Study On The Stability Of High-speed Railway Roadbed In Deep Seasonally Frozen Region

Along with the construction and operation of high-speed railway (HSR) in seasonally frozen regions, the problem of roadbed stability is increasingly prominent. The roadbed stability and its development under freezing-thawing cycles and train load should be paid attention to, thus could provide reference for the HSR safety. With the support of science and technology R&D project (2008G006) of Railway Ministry and the National Key Technology R&D Program (2012BAG05B01), based on the research of scholars home and abroad, Harbin-Dalian HSR was taken as the study object, using in-situ monitoring, empirical method, theoretical analysis and numerical methods. The temperature distribution was analyzed, the development of the temperature field, stress field and deformation field of roadbed were investigated. The results are beneficial to understand the roadbed stability of high-speed railway in deep seasonally frozen regions and will be conductive for the improvement of design and remediation measures of HSR frost damage in seasonally frozen regions. The main achievements include the following parts:(1) The design and operation characteristics of HSR roadbed were studied. Then the monitoring indices and plan were determined. Based on the monitoring data obtained, the distribution and change rules of monitoring indices, including temperature, water content and deformation, were studied. It is indicated that the temperature gradient changes the original existence of soil water. The water was removed, and then the soil mechanical properties were changed. The stress and deformation field were also influenced.(2) Considering changes of the average temperature, temperature amplitude, phase difference and soil thermal characteristics, the temperature estimation model were built based on field monitoring data. Then the temperature distributions, i.e., the frost penetration development, transverse temperature difference, freezing conditions, were detailed. The results indicated that:the transverse temperature differences are widespread. The freezing conditions are mainly influenced by the atmosphere temperature and the roadbed above layers. Geographical location and roadbed height are the key influencing factors of roadbed temperature.(3) The empirical and theoretical determination methods of frost penetration home and abroad were analyzed. It is indicated that the methods at present are not suitable to choose the anti-freezing measures of the HSR roadbed in seasonally frozen regions. Therefore, on the basis of the temperature estimation model, the empirical formulas between the frost penetration and freezing indices, both of air and subgrade surface, were attained. The main influence factors can be considered through the parameters and correction coefficients of the formula. Thus, the computation method of the frost penetration suitable for HSR roadbed in seasonally frozen regions was put forward.(4) The temperature field boundary conditions were determined based on the temperature estimation model. The mathematical model of unsteady phase transition geothermal field was employed to analyze temperature characteristics with time and distribution along depth, but also to predict geothermal field developing trends. The results indicate that:the thermal storage during construction gradually dissipates. The deeper, the time-consuming is longer and the process is more evident. Beyond the storage influence scope, the soil temperature is relatively stable. HSR roadbed of seasonal frozen regions will eventually form relatively stable seasonal frozen layer, stable temperature and asymmetric geothermal field.(5) Combing the actual constraint conditions of the numerical model, the thermo elasticity mathematical equations of frozen roadbed stress and deformation was reduced considering ice and water phase transition, roadbed mechanical finite element model were built which achieved continuous coupled calculation of roadbed geotemperature and deformation. Then considering different frozen-heave ratios of filling material, deformation and stress distribution characteristics were researched. The results indicate that:the vertical displacement, transverse uneven deformation difference, horizontal displacement and tensile stress increase with the frozen-heave ratio and frost depth. The vertical displacements of roadbed surface may exceed HSR permissible deformation, and the cracks maybe occur in the section of roadbed shoulder and slope for the tensile failure. Further analysis indicates that the frozen-heave, thaw compression ratio changes aroused by roadbed water redistribution under certain geotemperature field are the main factors of HSR roadbed deformation stability.(6) The indirect coupling method of thermology and mechanics was employed to study the transient dynamic problems under train load. According to the actual operation conditions, the dynamic response characteristics of displacement, velocity, acceleration and stress under single and dual train load during different seasons were compared; the long-term dynamic stability was evaluated and predicted. The results indicate that:the vertical displacement, velocity, acceleration and stress are changing quickly while the train is passing. Due to the asymmetric thermal regime in winter, the dynamic responses of the east part are slightly larger than the west part, thus may aggravate the transverse deformation difference of the roadbed. Under the train load, the long-term dynamics stability indices, including the maximum elastic deformation, the maximum dynamic stress and the additional settlement, satisfy the code requirements.