Research On The Mechanism And Analysis Method Of Cable Tunnel Fracture Damage

With the large-scale development of urban underground space,the mutual disturbance of urban underground engineering is increasing day by day.Taking existing cable tunnels in cities as an example,common external disturbance engineering includes excavation of foundation pit,municipal tunnel,subway tunnel,etc.Improper construction control or too close distance may lead to serious damage and cracking of cable tunnel lining.In this thesis,three typical external projects(underground tunnel,subway shield tunnel,urban highway tunnel)are orthogonal under the existing cable tunnel as examples.Based on fracture mechanics theory,the load-structure model is established by numerical simulation method.The damage process,crack cracking rule,disturbed damage cracking range and crack damage degree evaluation methods of existing cable tunnel secondary lining with different clean distance and cable tunnel size are studied.The main research contents and results are as follows:(1)It briefly expounds the basic theory of concrete fracture mechanics,and then introduces the principle of Extended Finite Element Method(XFEM)and Cohesive Zone Model(CZM)to simulate the appearance and development of cracks in concrete structures.(2)Based on relevant research,XFEM and CZM were used to carry out numerical simulation of three-point bending test of plain concrete,and the key parameters required for crack propagation were obtained.Based on the cracking parameters obtained,the three point bending test of reinforced concrete beam was carried out by using the two methods.The results obtained by the two methods were compared with the relevant research results.Considering the advantages and disadvantages of the two methods,CZM was used to simulate the tunnel lining cracking.The numerical simulation of four-point bending test of reinforced concrete beam was carried out by CZM,and the key parameters required for tunnel lining cracking were calibrated and the model validity verified.(3)ABAQUS finite element software was used to establish a load-structure model,and the mechanical characteristics and damage cracking process of the secondary lining structure of cable tunnel were numerically simulated under three underpenetration conditions: underground tunnel,shield tunnel and highway tunnel.The results show that,under the three underpenetration conditions,the bottom plate of the cable tunnel secondary lining suffers the greatest tensile stress,and the damage first appears at the bottom plate near the wall,which is the most dangerous position.In the damage process,the tensile damage of the structure first starts from the part of the bottom plate near the foot of the wall,and then gradually spreads to the middle of the bottom plate and the middle of the side wall.The damage initiation displacement,micro-crack initiation displacement and macro-crack development displacement of the secondary lining of cable tunnel under different working conditions are obtained.The damage cracking process can be summarized into three stages: initial damage stage,micro fracture connection stage and macro fracture development stage.(4)The crack types and crack geometric characteristics in different working conditions were analyzed,and it was found that the secondary lining cracking types were all type I open cracks(circumferential cracks),and the crack geometric size(such as crack width,length and depth)and the range of longitudinal cracking areas were different in different underpenetration conditions.(5)By referring to the relevant tunnel specifications and combining with the crack geometric characteristic data obtained by numerical simulation,the evaluation method suitable for the secondary lining fracture degree of cable tunnel is developed.According to the safety warning value,critical value and limit value of different fracture standards,the tunnel lining fracture degree is divided into four grades,and the evaluation method is used to grade the secondary lining fracture degree of cable tunnel under the construction disturbance conditions of urban dug tunnel,urban shield tunnel and urban road tunnel.