||A unified elastic–plastic–viscoplastic model is presented for argillites in this paper. Emphasis is put on the description of long-term hydromechanical behavior of argillites under unsaturated condition. Base on the experimental investigation, the material’s behavior is characterized by important plastic deformation coupled with damage on the short-term scale while important creep deformation has been observed for the long-term material’s response. Furthermore, the water content has an important impact on mechanical behavior of argillite at different time scales. Using the unified approach proposed by Jia (2006) and Zhou et al. (2008), a general constitutive model is proposed for the poromechanical behavior of argillites in both saturated and unsaturated conditions. Main features observed in experiments are taken into account, particularly the elastic degradation due to microcracks, coupling between plastic deformation and induced damage, creep deformation and influence of water content on plastic flow on different time scales. The performance of the model is examined by simulation of creep tests with different hydraulic states. Finally, the model is applied to study the hydromechanical coupling in a resaturation–desaturation test.