered for at all, even if it can be.

Way more important liquefactiondamage generally means sinking and this can generally be engineered around bycreating structures able to largely hold together through such an event.  Cellars do tend to act like displacementhulls and load balance is worth thinking about. If anything, such soils also act as dampers reducing the actual appliedshock.

What I find so remarkable aboutthis event is that the direct losses due to structural collapse outside thetsunami zone appears modest which is a testament to improving codes as Japanclearly learned from Kobe and obviously rethought the engineering on theirtraditional structures.  We have seen thesame thing happen in North America wereupgrading old buildings has been occurring slowly but surely and will certainlysave many lives when a major quake does occur.

On the other hand, people refuseto understand that land below fifty feet above high tide needs to be thoughtabout very carefully in terms of tsunami exposure.  There are plenty of building locales withcoastal views without the actual exposure. We all saw the town that understood their exposure sufficiently to builda thirty foot retaining wall or dike. They obviously needed to completely eliminate all building in the dangerzone instead.  It has now been done forthem and we hope they now do the obvious and create a memorial park in the lowarea.

And just how hard is it to sitean ocean side nuclear power plant behind a convenient barrier island in thefirst place?  Why can engineers notunderstand that a once in a five hundred year earthquake event is a ten to oneodds for a plant exposed for fifty years. Fukushimashould never have been situated in an exposed bay, yet I can almost hear thepresenters discounting the tsunami risk as a once in a thousand year event.

Liquefaction major culprit in Japan quake

by Staff Writers

Corvallis, Ore. (UPI) Apr 18, 2011 

http://www.terradaily.com/reports/Liquefaction_major_culprit_in_Japan_quake_999.html

U.S. seismologists studyingthe earthquake and tsunami disaster in Japan say the temblor caused significantsoil "liquefaction" responsible for much damage.

The broad geographic extent of the liquefaction over hundreds of mileswas surprising to experienced engineers accustomed to seeing earthquakedisaster sites, including the recent events in Chileand New Zealand.

"We've seen localized examples of soil liquefaction as extreme asthis before, but the distance and extent of damage in Japan were unusually severe," said ScottAshford, a professor of geotechnical engineering at Oregon State University and a memberof the research team.

"Entire structures were tilted and sinking into the sediments,even while they remained intact," Ashford said. "The shifts in soildestroyed water, sewer and gas pipelines, crippling the utilities and infrastructure thesecommunities need to function. We saw some places that sank as much as 4feet."

Some degree of soil liquefaction, in which saturated soils,particularly sand, gravel or fill, can lose much of their strength and flowduring an earthquake, is common in most incidents, an OSU release said.

However, the length of the Japanese earthquake -- almost 5 minutes --may require a reconsideration of the extent of liquefaction damage possible insimilar situations, researchers said.

"With such a long-lasting earthquake, we saw how structures thatmight have been OK after 30 seconds just continued to sink and tilt as theshaking continued for several more minutes," Ashford said.