Abstract: In order to accurately describe the evolution of plastic strain for soft soil, an improved plastic potential theory is proposed, in which a plastic potential surface with an inclination angle is employed. In addition, the plastic potential function shares the same form as the yield function. During deformation, the plastic flow direction varies following the rotation of the plastic potential surface until the limit state is reached. By analyzing the results of conventional triaxial tests, it can be found that during the shearing process, the initial and final values of the rotation angle of the plastic potential surface and , are basically stable and independent on the confining pressure. Considering such observations, a normalized rotation angle and a state parameter describing the stress state of the soil are introduced, in the case that the egg-shaped function is used as plastic potential function. A piecewise linear relationship can be obtained. Then an improved plastic flow rule is established, only two additional parameters are required. Finally, the proposed plastic flow model is verified, and the results show that the trend of plastic strain can be reflected well in this model.

Key words: non-associated plastic flow rule, rotational plastic potential surface, egg-shaped function, stress state