Effects of deviatoric stress and structural anisotropy on compressive creep behavior of a clayey rock

• Creep deformation of a clayey rock is investigated by a series of creep tests.
• The deviatoric stress level enhances the creep strain rate.
• The creep strain is also affected by the structural anisotropy of material.
• Creep deformation seems to induce an increase of rock long-term mechanical strength.

Clayey rocks are extensively investigated in France and other countries as a potential geological barrier for underground disposal of radioactive waste. In this context, it is necessary to study time-dependent deformation of clayey rocks. In this paper, we propose to perform a series of triaxial compressive creep tests on the Callovo-Oxfordian (Cox) argillite from the underground research laboratory of Andra at Bure in France. The main purpose is to investigate effects of deviatoric stress level and structural anisotropy on the creep deformation. To this end, creep tests are performed under different values of deviatoric stress for a same value of confining pressure. On the other hand, tested specimens are drilled respectively in the perpendicular and parallel directions to the bedding planes of the Cox argillite. The experimental results show that the increase of deviatoric stress enhances the creep strain which clearly exhibits an anisotropic behavior. Moreover, it is found that the inelastic creep deformation induces an increase of the long-term compressive strength of the argillite.