Mechanical properties and strength prediction model of anchored rock-mortar structural plane under triaxial compression

The rock wall-concrete shotcrete layer, which is supported by anchor and shotcrete in a mining environment, represents a typical example of an anisotropic structural plane anchored by rock mortar. The objective of this study is to investigate the mechanical behaviour of the rock wall-spray layer interface supported by bolting and shotcreting. To this end, the findings of an experimental study on the mechanical properties of rock-mortar non-anchor and anchored anisotropic structural plane specimens under triaxial compression are presented herein. The mechanical properties of rock-mortar unanchored and anchored anisotropic structural plane specimens under triaxial compression were investigated utilising the MTS-815 rock mechanics test system. The confining pressure of the test was set at 10, 15, 20 and 25 MPa. The rock-mortar anisotropic structural plane is defined as an isosceles triangular serrated surface. The undulating angles are 30°, 45°, and 60°. This study analyses the influence of bolt anchorage and confining pressure on the mechanical properties and failure mode of rock-mortar structural planes. Additionally, the acoustic emission signal in the compression test process is analysed. Subsequently, the three-dimensional morphology of the structural plane following fracture is scanned. Finally, the compressive strength prediction model of rock with anchored inclined anisotropic structural plane is established. The findings indicate that the peak strength of rock-mortar anisotropic structural plane specimens exhibits an increase with the rise in confining pressure and undulation angle of the structural plane. The mortar matrix exerts a dominant influence on the trend observed in the deviatoric stress-strain curve of rock-mortar non-anchor anisotropic structural plane specimens. Following the anchoring process, the peak strength of the rock-mortar anisotropic structural plane specimen is demonstrably enhanced, accompanied by a notable increase in the specimen's capacity to withstand plastic deformation. Following the fracture of the specimen, it was observed that the material was still capable of withstanding significant stress. Additionally, the deviatoric stress-strain curve exhibited double-peak characteristics. A comparative analysis of the failure modes exhibited by rock-mortar anisotropic structural plane specimens, both before and after anchoring, reveals that the unanchored specimens display shear slip failure along the anisotropic structural plane. Conversely, specimens that have been anchored demonstrate 'X'-shaped conjugate shear failure. Comparing the acoustic emission characteristics of rock-mortar anisotropic structural plane specimens before and after anchoring under triaxial compression, it is found that the acoustic emission signal of the specimen after anchoring is sparse in the early stage of loading, but the ringing count signal is more intensive after the macroscopic fracture of the specimen, which means that the bolt after anchoring is very good before the macroscopic fracture of the specimen, which limits the generation of small cracks inside the specimen. After the macroscopic fracture, the bolt better exerts the performance of the structural plane itself. Three-dimensional morphological scanning of the structural plane following fracture revealed that the degree of wear on the rock structural plane was less than that observed on the mortar structural plane. Furthermore, the degree of wear on the structural plane was found to decrease following anchoring. Comparing the test results with the strength prediction model results, the average error of the predicted strength under different structural surface roughness is 6.64 %, which verifies the reliability of the prediction model.