Experimental study on the failure mechanism of sandstone containing en-echelon joints and anchoring control effect of NPR bolt

This study systematically investigates the failure mechanisms of en-echelon jointed rock with varying joint dip angles and the bolt anchoring control effects through uniaxial compression tests combined with acoustic emission monitoring and digital image correlation techniques. The results demonstrate that unanchored specimens exhibited through-going failure characteristics, with the severity of failure significantly increasing as the joint dip angle increases. When the joint dip angle ranges from 15° to 45°, the specimens exhibit tensile cracks through-going failure, with AE events being dispersed and strain concentration zones localized around the joints. In contrast, at higher dip angles (60°–75°), the specimens exhibit shear cracks through-going failure, with AE events concentrating during the failure stage and showing higher peak values. Bolt anchoring significantly improves the mechanical properties of en-echelon rock, effectively reducing the degree of failure and strain values. Furthermore, strain values in the anchored zone are lower than in the distal jointed regions. Meanwhile, it leads to more dispersed AE events with lower peak amplitudes during the failure stage. At higher joint dip angles, bolt anchoring transforms the specimen’s failure mode from through-going failure to block failure. Negative Poisson’s Ratio (NPR) bolt anchoring provides better anchoring effects and reduces the transition angle of failure modes. Bolt restraint is pronounced under low dip angle conditions, with axial force increasing sharply during the plastic stage. Under high dip angle conditions, axial force increases sharply only during the failure stage. These findings provide critical insights for optimizing support strategies in en-echelon jointed rock engineering.