Study of hydro-mechanical-damage coupled creep constitutive model of mudstone, Part I: Theoretical model

Based on the laboratory creep experiments and in-situ tests of mudstone, a nonlinear creep constitutive model with the Mohr-Coulomb creep potential considering hydro-mechanical coupling is put forward by analysis of creep deformation mechanisms of clay; and the damage evolution equation is set up by constructing the relation between creep damage and creep strain. A fully coupled hydro-mechanical model of mudstone is established to describe the evolution of porosity and permeability with strain and damage. The self- sealing mechanism of mudstone is investigated, which is effected mainly by the confining pressure, pore water pressure , time for saturation and so on. A permeability sealing model is established by introducing the conceptions of sealing stress and hydro-chemical factor. The results show that the creep strain rate of mudstone is not only related to time and stress, but also to the cumulative creep strain. The creep damage of mudstone is caused by the progress damage of the fabric. The presented creep damage model can well describe the creep process : attenuation, stabilization and the speedup phases with limited numbers of material parameters.