Dynamic Response And Long-term Dynamic Stability Of Closely Spaced Transition Sections Subgrade For High-speed Railway

Currently China has the world’s largest high-speed rail network, and the operating speed is also the highest of the world. The track structure of high speed railway demands for high ride comfort and high stability. The long-term dynamic stability and subsequent settlement of the roadbed, especially the transition section between soil subgrade and rigid structure, play a controlling role in the motion of train with high velocity. Transition sections are prone to problems. Therefore, Transition sections are the important structures of high-speed railyway subgrade need to study. And the dynamic characteristics of the interaction between closely spaced transition sections need in-depth analysis. In this dissertation, status quo of researches on the subgrade dynamic response and problems of transition sections at home and abroad were summarized. Based on which, also combined with the project of National Natural Science Foundation of China and the major project from Ministry of Railways’science and technology research plan, the dynamic response characteristics, dynamic stability and long-term deformation characteristics of closely spaced transition sections subgrade for Wuhan-Guangzhou high speed railway, were deep researched by means of in-situ parametric tests, in-situ train-induced vibration tests, laboratory dynamic experiments, theoretical analysis and simulation. The main research work and results of this thesis are as follows:(1) For roadbed of closely spaced culvert-subgrade transition sections, the basic dynamic parameters of each structure layer such as dynamic modulus, shear wave velocity, dynamic Poisson’s ratio and so on, were obtained through in-situ wave-velocity test. And for the typical sections, the dynamic stiffness, damping ratio and undamped natural frequency were acquired via in-situ vibration-exciting experiments. The integrated dynamic stiffness ratio of roadbeds for culvert/transition section/soil subgrade show that the existing transition sections have been ensuring the smooth transition from culverts to soil subgrades. Although the natural frequency of subgrade goes up in proportion to the dynamic stiffness, there is not essential difference in natural frequency between subgrade sections with different dynamic stiffness.(2) Based on random vibration theory and signal processing techniques including the Empirical Mode Decomposition method (EMD) and Hilbert transform, an identification method of subgrade natural frequency using the vibration signals induced by ambient excitation was proposed. Then the natural frequencies of the typical section in the closely spaced culvert-subgrade transition sections were identified by the proposed method. The identification results were compared with results from other methods. Comparisons demonstrate that the proposed method is reliable and facilitaes operate, and the identification results are more comprehensive than results from other methods.(3) In-situ train-induced vibration tests were twice performed in the closely spaced culvert-subgrade transition sections for Wuhan-Guangzhou high-speed railway. One was in the original trial operation period; the other was at twenty months after formal operation. The original vibration signals of subgrade were firstly processed by the EMD to eliminate noise and trend items. Then the effective signals were filtered from the whole by Hypothesis testing. On basis of which, the dynamic response and basic spectrum characteristics of the subgrade were analyzed respectively in time and frequency domains. Furthermore, wavelet analysis method was introduced to perform in-depth analysis of subgrade’s vibration characteristics. Detailed analyses include:①The longitudinal distribution law of the subgrade dynamic response (dynamic stress, vibration acceleration, vibration velocity, dynamic displacement);②The effects of driving factors including train speed, axle load and so on, as well as effects of subgrade stiffness on the subgrade dynamic response and the distribution of subgrade vibration energy. Also the major excitation sourses causing subgrade vibration were analyzed combined with the identified natural frequencies;③The superimposed effects of two closely spaced culverts on the dynamic responses and vibration energies of the soil subgrade between the two culverts;④The dynamic responses and vibration energy charactristics of different subgrade structure layers were analyzed and compared. Base on all the analyses above, the transition effect on the existing transition section was evaluated, and design suggestions about transition section were put forward;⑤The dynamic responses, frequency characteristics and vibration energy distribution of the subgrade obtained in the two train-induced vibration tests were compared. The results proved that the subgrade of the closely spaced culvert-subgrade transition sections is stable and smooth longitudinally along the railway line. Based on the changes of dynamic responses and vibration energies in magnitude, distribution law and decay law before and after operation, the suggestion that the stiffness of upper layer of subgrade should match the one of lower layer was put forward.(4) Based on the laboratory and in-situ parametric experiments, as well as in-situ train-induced vibration tests in formal operation period, the dynamic stability of the subgrade of the closely spaced culvert-subgrade transition sections was checked from the aspects of critical dynamic stress, effective vibration velocity and dynamic shear strain. Then, taking into account the comparison of dynamic responses got from the two in-situ train-induced vibration tests and the additional settlements during the formal peration period, the long-term dynamic stability of the subgrade of the closely spaced culvert-subgrade transition sections was evaluated.(5) The formula of the relationship between cumulative plastic strain and loadings times, as well as the parameters needed in the formula was obtained by laboratory dynamic experiments. Combined with the dynamic deviator stresses calculated through the simulation model established by the three dimensional differential calculation programmer FLAC3D, the cumulative plastic deformations of the sugrade were obtained. The calculated results were compared with the results from the in-situ cyclic vibration test. The comparison demonstrated that the thinking based on both simulation calculation and laboratory experiment was reasonable and applied to predict the cumulative plastic deformations of subgrade for high-speed railway.(6) The dynamic finite element analysis model of the typical section for Wuhan-Guangzhou high-speed railway was established. The relationships between dynamic shear modulus, dynamic damping ratio and dynamic shear strain obtained through laboratory dynamic experiments of the A and B group fillings, were applied in the analysis model as paramaters of the equivalent nonlinear constitutive relation of the corresponding subgrade structure layers. Using the model, dynamic responses of subgrade were calculated144times when the dynamic calculation parameters ranged. Based on which, the effects of dynamic calculation parameters on subgrade dynamic response were analyzed, and the nonlinear formulas of relationships between them were deduced. The design suggestion for subgrade, that the dynamic elastic modulus and dynamic damping ration of every structure layer should match each other, was put forward.