Shizuoka Vice Gov. Takashi Namba meets reporters at the Shizuoka Prefectural Government headquarters on July 7, 2021. (Mainichi/Ayaka Sasaki)

       SHIZUOKA -- Soil mounds near the origin of a deadly mudslide that engulfed residential areas in the Izusan district of Atami, Shizuoka Prefecture, were inappropriately piled up some 50 meters, over three times the acceptable level, Shizuoka Vice Gov. Takashi Namba has disclosed.

       "The construction method was inappropriate. The earth and sand from the soil mounds aggravated the disaster," Namba told reporters at a hastily assembled press conference on July 7.

       While a company that previously owned the land where the accumulated soil was located had presented a plan to raise the ground level with a 15-meter layer of soil in 2009, laser-based terrain data collected by the prefectural government in 2000 showed that the soil mounds were about 50 meters thick. Under the prefecture's technical standards, soil mounds should in principle be no more than 15 meters thick. The vice governor said the prefectural government had repeatedly issued administrative guidance to the company.

       After the company purchased the land in 2006, it submitted a plan to the Atami Municipal Government the following year, stating that the firm would raise the ground level with some 36,000 cubic meters of soil. The city accepted the plan. Subsequently, it emerged that the firm had violated forest land development permission, prompting the prefectural government to order the company to correct its work. After the company took remedial measures, it started to transport soil to the site in 2009.

       The following year, however, it turned out that woodchips from industrial waste were mixed in the accumulated soil. The prefectural and municipal governments asked the company to remove the soil, and the municipal government also told the firm to suspend its work, among other requirements. However, the company did not comply with the request for suspension, and the land's ownership subsequently switched hands to an individual in 2011.

       "There's no way soil 50 meters thick had a chance of holding out when the design for durability set a thickness of 15 meters," Vice Gov. Namba said.

       He revealed that during his visit to the area in the aftermath of the mudslide, he saw water coming out of the section where the soil had given way.

       "The mounded soil acted as a lid (on the valley), and water built up inside it," he said, adding, "Due to the prolonged rain, water had accumulated to the point the soil mounds were no longer able to withstand a collapse, causing them to fall apart."

       (Japanese original by Hideyuki Yamada and Ayaka Sasaki, Shizuoka Bureau, and Eiji Nagasawa, Fuji Local Bureau)

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