Experimental study of the influence of drainage conditions on sandstone mechanics and its deformation localization characteristics

In order to study the deformation localization failure characteristics of sandstone under the coupling action of seepage and stress, the triaxial compression tests of sandstone under different drainage conditions were carried out using the visualized triaxial servo control test system combined with three-dimensional digital image correlation (3D-DIC). The rock mechanics, seepage and deformation localization characteristics were analyzed, and the micro-morphology of fracture surface after sandstone failure was analyzed using electron microscope scanning. The results shown that the peak strength and elastic modulus of sandstone under drainage condition were higher than those under undrained condition, the peak strength, elastic modulus and Poisson’s ratio of sandstone increased with the increasing seepage water pressure, the time point of penetration crack and maximum permeability would be advanced. When the seepage water pressure was the same, the localized zone of deformation field cloud map of the sand surface was wider in the undrained condition than that in the drained condition, i.e., the macroscopic crack of rock was more obvious. Water flow under drained condition took away the mineral particles inside the rock and formed holes, the rupture surface of which was smoother than that under undrained condition, whereas flakes cuttings were obviously attached to the surface of the particles under undrained condition. The initiation point of radial deformation localization was consistently higher than that of axial deformation localization for all drainage conditions, with an average increase of 1.23%. The initiation stress levels of the radial and axial deformation localization increased with increasing seepage water pressure, i.e., the initiation time point was earlier. The radial and axial initiation stress level of sandstone under drainage condition were higher than those under undrained condition, with an average increase of 1.85% and 2.21% respectively. When the water pressure was the same, the initiation stress and stress level were more significantly affected by water pressure in the undrained condition than in the drained condition.