For the dredging operations at hard strata and rocks in the sea-bottom, special dredgers are used today, but the operation of these dredgers are often insufficient and there are many areas in which it is difficult for these dredgers to work.
　Accordingly, we have to develope some other methods of efficient destruction of rocks in the sea-bottom.
　In this report, the author presents the method of destructing rocks by high-pressure water jets and showed some experimental results of them. From the experimental results, we got following experimental equation
　l≒(-0.39L-3.1v+11.8)・(2Ho/σc-0.7)・d1.7・N0.38
where
　l=Avarange excavated depth (cm) (l>0)
　Ho=Pressure of the water jets at the tips of the nozzles(kg/cm2) (10≦Ho≦47)
　d=Diameter of nozzle(cm) (0.6≦d≦2.0)
　L=Distance between tip of the nozzle and the surface of the test-pieces.(1≦L≦20)
　σc=Moving speeds of nozzle (cm/sec) (0.25≦υ≦3)Compressive strength of the model test-pieces (kg/cm2).
　N=Number of the times of excavation operations.