The washout resistance of foamed treated which was made of dredged slurry and casted underwater is studied. The washout resistance test was newly developed in this research, and the following conclusions are obtained by a series of tests:
　1)In a washout resistance test of casting velocity control type, the density of suspended solid (SS) of water , pH value of water and the strength of soil after curing can be indexes of the washout resistance.
　2)As guideline of the washout resistance test, following values are given:
　　　SS<100 parts per million
　　　pH<10.5
　　　Strength ratio of sample casted in water to that casted in
　　　air>0.5
　　　Water content increase of sample casted in water<10%
3)The results of the washout resistant test shows that, as for the light weight treated soil used in this study, the flow value of 180～290 mm and casting velocity less than 100 cm/sec were recommended.