Photo/IllutrationThe Asahi Shimbun

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Opponents of moves to build a new U.S. military base in Okinawa Prefecture have been handed a new card in their long campaign to halt the project.

The central government will have to modify its design plan because the seafloor is too soft in some portions of the reclamation site off the coast of the Henoko district of Nago in the prefecture, said a senior official of the Abe administration.

That means it will have to seek approval from the prefectural government headed by Denny Tamaki, a fierce opponent of the plan to construct a partially offshore airfield to take over the functions of the U.S. Marine Corps Air Station Futenma in Ginowan, also in the prefecture.

Given that the governor's approval is highly unlikely, the matter may well end up in the courts, raising the prospect of a prolonged showdown between the central and prefectural governments over the project.

A V-shaped airstrip will be built on a 160-hectare site of reclaimed land that is more than three times larger than Tokyo Disneyland.

Work got under way Dec. 14 to dump tons of dirt and sand in a 6.3-hectare area.

The porous seafloor, described earlier as soft as mayonnaise, was detected in the construction site during drilling work by the Defense Ministry’s Okinawa Defense Bureau between 2014 and 2016, according to a central government official.

The Defense Ministry is now doing additional survey work in the problem area. It submitted an interim report on the study to the Ministry of Land, Infrastructure, Transport and Tourism late last month.

The seafloor was confirmed to be soft at several locations to northeast of the site where the reclamation work is under way, according to the official.

The ministry is expected to compile a final report in March after the study wraps up.

The central government will be forced to change the design plan and carry out reinforcement work aimed at strengthening the weak foundation because of the risk of the land sinking if it goes ahead with the original plan authorized by Okinawa Prefecture, according to the official.

Tokyo is expected to submit documents by the end of the year seeking approval for the change from the prefectural government.

Tamaki is bound to reject any such request.

One option for the central government is to file a lawsuit against the prefectural government over the legality of Okinawa’s near-certain refusal to allow the design change.

“The prefectural government’s refusal will likely result in another legal battle,” the central government official said.

At present, Okinawa officials estimate it will take more than 10 years for the new facility to be completed.

But it could take considerably longer if a court battle is fought.

The soft seafloor also means that more taxpayer money will be spent on the project.

Prefectural authorities project the final tab will come to 2.5 trillion yen ($22.8 billion), including reinforcement work, nearly 10 times as much as the Defense Ministry’s initial estimate.

The prefectural government authorized Tokyo's reclamation plan in December 2013 when Hirokazu Nakaima was governor.

(This article was written by Narumi Ota and Natsuki Okamura.)