Study On Longitudinal Crack Propagation Law And Fatigue Life Of Concrete In Tunnel Bottom

Concrete is a composite material composed of cement,aggregate and various admixtures.At the beginning of its formation,the interior of the concrete has the initial defects of randomly distributed micro-cracks.As a important part of the tunnel support structure,the foundation not only bears The effect of surrounding rock pressure is affected by the long-lasting repeated effects of train loads.Therefore,the bottom of the tunnel is more common,especially the cracking of the concrete.The research on the crack propagation law and fatigue life of the bottom concrete under combined load has important practical significance for the maintenance and treatment of cracks in the tunnel operation process.Based on the literature investigation and the cracking of the concrete under the tunnel foundation,the crack initiation location prediction model and the crack propagation characteristic analysis model are established based on the fracture mechanics theory.On this basis,the expansion law of single crack and combined crack of the bottom concrete is studied.Based on the fatigue damage theory,the fatigue life of the bottom concrete under the dynamic load of the train is analyzed by the fatigue analysis software.main tasks as follows:(1)The system investigates the longitudinal crack propagation of existing concrete,especially the distribution of cracks and crack characteristics,and points out the main causes and hazards of crack propagation.(2)Field observation,radar scanning and core drilling were carried out on the cracks of the tunnel,and the load distribution of the train was tested and analyzed.Based on the bottoming situation,the finite element model is established.Combined with the above work,the crack initiation position of the foundation structure under the combined effect of surrounding rock pressure and train dynamic load is analyzed.It is believed that cracks are easily generated at three locations at the center of the top surface of the foundation,at the track,and at the end of the sleeper.(3)Based on the theory of elastic fracture mechanics of concrete,a model with preset cracks(single crack and combined crack)at the center of the concrete,at the track and at the sleeper end is established.The crack position,angle and depth are studied.The characteristic parameters and the influence of the extended path give the critical depth of the crack expansion of the three locations at the bottom of the foundation.The mechanism of the three-position crack propagation is dominated by the open-type extension and the sliding-type expansion.The minimum depth when there is an effect between the combined cracks is given.(4)Based on the fatigue damage theory of concrete,the fatigue life of concrete beams in existing literature is analyzed,and the feasibility of using Fe-safe software to analyze the fatigue life of concrete is verified.Then,the single crack and combined crack of the bottom concrete are studied under train cyclic load.The fatigue life is obtained,the single crack has the shortest fatigue life at the track and the fastest expansion rate.The combined crack has a different degree of reduction compared with the minimum fatigue life of the single crack.