Triaxial unloading tests on rupture characteristics of sandstone under hydro-mechanical coupling conditions

The strong and sudden properties of the bulking and deformation behaviors of the sandstones under the hydro-mechanical (H-M) function are studied. The evolution of micro-defects under high geo-stress and hydraulic coupling conditions are critically important to reveal different stages of the micro-cracks evolving from the initiation to the failure and to evaluate the stability of the surrounding rocks. The MTS815 rock mechanical system is used to test sandstone samples collected from Bamiao Tunnel in Sichuan of China, and the H-M coupling experiment is performed under the fixed confining pressure and varied hydraulic pressure. The findings reveal that the rock deformation is compressive before unloading, though the change is small. However, rapid dilatancy proceeds to the failure stages shortly after the unloading. The stress-strain curves under H-M coupling conditions are generally higher than those without hydraulic pressure, implying that the strain energy is enhanced. In addition, the experienced time lasts shorter from the initial dilatancy point to the peak, and the slope of the curves becomes steeper. Furthermore, with the increase of the hydraulic pressure, the key characteristic stress decreases, which is reflected under lower fracture initiation conditions, reduced compression limits and advanced bulking time. Accordingly, the brittle characteristics of the sandstones are further enhanced while the ability to resist deformation and failure gradually decreases. Finally, a polynomial regression relationship is established by calculating the relative expansion strains after the initial expansion points and their corresponding deformation modulus difference values. In conclusion, the findings of this study may provide advices for the forecast and control of the deformation of the surrounding rocks under multi-conditions.