Soil and water

A foundation flaw in development considered a factor in the Atami landslide: the Asahi Shimbun

Insufficient drainage in the foundation of the material used to fill a the valley above Atami may have helped trigger a massive landslide on July 3 that left at least four dead and around 20 missing.

Even after a valley has been filled with earth and earth, it will continue to collect groundwater, said Toshitaka Kamai, professor of applied geology and director of the Landslide Research Center at the Institute for Research on Earth. Kyoto University disaster prevention.

If the foundation were to weaken after absorbing a large volume of water, a landslide could trigger along the areas that had been filled in, Kamai added.

“Filling a valley is like building a dam,” Kamai said. “Unless action is taken to release water from the landfill, it could lead to a landslide if the embankment collapsed after filling with water. “

He added that not only did the collapsed embankment enlarge the scale of the landslide by adding soil and dirt along the way, it is possible that its collapse was the main cause of the landslide.

According to officials in Atami City, a real estate company based in Odawara, Kanagawa Prefecture, purchased the approximately 116 hectare area in September 2006. At that time, the area was to be developed into a residential site.

About six months later, the company applied for the development of approximately 1 hectare of land. The surrounding trees were cut down and sand and dirt brought in to fill a valley.

But when city government officials learned that industrial waste, such as wood chips and tiles, was also buried in the area, they repeatedly asked the company to rectify the problem.

The company ignored these instructions and decided in 2011 to abandon the development project.

In February 2011, the owner of a Tokyo-based holding company that included a construction company among the group companies, purchased the site. In accordance with instructions from the city government, the site was filled in with the remains of land left in the area.

Regarding this instruction, a city government official said the remaining dirt was used as the huge volume meant it would have been much more difficult to remove it from the area.

The official added that at that time the foundation in the area was considered sufficiently strong, but added that following the landslide, no conclusions could be drawn about the stability of the foundation. The official said an assessment would be conducted to determine whether the 2011 instructions were appropriate.

In response to questions from The Asahi Shimbun, an attorney for the business owner denied filling the area with dirt and dirt.

What is clear is that areas that have been leveled by backfilling land to build houses and roads are vulnerable to landslides when earthquakes strike or heavy rainfall is recorded.

Heavy rains triggered by a typhoon in October 2014 led to the collapse of an unauthorized embankment in the Midori district of Yokohama. A man living in an apartment at the foot of the cliff was killed.

In October 2017, a typhoon caused an embankment collapse in Kishiwada, Osaka Prefecture, which dammed a river and killed a woman trapped in a submerged car.

(Shigeko Segawa and Takahiro Takenouchi contributed to this article.)

The Asahi Shimbun

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