Effect of normal boundary conditions on the shear mechanics and acoustic emission characteristics of anchored rock joints

Understanding the shear mechanical behavior of anchored rock joints is crucial for controlling of the stability of rock engineering structures. This study conducted direct shear tests on anchored and unanchored rock joints under constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. We also adopted the acoustic emission (AE) technology to analyze the shear mechanical characteristics, AE characteristics, damage and failure mechanisms of jointed rock mass under initial normal stress and normal stiffness. Results show that 1) under the CNL boundary, the shear stress-shear displacement curve exhibited explicit stress softening, while under the CNS boundary, the curve showed transition from stress softening to stress hardening. 2) both τ1 and τ2 increased with increasing normal stress, and the normal stiffness significantly enhanced the magnitude of τ2.3) Under the CNS boundary, the normal displacement of the sample showed transition from shear contraction to shear expansion, and the shear contraction and expansion were more pronounced in unanchored joint samples.4) The AE energy and cumulative AE energy of the anchored joint samples were significantly higher than those of the unanchored joint samples.5) AE localization points were mainly distributed at the intersections of the joint surfaces and near the anchor bars, and the distribution of AE points was more concentrated under the CNS boundary conditions.6) As the normal stress and normal stiffness increased, the damage area of the joint surface expanded from the vicinity of the hole wall to the surrounding area, the damage range of the anchor hole wall decreased, and the anchor bars exhibited "Z" type failure.