Abstract: In order to study the influence of water pressure on incipient velocity of undisturbed clay, we carried out test on the incipient velocity by using a closed rectangular pipe which could provide different water pressures. Results show that incipient frictional velocity of undisturbed clay in the presence of 4.35 m water column height is 2.52-3.06 times that under 0.35 m height. In other words, incipient velocity of undisturbed clay increases with the increasing of water pressure. Besides, water column height, grain gradation and compactness of undisturbed clay are also main factors affecting the incipient frictional velocity.The experimental results can provide reference for further studies on incipient velocity of cohesive undisturbed soil.

Key words: water pressure, undisturbed clay, incipient velocity, gradation of grain, dry density