PURPOSE:To enable dredging of medium and small rivers running through a city, by a method wherein mud in a caisson driven from a ship is sucked and divided and classified by a spiral classifier, and after it is temporarily storaged in a slug tank, it is caused to self-run. CONSTITUTION:A suction device 20, a crane device 5, a caisson 6, a rotary film device 9, and a spiral classifier 10 are mounted to a primary ship 1 of a ship hull, to which primary, secondary, and tertiary ships 1, (the secondary ship not shown), and 3 are detachably coupled together, a dehydrating device 14 to the secondary ship, and a slug tank 19 and a propelling device 22 to the tertiary ship 3, and conveyors 11 and 12 are installed to the primary ship 1, the secondary ship, and the tertiary ship 2. Furthermore, a pipe 205 of the device 20 is inserted in the caisson 6 driven down to the mud base, the mud, sucked in an intermittent discharge tank 204 of the device 20, is divided by means of the device 9 and the classifier 10, and after the mud is temporarily stored in the tank 19 through the devices 11 and 12, the ship 3 is caused to self-run, a moisture content is discharged through the working of the device 14, and bulky matters are removed by means of a bucket 503 of the crane device 5.
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