Thoughts of the long-anticipated Tokai Earthquake hitting Aichi Prefecture have worried a company employee, 40, who is having a two-story wooden house built there. Ground liquefaction in a major earthquake can cause a house to tilt--or worse.
To avoid such an outcome, he asked experts to check the condition of the ground at the site before construction of his new house began. The result showed that a big earthquake might indeed cause liquefaction of land there.
The man is one of many would-be individual home builders in the nation who became interested in strengthening the ground at their building sites after the Great East Japan Earthquake caused liquefaction in many places.
Measures against liquefaction are legally mandated in the construction of condominiums, but the requirement does not apply to building detached houses in most cases.
If people want to check the ground strength or apply measures against liquefaction before building a house, they must enlist the help of specialists through housing contractors.
The man in Aichi Prefecture asked such a business to improve the ground conditions at his site. It cost him a total of 700,000 yen for the inspection and the reinforcement work. "I cannot feel safe unless not only the building itself but also the ground is strengthened," he said.
In September, Media Interactive Inc., a Tokyo-based survey company, questioned a total of 500 men and women over 30 years old about factors they would weigh heavily in buying a house after the March 11 disaster. According to the poll, 41 percent answered "the ground condition of the building," second only to "the quake resistance of the building," which was cited by 47 percent.
HyAS & Co. Inc., a company that reinforces ground, has been nationally promoting its "high-speed construction method," which refers to reinforcing the ground at a housing site by burying crushed stones in the soil in a pillarlike structure.
An official at the company said, "Since the disaster, we've received an increasing number of orders from customers who want us to strengthen the ground to avoid liquefaction."
In case of building condominiums, it is mandatory to reinforce the ground if there is a possibility that liquefaction would occur by a big earthquake, according to the Building Standards Law.
However, it is not required in building one-story wooden houses or two-story houses if they are smaller than a certain standard. This is because the envisioned damage to a house from liquefaction is not as serious as the damage to a condominium building and also because it would likely be a great burden on individuals who wish to build houses for themselves.
Tokai University Prof. Mamoru Fujii, an expert on liquefaction, said the liquefaction-induced tilting of a house can be at least partly prevented if the ground condition is strengthened. "However, it's necessary to decide the degree to which such measures should be taken based on [the household's] financial considerations and the scale of the envisioned earthquake," he said.
The first step is to determine whether the ground can be expected to cause liquefaction. Liquefaction is likeliest in sandy ground with a high level of groundwater. Reclaimed land, drained land and areas that used to be rivers belong to this category.
To check the history of a plot of land, old maps, past aerial photos and charts that show the structure of the ground are useful. They can be viewed on the website of the Geospatial Information Authority.
According to Jutaku Jiban Hinshitsu Kyokai, a Tokyo-based nonprofit organization for assessing ground quality, it costs between 100,000 yen and 300,000 yen for an inspection of ground conditions.
Common reinforcement work when the ground is judged to be prone to liquefaction includes firming up the ground by mixing cement with soil, according to the organization. The cost can run from about 800,000 yen to 2.5 million yen depending on construction methods for a two-story house with a footprint of 50 to 70 square meters.
Information about providers is available on the organization's website: www.juhinkyo.jp.
(Dec. 14, 2011)