Failure characteristics of sandstone containing a single flaw under different σ2 and σ3 conditions

In deep underground geotechnical engineering, flaws emerge as a critical factor influencing the stability of engineering rock masses. This study delves into the macroscopic mechanical behavior and fracture characteristics of sandstone specimens containing a single flaw under true triaxial stress states. The main objective is to systematically elucidate the impact of the intermediate principal stress sigma(2) and minimum principal stress sigma(3) on the stress-strain response, triaxial strength, brittleness index, and cracking pattern. The results demonstrate that the stress-strain curves of flawed sandstone specimens exhibit multiple fluctuations with rises and falls, and the post-peak recovery stresses are significantly influenced by sigma(3). The peak strength, crack initiation stress, and crack damage stress under true triaxial compression initially increase and then decrease as sigma(2) increases, while the increase in sigma(3) has a pronounced enhancement effect on the characteristic strength of flawed sandstone specimens. Based on the total stress-strain curve, an evaluation index of rock brittleness characterized by fracture energy is established to determine the quantitative relationship between principal stress and brittleness of flawed sandstone specimens. The flawed sandstone specimens typically fail due to flaw-dominated tensile-shear mixed cracks, regardless of sigma(2). Conversely, when sigma(3) is low, the failure of the flawed sandstone specimen is dominated by tensile-shear mixed cracks with a tensile-dominated mechanism. As sigma(3) increases, the failure mode gradually transitions to flaw-dominated tensile-shear mixed failure. When sigma(3) is high, the failure mode is characterized by stress-dominated tensile-shear mixed cracks. Compared with sigma(2), the failure mode of flawed sandstone specimens is more sensitive to sigma(3).