Study On Mechanical Properties Of Double-block Ballastless Track Under Differential Settlement Of Subgrade

There are many types of ballastless tracks,and its structure is beautiful,stable and easy to maintain,and it has gradually become the structural type adopted by high-speed railways.Among them,the double-block ballastless track has became one of the main structural types of high-speed railway ballastless track in China because of its high integrity and good durability.The subgrade is the foundation of ballastless tracks,and its stability is crucial to the safety of train operation,and the engineering community is particularly concerned about track deformation caused by uneven settlement.In addition to the influence of self-weight and uneven filling,the roadbed is also affected by external factors such as train load and water erosion,and its deformation will continue to accumulate,resulting in uneven settlement.When the roadbed is unevenly settled,the smoothness of the track structure is affected,and even the phenomenon of empty crane occurs,which affects the safe and smooth operation of the train and the service performance of the track.In view of this,this paper establishes an analytical model of tensile-free Pasternak foundation superimposed beam and a three-dimensional beam-body space finite element model for the double-block ballastless track of soil subgrade,and studies the static characteristics of double-block ballastless track under uneven settlement of subgrade,so as to provide theoretical reference for engineering problems such as uneven settlement of high-speed railway subgrade.The main work and conclusions of this paper are as follows:(1)This article summarizes the development status of ballastless tracks at home and abroad,introduces the causes and hazards of subgrade settlement,and summarizes the relevant research on uneven settlement of subgrade.The advantages and disadvantages of the superimposed beam model,beam-plate model and beam-body model are discussed,and several solution methods for the elastic foundation superimposed beam model of ballastless track are further introduced and derived.(2)The analytical model of the superimposed beam of the Pasternak foundation without tensile force was established,the stress and deformation characteristics of the double-block ballastless track under the uneven settlement of the subgrade were analyzed,and a three-dimensional beam-body space finite element model was established for comparative verification,analysis,and applicability of the analytical model and its scope of application.It is found that when the subgrade settlement is relatively flat,there is almost no difference between the prediction results of the analytic model and the finite element model on the rail surface deformation,and the deviation of the prediction results of the two models gradually increases with the intensification of subgrade settlement.When the sedimentation amplitude exceeds 21 mm under the condition of subgrade sedimentation wavelength of 20 m,the analytical model is no longer applicable.Given the subgrade settlement amplitude of 20 mm,the analytical model is suitable for subgrade settlement conditions with sedimentation wavelength greater than 19.8 m.(3)Using the established analytical model of Pasternak foundation superimposed beam and the three-dimensional beam-body space finite element model,within the allowable error range,the corresponding model is used to analyze the force and deformation results of double-block ballastless track under different subgrade settlement conditions.The results show that the ballastless track will undergo subsequent subsidence deformation under the action of uneven settlement of the subgrade,and the rail will have a slight local up-arching phenomenon at both ends of the settlement zone,and the smoothness of the track will be reduced.In addition,the stiffness of the ballastless track and the roadbed is quite different,so that the settlement deformation of the two cannot be coordinated,and the orbit derailment phenomenon occurs.The force and deformation of ballastless track have a potential relationship with the subgrade settlement wavelength and amplitude,and the maximum deformation of the rail will increase regardless of the sedimentation wavelength or amplitude.However,the magnitude of the maximum deformation of the rail varies with the settlement amplitude.And under a certain settlement amplitude,the maximum deformation of the rail will appear a critical wavelength with the change of the sedimentation wavelength,when the sedimentation wavelength is less than the critical sedimentation wavelength,the maximum deformation of the rail increases with the increase of the sedimentation wavelength,and when the sedimentation wavelength exceeds the critical wavelength,the maximum deformation of the rail basically no longer changes.The longitudinal maximum tensile and compressive stress of the track bed plate will also appear a critical wavelength with the change of the sedimentation wavelength,and the longitudinal maximum tensile and compressive stress of the track bed plate will increase with the increase of the subgrade sedimentation wavelength before the sedimentation wavelength reaches the critical wavelength,and the sedimentation wavelength exceeds the critical wavelength and the trend of decrease,and the critical wavelength corresponding to different sedimentation amplitudes is different,and the different critical wavelengths will basically appear in the range of sedimentation wavelength of 20m～25 m when the subgrade settlement amplitude is within 40 mm.Generally,at this critical wavelength,the sedimentation amplitude has a significant effect on the increase of stress on the track bed plate.Therefore,when the subgrade settlement amplitude is within 40 mm,the subgrade settlement at the wavelength of 20 m～25 m should be focused on,and effective engineering measures can be taken if necessary.(4)The influence of track structure parameters on the static characteristics of double-block ballastless track under uneven settlement of subgrade was analyzed by using the three-dimensional beam-body space finite element model.The results show that the increase of the thickness of the track structure layer can reduce the deformation of the track structure caused by subgrade settlement.The change of the thickness of each structural layer of the track has different degrees of influence on the stress of the track bed plate and the support layer.The reduction of the elastic modulus of the track structure has a great influence on the force deformation of the double-block ballastless track under the uneven settlement of the subgrade,so the engineering problems such as the reduction of the elastic modulus of the track caused by the long-term service of the track should attract sufficient attention.