Study On Crack Damage Of Double Block Ballastless Track Roadbed Slab In Tunnel Under Wate Pressure

Double-block ballastless track with its structural stability,good integrity,simple construction and other advantages,has been widely used in railway construction of our country.However,under the influence of groundwater and shrinkage load for a long time,the double-block ballastless track appears a large number of cracks in the construction stage,which affects the bearing capacity and durability of the track structure.This thesis takes the double block ballastless track roadbed slab in a tunnel under construction of Chenglan Railway as the research object.Based on the theory of concrete fracture mechanics,the instability and propagation of cracks in the roadbed slab are studied as follows.(1)Combined with the field investigation and domestic and foreign literatures on ballastless track injury,analyzed and summarized the causes of roadbed slab cracking and the process of crack growth.(2)A three-dimensional solid model of tunnel-double block ballastless track was established by using finite element software,and the reliability of the finite element model was verified.The values of hydraulic pressure load and shrinkage load under invert are considered according to the actual conditions in the tunnel.The simulation calculation of tunnel-double block ballastless track under different conditions was carried out,and the mechanical characteristics of track bed slab structure under the action of shrinkage load and water pressure load were analyzed,and the cracking of track roadbed slab was judged according to the stress strength criterion.The results show that the shrinkage load is an important factor for the cracking of roadbed slab,and the maintenance measures can be taken to reduce the shrinkage cracking of roadbed slab concrete to the greatest extent during the construction stage.Hydraulic pressure load is the key factor of the crack of the roadbed slab.When the drainage system is completely blocked,some measures of drainage and pressure relief should be taken to control the water pressure below0.3MPa to ensure the durability of the roadbed slab.The joint action of shrinkage load and water pressure load is the fundamental cause of cracking of double block ballastless track roadbed slab in tunnel during construction.(3)Based on the results of finite element simulation,FRANC3 D software was used to insert three-dimensional cracks of different shapes in the maximum stress region of the roadbed slab,and the influence of crack length and depth on the crack growth of the roadbed slab was studied.The stress intensity factor of crack tip under different conditions was calculated,and compared with the critical value of fracture toughness of roadbed slab concrete,so as to judge whether the crack is unstable and expand,and summarize the factors affecting the existing crack growth.The results show that under the action of shrinkage load,when the crack length is less than 1000 mm and the depth is less than 200 mm,the crack will not propagate along the transverse instability.Under the action of water pressure load(the drain pipe is completely blocked),when the length of the crack exceeds 700 mm and the depth exceeds 100 mm,the crack will propagate along the transverse instability;The joint action of shrinkage load and hydraulic load is the main driving force of crack propagation.