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.