Wednesday, April 27, 2011
Liquefaction in Japan
disaster has obscured the full
extent of the earthquake disaster itself and the all important lessons
learned. The big lesson is that big
quakes release their energy over time as happened in the big Fukushima quake. Thus movement will shake structure over an
extended period which is rarely engineered for at all, even if it can be. Mexico City
Way more important liquefaction damage generally means sinking and this can generally be engineered around by creating structures able to largely hold together through such an event. Cellars do tend to act like displacement hulls and load balance is worth thinking about. If anything, such soils also act as dampers reducing the actual applied shock.
What I find so remarkable about this event is that the direct losses due to structural collapse outside the tsunami zone appears modest which is a testament to improving codes as Japan clearly learned from Kobe and obviously rethought the engineering on their traditional structures. We have seen the same thing happen in
North America were
upgrading old buildings has been occurring slowly but surely and will certainly
save many lives when a major quake does occur.
On the other hand, people refuse to understand that land below fifty feet above high tide needs to be thought about very carefully in terms of tsunami exposure. There are plenty of building locales with coastal views without the actual exposure. We all saw the town that understood their exposure sufficiently to build a thirty foot retaining wall or dike. They obviously needed to completely eliminate all building in the danger zone instead. It has now been done for them and we hope they now do the obvious and create a memorial park in the low area.
And just how hard is it to site an ocean side nuclear power plant behind a convenient barrier island in the first place? Why can engineers not understand that a once in a five hundred year earthquake event is a ten to one odds for a plant exposed for fifty years.
should never have been situated in an exposed bay, yet I can almost hear the
presenters discounting the tsunami risk as a once in a thousand year event. Fukushima
Liquefaction major culprit in
by Staff Writers
The broad geographic extent of the liquefaction over hundreds of miles was surprising to experienced engineers accustomed to seeing earthquake disaster sites, including the recent events in
and . New Zealand
"We've seen localized examples of soil liquefaction as extreme as this before, but the distance and extent of damage in
Japan were unusually severe," said Scott
Ashford, a professor of geotechnical engineering at and a member
of the research team. Oregon State
"Entire structures were tilted and sinking into the sediments, even while they remained intact," Ashford said. "The shifts in soil destroyed water, sewer and gas pipelines, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as much as 4 feet."
Some degree of soil liquefaction, in which saturated soils, particularly sand, gravel or fill, can lose much of their strength and flow during 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 in similar situations, researchers said.
"With such a long-lasting earthquake, we saw how structures that might have been OK after 30 seconds just continued to sink and tilt as the shaking continued for several more minutes," Ashford said.