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-healing mechanism of mudstone fractures is investigated

which is effected mainly by the confining pressure

pore water

time for saturated state and so on. A permeability healing model is established by introducing the conceptions of healing 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 initiation and propagation of new fractures

which is the comprehensive expression of the coupling between deformation and time effect. The presented creep damage model can well describe the creep process of attenuation phase

stabilization phase and the speedup phase and the damage process with less material parameters. The numerical algorithm

numerical implementation and further application of the model are provided in Part Ⅱ of this paper series.