An original constitutive law for Callovo-Oxfordian argillite, a two-scale double-porosity material

Abstract Callovo-Oxfordian argillite exhibits a mechanical behavior that is not typical for a porous material. Its stiffness for example is found to decrease with an increasing saturation degree. Time to reach strain equilibrium at high mass water content is much longer after a purely hydric loading than after a purely mechanical loading. This behavior is a source of questions and these experimental observations question in particular the interaction between liquid in the pore space and the solid particles of argillite. Based on the analysis of the microstructure, some pores are revealed inside the solid particles. This paper assumes that these intraparticle pores are in thermodynamical equilibrium with the pores in the interparticular space. Micro-mechanics integrates the assumption of this connection and leads to an original constitutive law for Callovo-Oxfordian argillite. Based on micro-mechanics, an explanation is proposed for the origin of the elasticity tensor of Callovo-Oxfordian argillite. This state equation is then checked against some experimental data, and the relevance of this original stress state equation is emphasized. In particular, the fact that Callovo-Oxfordian argillite behaves like a Terzaghi's material at constant saturation degree is consistent with the proposed constitutive law. As in Biot's theory, Biot's tensor in the proposed stress state equation of Callovo-Oxfordian argillite weighs in a hydro-mechanical coupling term the pressure variation in the interparticle pores. Because of the particular microstructure of Callovo-Oxfordian argillite, Biot's tensor cannot be determined by a classic triaxial “change in pore” pressure poromechanical test. An original methodology is proposed to determine this tensor, which uses the monitoring of the length variation of a sample during a free-drying. Biot's tensor is found to be almost isotropic, with a Biot's coefficient equal to 0.84.