The effect of pegmatite veins on fracture behaviors of granite in a deep large underground cavern

The fracture behavior of granite with pegmatite veins, which influences brittle spalling at the Shuangjiangkou Hydropower Station’s underground powerhouse in China, was investigated. This study explored the fracture characteristics of veined granite through three approaches: an in-situ geological survey, true triaxial compression experiments, and microscopic fracture analysis. The results revealed that pegmatite veins, due to their high brittleness, are prone to spalling and often serve as boundaries for rock mass instability. Under true triaxial compression, the stress–strain curve of granite shifted from Class I to Class II in the presence of veins, with thicker veins resulting in lower overall granite strength. In granite with gently dipping veins, deflection of the main fracture at the vein–granite interface was observed, while steeply dipping veins led to fracture propagation either within the veins or along the lithologic interface. Factors influencing the fractures included differences in elastic modulus and Poisson’s ratio between the veins and granite near the lithological interface, as well as mineral composition and structural characteristics. A greater aggregation of shear fracture signals was observed in specimens with thicker veins before reaching the damage stress threshold. Stress-relieving measures are necessary to reduce stress concentration near veined rock masses, and microseismic techniques are recommended for monitoring shear fractures to provide timely warnings of rock mass instability when excavating granite with steep and thick veins in a deep underground cavern.