Weak cementation characteristics of Jurassic sandy mudstone and its creep behavior under multi-axial loadings

Abstract Jurassic sandy mudstone formations are common in the Ordos mining area, and the diagenetic period is relatively late. The lithological characteristics and creep mechanical properties of this formation form the basis for the long-term stability of excavated rock layers. In this study, the microscopic fabric, basic physical properties, and creep mechanical behavior of Jurassic sandy mudstone are experimentally studied. The results indicated that the porosity of the sandy mudstone is 9.7%, the mineral particles are small and the clay mineral content is as high as 70%. The clay mineral content and porosity affect the uniaxial strength, elastic modulus, and cohesion of rocks. The mineral content of the matrix affect the internal friction angle. These phenomena collectively indicate that the Jurassic sandy mudstone belongs to the category of weakly cemented rock. However, sandy mudstone undergoes creep attenuation under low deviatoric stress, and the creep isochronous curve approximates a straight line, indicating poor time dependence. The time dependence of the deformation is evident only when the deviatoric stress approaches the long-term strength. The rock volume continues to compress throughout the creep process, and this creeping behavior is similar to that of non-weakly cemented rocks. Similar to weakly cemented rocks, the steady-state creep rate of sandy mudstone exhibits an exponential relationship with the deviatoric stress, and the Burgers creep model with damage expressed in an exponential form can describe its creep behavior. This reminds us that the traditional criteria for classifying weakly cemented rocks need to be optimized, and the creep performance indicators of rocks cannot be ignored.