Water jet is utilized in various industrial application such as cutting carious materials and soil. In particular, jet grouting for soil improvement is one of the most important application of water jet under high pressure and high liquid flow rate. Such technology is already in practical use in civil engineering. In order to improve the efficiency and performance of jet grouting, it is quite important to clarify the hydrodynamic structure of water jet under high pressure and high flow rate. However, basic researches on this subject are quite insufficient both experimentally and analytically. Water jet utilized in jet grouting is usually very high speed ( up to 500 m/s) two-phase dispersed flow. Therefore, it is quite difficult to make visual observation of such water jet using conventional optical methods. In the present work, the authors utilized high sensitive CCD camera with image intensifier of which gate speed is 10 ns. Using this optical device, the authors obtained still image of high speed water jet for the first time. Visual observation revealed that high speed water jet is composed of very fine droplets and has complicated structures such as swirl and cluster of droplets. Velocity of water jet was also measured using two consecutive images taken between 10 microsecond. Such direct measurement of jet velocity of high speed water jet has not been carried out so far. The result indicated that jet speed (droplet speed) is even very high even at the boundary of water jet. Measure water jet velocity was reasonably correlated with jet velocity at the exit of nozzle.

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