| Due to the long-term effect of the dynamic load,the traditional ballast track structure is prone to occur diseases like ballast and subgrade deformation,leading to track irregularities and vibration increase,and affecting the smoothness of train operation.As a viscoelastic material,asphalt concrete can improve the bearing capacity and stability of the structure used in the track foundation.There is few research on the application of asphalt concrete in ballasted tracks in our country.Based on the analysis of dynamic characteristics,this paper studies the improvement of bituminous subballast on the mechanical response of ballasted track structures by means of theoretical research and finite element numerical simulation.A design method is proposed to provide theoretical support in the optimization of the underlying structure of ballasted track.Firstly,the research theory of rail-subgrade interaction and asphalt concrete track foundation is summarized.A generalized Maxwell model is used to characterize the constitutive model of asphalt concrete materials,and the viscoelastic parameters of the material are fitted by using the results of dynamic modulus test.The vertical load simplified as concentrated excitation load moving in the direction of the rail.The dynamic indexes used in the analysis of dynamic characteristics include vertical stress,vertical acceleration,vertical deformation,and bending strain at the bottom of the asphalt layer.The dynamic response of a ballasted track structure model established by ABAQUS is compared with the measured results in the relevant literature to verify the reliability of the model.Secondly,the time-course changes as well as distribution of dynamic response in the standard asphalt subballast model are analyzed.It is found that each dynamic response analysis index increases as the train load approaches.The time when the mechanical response reaches its maximum lags is usually later than the time when the train load reaches the index extraction point.Both the vertical deformation and stress are symmetrically distributed and have maximum values below the rail.Asphalt subballast is the main stress bearing layer,and the dynamic stress on the surface of the subgrade is only 12% of the asphalt layer.During the operation of the train,the bottom strain of the asphalt concrete layer is maintained as tensile strain.Comparing with traditional ballast track,the dynamic response of the under-track foundation in the structure using asphalt concrete reduces by over 30%,which proves the advantages of asphalt concrete layer.The analysis results of various influencing factors show that the dynamic response analysis index decreases with the increase of the asphalt concrete layer thickness.When the thickness of the asphalt concrete layer is greater than 16 cm,the reduction rate obviously slows down.The increase of the ballast layer thickness is beneficial to the reduction of the dynamic response of the structure.The mechanical properties of asphalt layer perform better in high modulus.The effect of the subgrade modulus on the dynamic response is small.The greater the train speed,the greater the dynamic response indicators.Comparing different structural thickness combinations,considering the combination of dynamic response and economic cost,the thickness of asphalt layer is better less than 20 cm.Finally,the design method of the track structure using bituminous subballast is put forward,mainly controling the fatigue cracking and deformation of the structure.Fatigue life standard calculation formula is then established using tensile strain as the control index.According to the design example,the thickness of the asphalt layer is better more than 14 cm,so that the service life of the structure can reach 60 years.Considering the dynamic characteristics analysis and service life requirements,it is recommended that the thickness of the bituminous subballast be 14~16cm... |
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