| The Qinghai-Tibet Railway (QTR) is experiencing severe subgrade damagesresulting from train-induced vibrations, especially for some sections located in theice-rich warm permafrost site. The operation of passenger and freight trains and thespeed-increasing scheme of QTR may therefore be interrupted. Meanwhile thestablility of permafrost subgrage, without thermal-insulation-measures taken, willbe disturbed with the evolution of global warming. This will gradually lead to thegrowth of unstable warm permafrost. The present thesis attempts to evaluate thelong term operating performance of the ice-rich permafrost subgrade along the QTR,when subjected to traffic loads and annually increasing temperature. A completethree dimensional FEM model of the ice-rich permafrost subgrade is accordinglyestablished, taking thermal effect into account. Thermal-stress coupling isconsidered by simultaneously applying temperature field and the calculated track-sleeper force to the proposed model. The adopted research approach is becomingmore systematic with the implementation of laboratory tests and in situ monitoring.The original work is presented in detail as follows.1. Based on the boundary layer principle, a phase-based transient temperatureforecasting model of permafrost subgrade is built, with a proper consideration ofconstruction period. The temperature evolution of subgrade in the future50years uponthe completion of embankment construction is then predicted, considering the climateand geologic structure of the ice-rich Beiluhe section along the QTR. The numericalresults are in good agreement with the reference data.2. Considering the organization of passenger and freight trains and the trackstructure of the QTR, track-sleeper force induced by passing trains is more accuratelysimulated on the basis of the program ZL-TNTLM, taking also into account the verticaltrack irregularities, track gap and unstable vibration excitation. Special attention is paidto the effects of vehicle weight and running speed on the vibration loads.3. Cryo-dynamic triaxial tests of ice samples are conducted, with the aim to obtainthe required parameters for later research, as well as to carry out the correspondingdeformation-prediction and its influencing factors analysis. Ice could be well characterized by the visco-elasticity constitutive model according to the test results.4. Taking the well-calculated track-sleeper force as vibration input, a threedimensional track-subgrade coupling FEM model is initially built by using infiniteelement boundaries. Based on equivalent linear constitutive model, the obtainedtemperature field is simultaneously employed to the model to simulate thermal-stresscoupling effect, so that the freezing-thawing state of subgrade is available. Traffic loadsare shown to be well characterized by groups of multi-supported dynamic track-sleeperforces. The numerical model is further validated by the monitored acceleration anddynamic stress response at ground surface.5. Using the above well-validated numerical model, one can obtain the train-induced vibration response of the permafrost subgrade along the QTR. Based on thecomputed vertical acceleration and stress time histories, corresponding1/3octavebandwidth frequency response are also calculated. The vibration response of ice-richwarm permafrost subgrade induced by passing trains, and the effects led by differentfactors, are accordingly analyzed in both time domain and frenquency domain. Alsogiven interest is to the stress path of soil element subjected to traffic loading and itsinfluencing factors.The present thesis is aimed to provide preliminary sights into degradation tendencyof ice-rich permafrost subgrade with low temperature, and train-induced vibrationmechanism of warm permafrost subgrade with subsurface ice layer and unfrozeninterlayer. The proposed approach is complete, systematic and reliable, and is of interestin relation to the construction of permafrost subgrade along the QTR. |
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