Pull-out debonding characteristics of rockbolt with prefabricated cracks in rock: a numerical study based on particle flow code

The bonding damage of the bolt-rock mass interface determines the stability of supported structures. To investigate the pull-out debonding characteristics of rockbolt, a rockbolt pull-out model considering the physical existence of ribs and the distribution of pre-cracks in cement mortar (rock like material) was established in this study using PFC2D software. Through theoretical analysis and numerical simulation, the influence of distributed cracks on the debonding characteristics of rockbolts, such as crack development, peak load, failure scheme, and contact forces between cement mortar particles and rockbolt, were investigated. Cracks asymmetrically distributed around the rockbolt were also examined. Results show that (1) With symmetrical prefabricated distributed cracks, the anti-pull load of the rockbolts has weak correlation with the uniaxial compressive strength (UCS) of the standard specimen. (2) The peak load of rockbolt-cement mortar with symmetrical prefabricated crack depend on the strength of cement mortar in direct contact with the rockbolt and the overall strength of the coupled cement mortar, which is related to the size, number and distribution of cracks, so is the failure mode. The pull-out stiffness reduces with decreasing model's pull-out bearing capacity. (3) For the normal contact force, its value and overall distribution direction have a positive correlation with that of the rockbolt load. For the shear contact force, its value has a positive correlation with that of the bolt load, but its overall distribution direction has a weak correlation with that of the bolt load. (4) With asymmetrical pre-cracks, the overall strength and the failure characteristics of the rockbolt-cement mortar are predominantly determined by the strength and crack characteristics of the weaker side (i.e., with more or larger cracks), as revealed by the analysis of crack distribution and contact force at failure. The asymmetrical distribution of the cracks also introduces asymmetry in the crack development, with new cracks growing faster along the stronger side of the cement mortar.