Abstract:The experimental results suggest that the mechanical behaviours of chalk can be modeled in terms of elastoplastic model with two plastic deformation mechanisms:plastic compaction(pore collapse) and plastic shear. From this observation,a constitutive model based on Gurson criterion and Pietruszczak criterion is proposed to study the mechanicals behaviour of chalk. The model contains two yield functions and corresponding plastic flow rules. The performance of the model has been verified for the chalk in drained hydrostatic and triaxial tests;and a good agreement has been obtained. It is well known that minerals,structure and saturation conditions have important influences on the mechanical behaviours of chalk. Moreover,the influence of pore pressure is also very important. Consequently,pore pressure should be taken into account in the modelling of mechanical behaviour of saturated porous chalk. Coussy¢s poroplastic theory is used for the extension of this model to describe poromechanical coupling of chalk. According to the general framework of poro-elasto-plastic modelling,the concepts of the effective stress tensors,especially the concept of plastic effective stress tensor,are discussed. In order to study the poromechanical behaviour of chalk in consideration of pore pressure variation,two yield functions and plastic potentials are expressed as the function of the plastic effective stress tensor. Then the plastic behaviour of saturated porous chalk can be studied in undrained conditions,especially in undrained triaxial compression condition. In general,the model¢s predictions are in satisfactory agreement with the experimental results. It is shown that the proposed model is able to describe the poromechanical behaviour of saturated porous chalk by using the concept of effective stress.