To secure stability by rapidly dealing with predictions about the occurrence of an abnormal external force.
A cofferdam structure 10 is constructed when the repair, reinforcement and new installation of an underwater structure are performed in a state wherein water is discharged from inside by surrounding the outer periphery of the underwater structure such as a bridge foundation. A water injection valve 28 capable of taking in external water is installed on the side surface of the cofferdam structure 10. When the abnormal external force caused by a high water level, high flowing water, etc. acts on the cofferdam structure 10 or when the action of the external force is predicted, the valve 28 is opened so that the external water can be taken into the cofferdam structure 10. When the external water is taken in via the valve 28, pressures are balanced inside and outside the cofferdam structure 10, and the abnormal external force does not act on the cofferdam structure 10, so that the cofferdam structure 10 can be stabilized.
KATO SHOZO
UENO KOJI
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