Evolution of Reversible and Irreversible Deformations of Compact Sandstone Under Different Modes of Triaxial Loading

Abstract Uniaxial or standard triaxial tests, which are the most frequently used in geotechnical practice, do not always simulate in-situ conditions of rocks well. Therefore, the deformations of rocks are now investigated even along alternative loading paths in the stress space. However, these alternatives are mainly used to study the conditions of the ultimate failure of rocks. Their deformational behavior along the alternative paths in the anticipated quasi-elastic stress region and corresponding stiffness moduli are rarely analyzed. Our recent study of the Brenna sandstone therefore compared deformational responses to monotonic loading along selected alternative paths in this stress region. The present new study aimed to evaluate the role of irreversible processes in deformation behavior under the different triaxial loading modes. The experimental methods included the following tests: cyclic isotropic compression, cyclic triaxial pure shear, and cyclic conventional triaxial compression. Irreversible deformation processes representing gradual breakage of rock were confirmed throughout the hypothetical quasi-elastic region. These processes, which may include damage to inter-grain connections or permanent displacements at grain contacts in cracks, are dependent on the stress path. The results also indicate a slight material anisotropy associated with stratification. The reversible and irreversible strains along several alternative triaxial stress paths were estimated. The study’s results are significant for predicting the deformation response of sedimentary rocks to in situ stress. Identification of the extent of the irreversible processes is crucial for understanding the mechanical behavior of these rocks during different time evolutions of in-situ stress or conditions of repeated loading/unloading.