Abstract: In this paper, it was proposed the dilatancy occurred in soil mass is a second power effect of applied deviator stress. Based on this assumption, a formula for computing pore water pressure was developed. Analyses were carried out of the undrained triaxial compression test results of several normally consolidated saturated clays. It was shown that the new proposed formula is reasonable to reveal the dilatancy characteristics of these clays. Failure criterion of soil masses was also briefly discissed in this paper.