Geostress environment and fracture distribution both exert important influences on the mechanical properties and failure modes of fissured rock masses. Laboratory test results are presented here to simulate particle flow code (PFC) in externally double-fissured sandstone samples. Mechanical responses of confining pressure and rock bridge angle on stress-strain curves as well as macroscopic damage and fracture propagation in these samples were studied in order to elucidate energy dissipation

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... anisms. The results of this analysis show that fissured sandstone peak strength and elastic modulus as well as peak axial and lateral strain increase significantly as rock bridge angle decreases while peak strength increases slightly in concert with confining pressure. Rock bridge angle exerts an important influence on macro fracturing patterns; when β = 0°, wing cracks from two pre-existing external fissures propagate in opposite directions, but when β = 60°, the inner tips of two external fissures become directly connected. The evolution of specimen fracturing passes through four main stages, elastic compression deformation, stable crack development, unstable crack development, and post-peak accelerated crack development. Internal contact forces reach maximum values at the peak stress point, while cracks are mainly tensile and shear examples are mostly distributed at orientations between 80° and 100°. Shear cracks are mainly generated along the direction of main stress, σ 1 , while pre-peak dissipated energy is small, and increases rapidly at the post-peak. As rock bridge angle decreases, peak strain and boundary energies both increase significantly. Data show that energy and rock bridge angle are approximately linearly positively correlated.