Investigation on the failure characteristics of fissured sandstone under true triaxial unloading and dynamic disturbance condition

In order to investigate the failure characteristics of fissured sandstone subjected to combined dynamic and static stress states，the true triaxial rockburst test system(QKX-YB200)，acoustic emission monitoring system and camera were used to conduct true triaxial unloading and dynamic disturbance tests on red sandstone under different fissure inclinations<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math></inline-formula>，initial static stresses<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mtext>s</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>and dynamic disturbance conditions(disturbance directions，disturbance amplitudes<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mtext>d</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>and disturbance frequencies <italic>f </italic>). The results indicate that：(1) The failure mode of fissured sandstone is tensile with shear failure under true triaxial unloading and dynamic disturbance condition，while the proportion of tension cracks and shear cracks varies depending on different influencing factors. (2) For the specimens that have undergone instability failure，the wing crack propagation at the tips of the fissure transforms from single wing cracks or a combination of wing and anti-wing cracks at <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math></inline-formula>&lt;90° to single anti-wing cracks at <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math></inline-formula>= 90°. (3) The fractal dimension and the failure severity of the sandstone both exhibit a trend of first decreasing and then increasing with the increase in <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math></inline-formula>. Additionally，these properties are positively correlated with<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mtext>s</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>and<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mtext>d</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>，but negatively correlated with <italic>f</italic>. When the dynamic disturbance shifts from the direction of the maximum principal stress to the direction of the intermediate principal stress，the overall instability failure of sandstone do not occur. (4) The wing crack propagation model of fissured sandstone under true triaxial unloading and dynamic disturbance condition was established. Additionally，the mechanism of wing crack propagation was investigated in relation to the duration of dynamic disturbance and the internal stress environment of sandstone. (5) By investigating the entire energy evolution process of fissured sandstone under true triaxial unloading and dynamic disturbance，the energy condition of triggering rockburst has been summarized. Additionally，the quantitative contributions of various influencing factors to the rockburst hazards on fissured sandstone were analyzed. The study is expected to provide theoretical guidance for the stability evaluation and disaster prevention of deep fissured hard rock in mining operations.