Study on the Fracture Evolution Characteristics of Existing Defect Lining under Unsymmetrical Load

In order to explore the fracture evolution characteristics of existing defect lining structures under unsymmetrical loads, unsymmetrical failure model tests of three working conditions, namely, intact lining, vault defect, and arch waist defect, were carried out. Acoustic emission (AE) technology was used to detect the lining cracks from microscopic to macroscopic. The relationship between the accumulated energy of AE and the relative load was established, and a fracture evolution model of lining based on the cumulative energy of AE was proposed. The results show that the failure process of the intact lining under unsymmetrical load has three stages: “initial crack cracking at the unsymmetrical position → development and formation of main crack → specimen cracking failure”. The load ratio of initial crack cracking is 16.5%, and the load ratio of main crack development and formation is 52.2%. The initial cracking position of the lining under unsymmetrical loads has nothing to do with the existing defect position, with defects occurring at the unsymmetrical position, and the remaining cracks are mainly distributed in the vault, inverted arch and arch foot position. Compared with the intact lining, the ultimate bearing capacity of the vault defect and arch waist defect decreased by 14.2% and 21.3%, and the maximum deformation decreased by 44.6% and 50.6%, respectively. The radial deformation degree in the unsymmetrical position at each stage is basically the same under unsymmetrical load conditions, and it is not affected by the position of the existing defects. The deformation proportion of each stage is concentrated in 5%, 40%, 53%, respectively. We derived the facture evolution of lining based on the accumulated energy of AE, and the quantitative relationship between the damage variable and deformation of the lining under unsymmetrical load was established. When the fracture degree reaches 0.04 and 0.35, the deformation of vault defect lining and the arch waist defect lining is 58.3%, 75.0%, 40.2% and 50.6% earlier than those of the complete lining, respectively. When the fracture degree exceeds 0.6, the growth rate of damage degree is no longer affected by the location of the existing defect.