Thursday, April 7, 2011

The Chiba Story (liquefaction) 2

Liquefaction at its most serious looks like Kobe after the Great Hanshin Earthquake: solid ground loses its solidity, and buildings topple. Now, Chiba prefecture has been afflicted by soil liquefaction.

What does it look like in Chiba?

It sounds cartoonish, but it is horribly serious.

For instance, you can only fill a glass half full. If it's on the table in your house, neither table nor glass is standing up straight because your house is leaning at a crazy angle. If you walk around outside in the affected areas, you might feel suddenly tall because heavy utility poles have sunk to a fraction of their former height. Or maybe you will feel suddenly dwarfed by odd mushroomlike structures sprouting from the streets, because manholes and their hollow concrete cylinders have floated up.

What do you do with a house that didn't fall down, but that won't let you sleep in it because you keep rolling out of bed? Let's hope there are smart engineers out there who can figure out a way to fix Chiba and the damage caused by soil liquefaction.

The Chiba Story (liquefaction)

Liquefaction! Do you picture the earth beneath your feet turning to water? Here's what happens, with a little help from Dr. Hazen, the first geologist to explain the phenomenon, back in the 20s.

When the soil is sandy and not well compacted, it's texture is grainy. If pressure is applied from above (for instance, a building is constructed on top of it) the soil can bear the pressure. Any water left in the sandy soil tends to be squeezed out. It moves from high pressure (under the building) to low pressure (anywhere not under the building).

But--if water is suddenly and repeatedly injected among the sand grains--instead of bearing the load, the grains roll around like well oiled ball bearings. The repeated shaking of unstabilized soil by earthquakes is one way water finds its way in. The sandy grains--separated from each other and the friction that holds them together--flow in a liquidy way.

This happened in San Francisco's Marina district in the 80s, in Alaska, in Kobe Port in the late 90s, and in New Zealand earlier this year. Now it is happening in Japan's Chiba prefecture.

Tuesday, April 5, 2011

Mangroves (2)

It is hard to love that which you do not know. Ocean Expo Park in the northern part of Okinawa does its best to show us the marine life that ecologists ask us to care about. We know we are supposed to protect mangrove forests because they guard the sub-tropical coastlines and provide a habitat where delicate creatures can be nurtured.

It is not hard to see mangrove forests above the waterline--tangled branches and lush leaves, not so different from other trees. But it is below the waterline that they do their best work. The photo below is from an exhibit at Ocean Expo Park.

Not many trees manage to shelter birds and fishes at the same time, but that is what mangroves do. In this exhibit, you can see them at work.