Deformation and failure characteristics and fracture evolution of cryptocrystalline basalt

Abstract

Cryptocrystalline basalt is one of the two major types of rocks exposed in the super large-scale underground powerhouse in Baihetan hydropower station in China. The rock of this type shows various site-specific mechanical responses (e.g. fragmentation, fracturing, and relaxation) during excavation. Using conventional triaxial testing facility MTS 815.03, we obtained the stress–strain curves, macroscopic failure characteristics, and strength characteristics of cryptocrystalline basalt. On this basis, evolution of crack initiation and propagation was explored using the finite-discrete element method (FDEM) to understand the failure mechanism of cryptocrystalline basalt. The test results showed that: (1) under different confining stresses, almost all the pre-peak stress–strain curves of cryptocrystalline basalt were linear and the post-peak stresses decreased rapidly; (2) the cryptocrystalline basalt showed a failure mode in a form of fragmentation under low and medium confining stresses while fragmentation-shear coupling failure dominated at high confining stresses; and (3) the initial strength ratio (σci/σf, where σci and σf are the crack initiation strength and peak strength, respectively) ranged from 0.45 to 0.55 and the damage strength ratio (σcd/σf, where σcd is the crack damage strength) exceeded 0.9. The stress–strain curve characteristics and failure modes of cryptocrystalline basalt could be reflected numerically. For this, FDEM simulation was employed to reveal the characteristics of cryptocrystalline basalt, including high σcd/σf values and rapid failure after σcd, with respect to the microscopic characteristics of mineral structures. The results showed that the fragmentation characteristics of cryptocrystalline basalt were closely related to the development of tensile cracks in rock samples prior to failure. Moreover, the decrease in degree of fragmentation with increasing confining stress was also correlated with the dominant effect of confining stress on the tensile cracks. Keywords: Baihetan hydropower station, Cryptocrystalline basalt, Deformation and failure characteristics, Characteristic strengths, Crack propagation evolution