Effects of relative humidity and mineral compositions on creep deformation and failure of a claystone under compression

During the last decades, clayey rocks have been widely investigated in many countries as a potential geological barrier for underground disposal of radioactive waste. Clayey rocks are heterogeneous materials at a micrometer scale. Mechanical behaviors of clayey rocks are generally influenced by mineralogical compositions and/or water content. A large number of studies have been so far performed on short term behaviors of clayey rocks. However, very limited data are available on time-dependent deformation and failure process. This paper is devoted to investigating the effects of relative humidity and mineralogical compositions on creep deformation of a hard claystone and the possibility of creep failure is studied. Influences of confining pressure and structural anisotropy are also investigated. For this purpose, four groups of creep tests are carried out under triaxial compression conditions on claystone samples drilled from the underground research laboratory at Bure in France. In the first group, creep tests are performed on samples with three different values of relative humidity (RH, 59%, 85% and 98%) under a confining stress of 6 MPa. In the second group, claystone samples with different mineralogical compositions drilled from different geological unities are investigated. In the third group, for a selected value of relative humidity (RH 59%), triaxial creep tests are performed under different values of confining stress (2, 6 and 12 MPa). Finally, effects of structural anisotropy of claystone at HR = 59% are investigated by considering samples respectively drilled in parallel and perpendicular directions to the bedding planes. A series of new experimental data are obtained and analyzed. It is found that at the tested level of relative humidity the confining stress seems to increase creep strain and rate of the claystone. The clay minerals especially the smectite can play an essential role in time-dependent deformation and increase creep strain of the claystone at RH = 59%. Further, creep strains are more important under higher values of relative humidity than lower ones. In very few cases, the failure due to acceleration creep was observed but for high deviatoric stress level. The experimental data obtained in this work can be used as a background for numerical modeling of time-dependent behavior of re/desaturated clayey rocks in the context of geological disposal of radioactive waste.