Abstract：According to the fact that the undrained shear strength decreases during field vane test，by applying the spherical expansion theory，and assuming that the saturated soft clay satisfies the Tresca yield criterion，the disturbance degree function is proposed on the basis of the sensibility of saturated soft clay. On the basis of the undrained shear strength being the logarithm function of the disturbance degree，the elasto-plastic solution of the cylindrical expansion is obtained. By comparing with the conventional spherical cavity expansion，the total stress，effective stress and excess pore pressure with considering the disturbed spherical cavity expansion decrease apparently.
