Experimental study on the influence of rock bridge dip angle on creep acoustic emission characteristics of double-fractured sandstone

To reveal the influence of the vertical overlapping degree between two prefabricated fissures on their failure characteristics, specimens with varying rock bridge dip angles (alpha = 75 degrees, 105 degrees, and 135 degrees) were prepared. Uniaxial compression creep and acoustic emission (AE) tests were conducted at a constant loading rate of 0.40 kN/s. The results showed that wing cracks first appear at the outer tips of two prefabricated fissures. Subsequently, wing cracks began to form at the inner tips. With the increase in stress, wing cracks at the outer tips extend toward the top and bottom surfaces of the specimen. For the specimen with alpha = 75 degrees, wing cracks at the inner tips coalesce, resulting in a mode of rock bridge penetration dominated by tensile failure. For the specimen with alpha = 105 degrees, the stress shielding effect causes the compressive contact force to concentrate less at the overlapping tip than at the non-overlapping tip. As a result, wing cracks dominate the rock bridge's penetration mode, exhibiting a tensile-shear compound failure. For the specimen with alpha = 135 degrees, tensile cracks form due to the accumulation of contact force between the tips of the two prefabricated fissures on the same side, resulting in creep failure. The rock bridge's penetration mode exhibits a more complex tensile-shear compound failure. With the increase in rock bridge dip angle, the rock bridge's failure mode transitions from tensile failure to a more complex, progressive tensile-shear compound failure, resulting in more cracks.