Mechanical properties of rock bolt and analysis for the full-process of sliding failure based on rock mass absolute displacement

Previous studies rarely involved the mechanical properties of anchorage system under the condition of internal absolute displacement of surrounding rock and the solution of important parameters of anchorage system. In this paper, the absolute displacement law of rock mass under the anchorage system is analyzed by the fiber grating (FBG) multi-point displacement meter and multidirectional displacement conversion formula. Based on the coordinated deformation principle for rock bolt and rock mass, the mechanical analysis model for rock bolt-rock mass system is established, and the influence rules of rock bolt length, rock bolt diameter and rock bolt preload on the internal force distribution of rock bolt is studied based on the stress analysis of full-length rock bolt in a tunnel engineering. The results show that: (1) with the increase of rock bolt length, the anchoring effect gradually enhances, but the improvement degree decreases gradually; (2) with the increase of rock bolt diameter, the anchoring effect is enhanced; at the same time, however, the shear stress of rock bolt is larger, and the possibility of rock bolt sliding also increases; and (3) with the increase of preload, the increase of "anchor length" makes the anti-sliding ability of rock bolt increase correspondingly, and the supporting effect of rock bolt increases gradually. Then the full-process analysis for the mechanical mechanism of rock bolt sliding failure and the iterative calculation method for rock bolt displacement are studied. At last, the numerical simulation for pull-out test is carried out through the finite element analysis to verify the calculation results of mechanical model, and according to the iterative operation, the two important parameters such as the interface shear stiffness K and the interface peak shear strength τ_P are solved. The analysis show that the numerical simulation results are in good agreement with the mechanical model calculation. This study can provide a theoretical basis for rock bolt support technology and support structure design.