PROBLEM TO BE SOLVED: To prevent a situation in which the construction of a rubble mound is delayed or the construction cost increases. SOLUTION: A polluted water diffusion prevention fence surrounding the periphery of an opening is suspended and attached to a rubble drop pontoon provided with an opening for dropping rubble thrown from above the sea to the seabed. The gut ship throws the loaded rubble into the opening of the rubble drop pontoon from above the sea with a heavy machine, and builds a rubble mound on the seabed. The heavy machinery installed on the gut ship throws rubble into the opening of the rubble drop pontoon without the gut ship coming alongside the rubble drop pontoon. [Selection diagram] Fig. 6
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