Study On Vertical Dynamic Response Of The Track/subgrade In High-speed Railway

As the train speed increases, the dynamic responses which are the bases of the design, construction, maintenance of the track/subgrade are more obvious than before. Therefore ,more and more researchers are paying attentions to this subject with the rapid development of the high-speed railway in our country. In this thesis, based on the achievements of the previous literature, the track/subgrade dynamic response in high-speed railway is investigated with the methods of theoretical study, numerical simulation and experimental research. The main research work of this dissertation are concluded as follows:1. Presented are the introduction of the published literature reviews on track/subgrade dynamic response.2. A semi-analytical mechanical model are developed for the prediction of track/subgrade dynamic response induced by trains loads. This model incorporates the necessary components of the railway system such as the track, railpad, sleeper, ballast and subgrade. The vehicles are described as a moving load. The track is modeled as multiple beams supported by vertical springs with consistent mass. The subgrade is regarded as a homogeneous viscous half-space. By mean of moving coordinate and Fourier Transform, the governing equations of the track/subgrade system are established and solved in the wavenumber domain. The Fast Fourier Transform technique is used to convert the responses of the track/subgrade from the wavenumber domain into the space domains. The effects of the train speed and the frequency of excitation on the subgrade displacement are investigated using this model.3. Three-dimensional dynamic finite element model of railway vehicle/track/subgrade system is developed to study the vertical dynamic response of the system. The vehicle is simulated by a lumped parameter system. The track is modeled as beam element on discrete pad-tie supports. The sleeper, ballast and subgrade are discretized into eight-node brick elements. The non-linear factors such as wheel/rail contact, non-linear constitutive relation of the ballast and subgrade are taken into account. A viscous-spring boundary is used to model the infinite domain of the subgrade. A numerical direct integration technique and separated iterative procedure isemployed to solve the equations of motions of the vehicle and track/subgrade systems. Dynamic three-dimensional finite element program ANSYS is chosen for creating the models to realize simulative analysis. The agreement between the numerical results and experimental data and published literature shows the validity of the created model, which suggests that it can be used as a tool for predicting of dynamic response of the system under the high-speed train load.4. Based on the data of track/subgrade of Qin-Shen passenger railway, some parameters are investigated which affect dynamic response of the system by utilizing this model, such as the train speed, track irregularities, depth of the ballast and subgrade, stiffness of the subgrade. Evaluation about the design parameters is made.5. In-situ measurements are performed on Qin-Shen passenger railway. During the tests, the train speed reaches to 300km/h, the data of subgrade dynamic responses under the high—speed train load are first obtained. On the basis of analysis about experimental data in time domain and frequency domain, some dynamic characteristics of different subgrade configuration are investigated, such as the amplitude of dynamic stress, acceleration and relations with the train speed. Some conclusions are made which provide reference for the design of high—speed railway subgrade.6. The contents are summarized and some questions on this subject which need to be deeply studied are put forward.