Unsaturated soils exhibit significant time-dependent characteristics under long-term loading and environmental changes. Accurate prediction of their rate-dependent behavior and time effects is crucial for deformation control and long-term stability analysis of engineering structures. Based on the isotach approach for unsaturated soils

a coupled relationship among yield stress

matric suction

and viscoplastic strain rate is established for unsaturated time-dependent soils. To address the discontinuity of viscoplastic rate-dependent parameters during the transition between saturated and unsaturated states

an improved parameter evolution law is proposed. Using bounding surface plasticity theory as the framework and extending the non-stationary flow rule to unsaturated conditions

a constitutive model applicable to triaxial stress states is developed. In this model

the evolution of bounding surface size is governed by viscoplastic volumetric strain

matric suction

and viscoplastic volumetric strain rate. This model effectively captures the coupled effects of viscoplastic strain rate and matric suction on the time-dependent behavior of unsaturated soils. The rationality and effectiveness of the model are verified by comparisons with experimental data from isotropic compression tests

triaxial shear tests

and creep tests on various time-dependent soils under different matric suction levels and axial strain rates. Validation results demonstrate that the model accurately predicts the rate-dependent behavior and creep characteristics of unsaturated soils under specified suction levels.