Effects Of Differential Subgrade Settlement And Its Evolution In High-Speed Railways On Mechanical Performance Of Vehicle-Track Coupled System

In the last decade,there has been a substantial boom in the development and construction of high-speed railway in China.The advantages of superior integrity,stability,uniform stiffness and low maintenance make the ballastless track the preferred track type.The increasing demands for higher speed and concerns over the running safety,ride comfort and lower track lifecycle cost have resulted in fresh challenges need to be addressed.As the basis of track structure and train load,the soil subgrade has relatively large compressibility and low permeability,which is sensitive to dynamic load and water erosion.Differential subgrade settlement becomes inevitable due to the non-uniform soil property,variation of groundwater-level and other defects of subgrade.It will directly lead to track degradation and change of supporting state,and further make an impact on dynamic performance of both train and track.Serious differential subgrade settlement may even pose a threat to the high-speed train's running safety.This makes the settlement control a key issue in construction of ballastless tracks on soil subgrade.In view of this,numerical studies on mechanical characteristics of high-speed vehicle?ballastless track system affected by differential subgrade settlement are carried out,to investigate the threshold value of the settlement and its long-term evolution from the perspective of dynamics,which may provide theoretical basis for the early design,operation and maintenance of high-speed railways.Primarily,based on the theory of composite beams on Winkler elastic foundation,model for mapping relationship between differential subgrade settlement and ballastless track deflection is established.Three types of ballastless tracks are considered according to the commonly used track structures in Chinese high-speed railways,the double-block ballastless track,the unit slab ballastless track,and the longitudinal connected slab ballastless track.Both analytical and semi-analytical solutions are adopted to determine whether the model considers interlayer contact or not,and the comparative verification is carried out by the corresponding finite element model.On this basis,the vertical high-speed vehicle?ballastless track?subgrade coupled dynamic model taking subgrade settlement into account is established by applying the vehicle?track coupled dynamics theory.In the model,the vehicle is simplified as a multibody system,and the track weight as well as the contacts between concrete track structures and between track and subgrade are considered in the track system,where the Heaviside function is introduced to describe the contact states.The differential subgrade settlement is input as a non-linear boundary condition,and the calculation procedure is programmed.Then,by applying the semi-analytical solution on the above-mentioned ballastless tracks,the additional track irregularities and contact characteristics between track and subgrade due to differential subgrade settlement under the action of track weight are analyzed and compared.Influences of settlement parameters(wavelength,amplitude)and types(cosine type,angular type,faulting type)on static track deflections and interlayer contact states are investigated in detail.Taking the results of statics study as the initial condition,the dynamic characteristics of the vehicle?ballastless track coupled system affected by differential subgrade settlement are simulated.Results indicate that the wheel?rail dynamic interaction is aggravated,but compared with the running safety,the car body vibration is more intense and attenuated slowly,which shows that the ride comfort is more susceptible to the differential settlement.The settlement-induced locally unsupported areas between track and subgrade have significant effect on the dynamic performance.When subjected to the vehicle load,the track vibration in these areas is exacerbated to an extent.The track and subgrade may resume contact or remain unsupported,but the gap size in the unsupported area is alleviated.Due to the long-term impacts of the high-speed trains,these unsupported areas will be compressed periodically,which is harmful to the fatigue life of the ballastless track.For the differential subgrade settlement in cosine type,there exists sensitive settlement wavelength(15~25m for the specific conditions considered in this dissertation)that significantly aggravates the vehicle dynamic response.The increases of the settlement amplitude and the vehicle speed also result in larger dynamic response of the coupled system.Besides,influence of differential settlement with angular type on dynamic performance is relatively gentle,while the faulting-type settlement gives rise to severe voids between track and subgrade,which leads to drastic influence on track vibration.Accordingly,research on the control value of the differential subgrade settlement in different ballastless track systems is conducted from the perspective of train's running safety,ride comfort and the dynamic performance of the track?subgrade system.It shows that compared with the index of running safety,differential subgrade settlement has more significant influences on ride comfort and dynamic interaction between vehicle and track.It turns out that the vehicle acceleration becomes the control index for subgrade settlement with wavelength greater than 20 m,and the dynamic performance of the track?subgrade system is crucial in the evaluation of settlement with wavelength below 20 m,especially the dynamic rail displacement.The control value of the differential subgrade settlement aiming at different settlement wavelengths can be preliminarily obtained,which are quite similar for all three types of ballastless tracks.The settlement with wavelength less than 10 m should be eliminated.For subgrade settlement with wavelength within 15~20m,20~30m or 30~40m,the settlement amplitude is recommended to be controlled below 10 mm,15mm or 20 mm,respectively.Further,the proposed vehicle?ballastless track coupled dynamic model with differential subgrade settlement is combined with an empirical power model for accumulated subgrade deformation.An iterative method is put forward to predict the long-term track degradation of ballastless track due to the evolution of the differential subgrade settlement under repeated traffic load.Taking the Wuhan-Guangzhou high-speed railway as an example,the long-term track degradations initiated by different differential subgrade settlements are simulated and compared.The variation of the dynamic characteristics of the vehicle?track system during the settlement evolution is also investigated.