Abstract: Developing the constitutive models of unsaturated soils is a main field of soil mechanics. Although a large number of approaches have been applied, only a few were reported to consider the explicit integration algorithms in FEM. In this paper, we consider the numerical explicit integration with sub-stepping error control implementation of an extended Cam-clay constitutive model for unsaturated soils in ABAQUS umat (user-defined material model) subroutine. The modified Euler method has been used to solve the ordinary differential equations associated with both of the saturation and the saturation increment obtained by usdfld subroutine from ABAQUS. The accuracy, stability and robustness of the schemes were examined by a set of triaxial compression tests between the modified Cam-clay model from ABAQUS and the extend Cam-clay unsaturated model. As a conclusion, the extended Cam-clay unsaturated model with the explicit integration scheme shows great performance in a series of stress pass numerical triaxial test. Finally, the new unsaturated soil umat was applied to analyze the dry-wet cycle triaxial consolidation tests with hydraulic hysteresis. The results show the material parameter b plays a significant role for accumulative irreversible volumetric strain in dry-wet cycle condition, in which the accumulative strain decreases as b reduces but rises with b increases. Those results provide a solid foundation for further studies.

Key words: unsaturated soils, explicit integration algorithm, extended Cam-clay model, hydraulic hysteresis, dry-wet cycle