Chengzhi Qi

Natural rock materials, containing micro-cracks and pore defects, significantly alter their mechanical behavior. This study investigated fracture interactions of red sandstone containing double close-round holes (diameter: 10 mm; bridge angle: 30°, 45°, 60°, 90°) using acoustic emission (AE) monitoring and the discrete element simulations method (DEM), which was a novel methodology for revealing dynamic failure mechanisms. The uniaxial compression tests showed that hole geometry critically controlled failure modes: specimens with 0° bridge exhibited elastic–brittle failure with intense AE energy releases and large fractures, while 45° arrangements displayed elastic–plastic behaviors with stable AE signal responses until collapse. The quantitative AE analysis revealed that the fracture-type coefficient k had a distinct temporal clustering characteristic, demonstrating the spatiotemporal synchronization of tensile and shear crack initiation and propagation. Furthermore, numerical simulations identified a critical stress redistribution phenomenon, that axial compressive force chains concentrated along the loading axis, forming continuous longitudinal compression zones, while radial tensile dispersion dominated hole peripheries. Crucially, specimens with 45° and 90° bridges induced prominently symmetric tensile fractures (85° to horizontal direction) and shear-dominated failure near junctions. These findings can advance damage prediction in discontinuous geological media and offer direct insights for optimizing excavation sequences and support design in cavern engineering.

Preexisting cracks are of great significance for the stability of rocks, they can influence the growth and propagation pattern of cracks. For revealing the failure characteristics of rocks with prefabricated cracks, the mechanical behaviors of red sandstone with two close-collinear-equal length cracks were studied by acoustic emission (AE) monitoring experiment. The results indicated that the stress of red sandstone specimens showed an obvious stepwise growth phenomenon, and AE ring counts can better characterize the growth of microcracks of rocks. Moreover, AE events were mainly distributed near the prefabricated cracks of rocks. Then, a novel modified Kachanov method for analyzing collinear-close cracks was proposed. The influences of crack length and crack spacing on the interaction force of cracks were discussed. Then, it can be found that this modified method had a good applicability for two close-collinear-equal length cracks of rocks under compression.

The evolution pattern of rock damage is a progressive failure process of rock materials. It is the basis for predicting failure time of rock materials. By theoretical and experimental analysis, the acoustic emission (AE) precursor characteristics of rock fracture and the gradual evolution pattern of rock damage were analyzed detailedly. Then, the time-to-failure of red sandstone was predicted and compared by several different methods. The results demonstrated that the failure process of red sandstone can be divided into the stable deformation stage and the critical acceleration failure stage. In the critical acceleration failure stage, the AE precursor of rock failure was easy to be observed, and the AE event rate occurred as jump-like increase phenomenon. Moreover, the gradual evolution pattern of rock damage obeyed an exponential function, and the damage acceleration phenomenon existed in the critical failure stage. Furthermore, the higher values of the average of rock damage was, the more obvious linear evolution pattern will be, which was beneficial to improve the prediction accuracy of time-to-failure of rocks. Clearly, the linear prediction results of rock failure time, after taking average values of five rock damage variables, had more higher accuracy when damage variable exceeded . The predicting result of specimen R1 was 0.2 s ahead of its actual failure time, and the predicting result of specimen R6 was 8.1 s ahead of its actual failure time. Therefore, this method is meaningful and it can be used for the early warning of rockburst.

A geological structural surface frequently appears in underground engineering and it significantly affects the stability of engineering structures. To elucidate the spatial-temporal failure characteristics of discontinuities between rocks, red sandstone with a firmly cemented discontinuity was investigated under uniaxial compression. The results revealed that adding a strong binder to rock discontinuities can improve the shear strength and stability of rock mass. The stress curve indicated that red sandstone specimens had elastic-brittle failure characteristics without obvious plastic deformation. The acoustic emission ring counts did not have large fluctuations, and its values were smaller (less than 2.5 × 10⁴/s) at elastic deformation stage, which was termed as the "quiet period". The acoustic emission ring counts initially exhibited an obvious jump increase phenomenon at the forthcoming brittle failure stage before stress curve produced an obvious turning point, and then, it entered the "relatively quiet period". Based on the acoustic emission index (rise angle and average frequency), at the initial destruction stage of rocks, the types of internal fractures were simple and clearly (tensile cracks or shear cracks), and the fracture location was also relatively scattered inside specimens. Subsequently, the internal fracture developed into mixed tensile and shear cracks, and then, the fracture location exhibited a progressive evolution trend in the vertical space of specimens.