Influence of amygdale on crack evolution and failure behavior of basalt

Abstract Amygdaloidal basalt is one of the two major types of surrounding rocks that are widely exposed in the super large-scale underground powerhouse caverns of Baihetan Hydropower Station, China. Excavation of the caverns induced a series of mechanical responses, including cracking and fracture relaxation. This study comprehensively explored the deformation and failure characteristics and the fracture evolution process of the amygdaloidal basalt using conventional MTS triaxial laboratory testing equipment and continuous–discontinuous modeling (FDEM) with the aim of clarifying its failure mechanism. The research results indicate that: (1) amygdales significantly changed the deformation and failure characteristics of basalt. Whereas the post-peak mechanical behavior of cryptocrystalline basalt was brittle, that of amygdaloidal basalt was brittle under low confining stresses and ductile under high confining stresses. Meanwhile, whereas the failure mode of cryptocrystalline basalt was fragmentation failure, that of amygdaloidal basalt was splitting failure at low confining stresses and shear failure at high confining stresses. (2) An FDEM model considering the heterogeneity of the rock mesostructure was established based on the statistical laws of the amygdales in real samples, and this confirmed the stress–strain curve characteristics, failure modes, and characteristic strengths measured in laboratory tests. (3) A complicated inhomogeneous stress field formed readily around amygdales and had a significant effect on the fracture path. (4) With an increase in confining stresses, the degree of tensile stress concentration at the upper and lower tips of amygdales decreased and the lateral displacement became restricted. Consequently, the total number of micro-cracks and the ratio of tensile cracks to shear cracks decreased, the crack initiation mode changed from tensional cracking to shear cracking at the tips of the amygdale, and the distribution of crack orientations transitioned from an approximately vertical direction to preferential concentration along fracture planes. The research results form a solid foundation for the accurate understanding of the mechanical response and fracture mechanism of the rock surrounding underground powerhouse caverns at Baihetan hydropower station, and also provide a reference for the prediction of the mechanical properties of rock with flaws and the stability control of engineering rockmasses.