Abstract: Cracks significantly influence the stability of rock engineering, and two parallel cracks are the severe distribution forms of cracks. A series of triaxial compression tests were conducted on sandstone specimens with two parallel cracks to investigate the effect of fracture morphology on its mechanical properties and failure modes. The results showed that strength, elastic modulus and Poisson’s ratio of fractured rock were influenced by both the trace length and the trace length ratio. It was also found that these parameters decreased first, then increased immediately with dip increments, and less varied with spacing between cracks. When the dip angle was less than 30°, the failure plane initiated from crack tips and propagated through the rock bridge along the direction of principal stress. Shear failures localised within one crack, when the angle was between 30°and 60°. For the rock specimen with small trace length and trace length ratio (continuity 75%), but the trace length ratio remained relatively small (trace length ratio <1). For the rock with moderate cracks (continuity falls between 25%~75%) and the trace length ratio of approximately 1, failure modes were controlled by two cracks.

Key words: two parallel fractured rock, fracture dip, fracture spacing, trace length, mechanical properties