Effect of natural defects on the fracture behaviors and failure mechanism of basalt through mesotesting and FDEM modeling

• A comprehensive mesotest and a FDEM model is applied to basalt with different defects. • The effects of hidden joints on the microfracture behaviors and failure mechanism of basalt were revealed. • The effects of amygdale heterogeneities on the microfracture behaviors and failure mechanism of basalt were revealed. As an important geological medium for engineering construction in Southwest China, basalt typically contains natural defects, such as hidden joints and amygdales, which significantly affect its mechanical properties. In this work, the fracture behaviors of basalt with different defects were studied in terms of its strength, deformation and failure characteristics, and crack evolution laws using a comprehensive mesotesting scheme and finite-discrete element method (FDEM) modeling, revealing the failure mechanism of basalt. The results showed that: (1) Intact specimens exhibiting high strength and smooth stress–strain curves burst instantly when loaded to the peak strength, and the corresponding acoustic emission events are abnormally concentrated, indicating a fragmentation failure mode. (2) The strength of fractured samples decreases significantly, the stress–strain curve is bimodal or multimodal, and the acoustic emission events are very active during the entire failure process, which is closely related to the local damage generated during each significant stress drop, with tension dominant, shear dominant, or tension–shear composite failure modes. (3) For amygdaloidal basalt samples with a serrated stress–strain curve, each small-scale stress drop is accompanied by the initiation, propagation, and coalescence of microcracks, resulting in relatively scattered acoustic emission events, indicating a splitting failure mode. (4) Because of the small particle size, dense arrangement, good uniformity, and high strength at the grain scale, intact basalt exhibits a relatively uniform stress field easily induced under loading, leading to an extremely high strength, fragmentation failure mode, and significant brittleness. However, a heterogeneous stress field near the original defects, such as hidden joints or amygdales, is prominent, and it significantly affects the fracture paths, which is the main reason for the strong randomness of the mechanical properties, evident strength reduction, and significant change in the failure modes. The research results can provide reference for revealing the occurrence mechanism, guiding warning systems, and formulating control methods for high-stress disasters in hard brittle rocks.