Recently the improvement of efficiency for dredging operation at hard strata or hard rocks are becoming one of the main problem in the construction of port and
harbours. Here in this report the authers presented the research on the water jet excavation (The pressure of water jet is 40 kg/cm2～50 kg/cm2), one of the rock
exavation method used today.
　As the excavation of the state rocks by water jets is very complicated, we made　clear only the first stage of the destruction of rocks by water jets. We analyzed as follow.
　The stresses created in the inner part of the rocks were given using Boussinesq's　theorem by the shocks which were produced by water jets when they collide with 　the surfaces of the rocks. On the asumptions that rocks are excavated by the　stresses according to Mohr's theory following relation was found for the rocks with
the poisson's ratio 0.15～0.30 and when their position from the tip of the nozzle is　away about quadruple of the nozzle diamter,
　σc≒2P
　where σ:Compressive strength of the rocks destructed by water jets.
　P:Pressure on the axis of water jets.
　And this relation was also confirmed by distruction experiments on the mortar-test pieces.