Effect Of Differential Subgrade Settlement On Dynamic Characteristics And Damage Behavior Of CRTSⅡ Slab Track

During the long-term service of high-speed railway,the subgrade is affected by factors such as the level change of groundwater,uneven soil quality,and near-field construction disturbances,which will inevitably lead to differential settlement.The differential subgrade settlement will lead to the changes in the geometry of the track structure,resulting in line irregularities,increasing the wheel-rail dynamic interaction,and generating additional stress in the ballastless track structure,which is likely to cause damage to the track structure and affect working state and service life of the track.However,the current researches on differential subgrade settlement are insufficient in considering the plastic damage constitutive and interface bonding of the track structure.And the influences of differential subgrade settlement on the dynamic characteristics and damage behavior of the track cannot be accurately revealed.In this paper,a nonlinear finite element model of ballastless track-subgrade system is established by taking the CRTSⅡ slab track,which has the characteristics of longitudinal connection and poor adaptability to differential subgrade settlement.On the basis of clarifying the influences of differential subgrade settlement on the deformation of track structure,the influences of the differential subgrade settlement on the dynamic performances of the vehicle-ballastless track system and the damage behavior of the track structure are analyzed.The damage mechanism and evolution law of the track structure under the differential subgrade settlement are revealed.The reasonable control limit of the differential subgrade settlement is proposed,which provides theoretical support and technical guidance for ensuring the long-term safe operation of high-speed railways in China.The specific research work and results are as follows:(1)Considering the plastic damage of track structure concrete and the cracking behavior of interface bond,a nonlinear finite element model of CRTSⅡ slab track-subgrade system is established.The concrete damaged plasticity(CDP)model is used to characterize the damage and deterioration behavior of the main structure of the ballastless track.The calculation parameters of the CDP model are obtained through laboratory tests.An improved bilinear mixed-mode cohesive zone model is proposed to simulate the mixed-mode cracking behavior of the interface in track structure.Considering the bond-slip effect between steel bars and concrete in the track slab,a bilinear kinematic intensification(BKIN)model of steel bars is constructed.Finally,the nonlinear finite element model of CRTSⅡ slab track-subgrade system is established,and the correctness of the model is verified.(2)The deformation characteristics of the CRTSⅡ slab track under differential subgrade settlement are studied.And the influences of the settlement wavelength,settlement amplitude and temperature load are analyzed.Based on the nonlinear finite element model of the CRTSⅡ slab track-subgrade system,the deformation characteristics and the interlayer contact state of the CRTSⅡ slab track under differential subgrade settlement are studied.The internal relationship between the settlement amplitude,settlement wavelength and the deformation behavior of track structure is discussed.The influence mechanism of temperature load and the coupling effect of temperature load and differential subgrade settlement on the deformation behavior of track structure are revealed.(3)The vehicle-ballastless track-subgrade coupling dynamic model is established.The influences of the differential subgrade settlement on the dynamic characteristics of the vehicle-ballastless track system are studied.The infinite transmission boundary of the subgrade structure is simulated by using infinite elements.And the vehicle-ballastless track-subgrade coupling dynamic model is established.The dynamic behaviors of vehicle-ballastless track system under the condition of differential subgrade settlement are analyzed,and the influence of settlement parameters is discussed.The settlement change rate index is proposed to evaluate the differential subgrade settlement,which realizes the comprehensive consideration of settlement amplitude and settlement wavelength,and overcomes the deficiency that the traditional single index(settlement amplitude)can not accurately evaluate the effect of differential subgrade settlement on vehicle dynamic characteristics.In addition,the relationship between subgrade settlement change rate and vehicle operation stability and safety is studied.And the fitting formula between vehicle dynamic response and settlement change rate is proposed.(4)The influences of differential subgrade settlement on the main structure damage and interface bond failure of CRTSⅡ slab track are studied.The damage mechanism of track structure under differential subgrade settlement is revealed.The concrete stiffness damage factor and the bonding damage factor are used respectively to characterize the damage degrees of the concrete and the interface bond.The damage behavior of the ballastless track under differential subgrade settlement is analyzed.And the damage mechanism of the track structure under differential subgrade settlement is revealed.The evolution law of track structure damage with the change of settlement type and settlement parameters is discussed.The influences of temperature load and differential subgrade settlement on the damage behavior of track structure are proved.(5)The reasonable control limit of the differential subgrade settlement of the CRTSⅡ slab track is proposed.Based on the dynamic response of vehicle-track system,a method for identifying differential subgrade settlement is proposed.An evaluation index suitable for the overall damage state of the track structure is proposed.Considering the influences of differential subgrade settlement on vehicle-track dynamic response and track structure damage deterioration,the control limit of subgrade settlement is proposed.The control standard of subgrade settlement under different wavelength conditions is improved.