Quakes Revive Liquefaction Woes
Jan. 31, 2017
A series of powerful earthquakes hit Kumamoto Prefecture in Western Japan last year.
A survey of the damage revealed that many buildings were not sufficiently prepared to deal with the loosening of soil, a process called liquefaction.
The quakes made nearly 3,000 buildings tilt or sink into the soil.
One of the people tasked with assessing the damage is Hiroshi Takamori.
He develops technologies to help make stronger foundations for buildings in Osaka.
More than 9 months after the quakes, Takamori still makes regular visits to the area. He offers advice on how to repair damaged houses.
He chose to go on this mission after the devastating earthquake that rocked Kobe, near Osaka, in 1995.
"If I had told more people about the risks of liquefaction and the importance of taking countermeasures, more would have been aware of the dangers," he says.
In the 1995 quake, liquefaction was especially bad in reclaimed land along the coast. Many housing units were damaged.
At the time, Takamori was an engineer for a housing construction firm. He witnessed for the first time just how much damage liquefaction can cause.
"I was speechless. I've never seen houses sinking. The first thing that came to my mind was: What can be done to prevent it?"
After the disaster, Takamori and other engineers in the housing industry teamed up to develop preventive measures.
When an earthquake shakes wet soil, the soil can weaken, causing buildings to sink or tilt.
One safety measure involves driving piles through the soft layers of soil and down to solid ground.
But a survey of Kumamoto revealed that even buildings on top of pilings suffered damage.
One resident of the hard-hit areas is Rumiko Ichikado.
Her house was built in 2003. At first glance, it seems fine. But it actually sustained serious damage.
Liquefaction occurred in the ground around the house. The entrance area fell 16 centimeters, requiring major repair work.
"I feel uneasy when I lie on the bed or the floor. When my head is on the south side, I feel like the ground slopes upward. I don't think that it's OK to live in this house any longer," says Ichikado.
The house was built 8 years after the 1995 quake. A survey was done before construction and it was decided that it would be built on pilings.
"I never imagined that the house would tilt, or liquefaction would occur in the first place, because piling work had been done, " says Ichikado.
Takamori examined the ground layers and found the measures that had been taken were not enough to prevent liquefaction.
The piles were driven 5 meters deep. There is a firm layer at that depth but the layer is only a meter thick. Below that is more soft soil.
The liquefied soft layers in between affected the firm layer. Takamori believes that is what caused the house to tilt.
He says there is a firm, thick layer of soil 10 meters below. If the pilings had reached that far, the house might have been saved from damage.
Takamori believes there are many other areas of alternating layers of firm and soft soil.
"I think there weren't enough machines or knowhow to conduct surveys. It's necessary to dig deeper into the ground to assess the risks of liquefaction to come up with effective preventive measures," he says.
Damage assessment in Kumamoto has shed new light on the challenges. Takamori wants more people to learn from the lessons drawn from previous disasters.
"Liquefaction won't kill anyone, but it ruins people's life plans, because they won't be able to live in their homes. To prevent liquefaction, a careful ground survey is highly important before building a new house," he says.
NHK World's Kyoko Tashiro in Osaka discusses liquefaction with NHK reporter Shu Morino.
Tashiro: Based on the experience of the 1995 quake, piles were driven down to a depth of about 5 meters. How much would it cost to do a survey even deeper than that?
Morino: To conduct a ground survey to find out if there is a firm layer of soil 10 meters down, it is said to cost around $1,700. To drive a pile to that depth, will require a maximum additional cost of about $17,000. It will be a heavy burden.
However, in order to repair houses damaged by liquefaction, it will usually cost over $43,000. In the worst case, houses must be rebuilt. Experts say, for areas with high risks of liquefaction, taking preventive measures will bring a sense of security to those living there.
Tashiro: When thinking about ground surveys, what kinds of areas are at high risk of liquefaction?
Morino: Risks are especially high in reclaimed lands near the coast and areas along the river. Six years ago, I covered liquefaction caused by an earthquake in Christchurch, New Zealand, and there too, more damage occurred in the riverside areas.
Earthquakes worldwide, for example, California, US, L'Aquila, Italy, and Taiwan, also caused liquefaction near the river. Cities are often built near big rivers so we must be careful.
Tashiro: But it's not only areas near water that are at risk, is it?
Morino: Caution is needed for reclaimed lands which were previously rivers and reservoirs. Just by looking, we won't be able to know the danger, but experts say heavily damaged areas in Kumamoto also involved reclaimed lands which used to be rivers.
Non-experts can also find out the softness of the soil layer by comparing past aerial photos and maps to current maps. Paying attention to the soil layer of your home is necessary to prevent losing your house by liquefaction.