With the large scale of dredging and reclamation works in port and harbour constructions in recent years, it has become more and more difficult for the present pump-dredgers to carry out dredging at hard strata of sea-dottom or the bottom of deep sea. So the need of new method of dredging and improvement of dredging apparatus in dredgers has become pressing.
　To meet these requirements, high-pressure water jets may bring about effective dredging operation, if we use them for the excavation of hard strata and hard rock.
　In the first half of this technical note, we present the results of experiments on the fundamental structure of water jet with the pressure of 40～50kg/cm2, spread out into water tank, and analyzed them comparing with the data of this kind of experiments carried out by other researches hitherto. And in the latter half, we showed excavation test by water-jet for the mortar test-pieces, changing the way of excavation, and excavation time by water jets. And some data on the excavated depth, excavated volume, and excavation limits were gained.
　Through the results of these experiments, the test-pieces, the compressive strength of which are about two times of the pressure of water jets are proved to be crashed by submerged water jet. But as these results are limited only to the test-pieces we made this time, we must make further deep study on the crash condition of each rock by sudmerged jets.