Abstract: In order to elucidate the expansion mechanism of spherical cavity in undrained unsaturated soils, based on the modified Cam Clay model considering suction hardening and a liquid phase equation, and by incorporating the associated flow rule, the logarithmic large-deformation geometric relation, and the equilibrium equation, the incremental governing equations of stress, matric suction, and void ratio in the cavity plastic zone with respect to an auxiliary variable were established. A novel plastic solution for spherical cavity expansion in undrained unsaturated soils was then iteratively obtained using the boundary condition at the elastic- plastic interface. The rationality of this solution, its radial variations, and the factors influencing the cavity expansion pressure were subsequently discussed. The results reveal that the novel solution not only reasonably accounts for cavity expansion ratio, suction hardening, and compactness, but also does not simplify the deviator stress and average stress. Its parameters have clear physical meanings, and its rationality is verified by comparison with both the undrained semi-analytical solution for spherical cavity expansion in saturated soils and the undrained similarity solution for spherical cavity expansion in unsaturated soils available in the literature. Consequently, it has theoretical significance and broad application prospects. In the plastic zone of the spherical cavity, the radial effective stress and circumferential effective stress decrease continuously, while the matric suction increases gradually, and the void ratio first increases approximately linearly and then slowly. In dense soils, volumetric dilatation occurs, and the void ratio of dense soils first increases and then decreases. The cavity radius ratio, initial suction, and initial void ratio all significantly affect the cavity expansion pressure, and the cavity expansion pressure curves are parallel to each other for different initial suctions or initial void ratios.

Key words: spherical cavity expansion, unsaturated soils, undrained condition, auxiliary variable, suction hardening