A constitutive model is developed within the framework of Perzyna's viscoplasticity for predicting the stress-strain-time behaviour of soft porous rocks. The model is based on the hyperelasticity and multisurface viscoplasticity with hardening. A time-stepping algorithm is presented for integrating the creep sensitive law. An example of application to one-dimensional consolidation is presented. The objectives are to: 1. present a soft rock model which is capable of taking into account the rate sensitivity, time effects and creep rupture; 2. to discuss the use of an incremental procedure for time stepping using large time increments and 3. to extend the finite element code Lagamine (MSM-ULg) for viscoplastic problems in geomechanics.
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