This report shows the effect for the reduction of driving wower of dredger for leveling the bottom of sea, through the combination of water-jet and blade, when a large quantity of sand and soil are piled up before the blade of dredger by laboratory test.
　The object of this laboratory test was to examine qualitatively the effect for the reduction of leveling power of blade, using model brade (length=1m, height=10cm), and jet nozzles (diameter=2, 4, 6, 8mmφ).
　Resistance power of blade is known to be proportioned to volume of piled sand and soil, so we investigate the effect of reduction of leveling power studying the volume of piled up sand soil when water-jet is thrown out to them.
　Accordingly, the effect of water-jet upon reduction of leveling power by blowing off the piled sand and soil before blade were proved to be the determined mainly by momentum of water-jet and speed of blade.
　Assuming the height of blade is infinitely large, horizontal power of lade (H) caused by the piled sand, when water-jet is thrown out, can be expressed theoretically by following equation.
　H=A0・DB2/(1+α・d2p/V){1+exp(-k1(1+αd2p/V)S/B)}
where, D, p, B, S, V and α are depth of excavation, pressure of jet, length of midel blade, leveling stroke,speed of blade and determined from the places where nozzle is attached number of nozzle; the nature of the soil, constant A0, coefficient of discharge of jet-nozzle.
　Constant A0 is obtained from the ratio of piled sand and soil to resistance power caused by the piled sand and soil, in case of leveling without jetting.