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.