Study On Temperature Fields And Frost Heave And Thaw Settlement Of Railway Culvert In Permafrost Region

The main problems are frost heave and thaw settlement duringrailway construction in permafrost area of Qinghai-Tibet plateau, culvertsare also confronted with the two problems. In this disseration, research onthe two key points and the main reasons leading to frost heave and thawsettlement, which are the effect of culverts on the existent conditions ofunderlying permafrost and the temperature field evolution feature ofnewly-built permafrost culverts, has been done. The characteristics ofunderlying ground thermal regime after constructing culvert andembankment, the features of frost heave and thaw settlement alongculvert's axis are analyzed by in-situ experiment. Finite element model isestablished to simulate different temperature fields on the basis ofhydromechanics and thermodynamics theory to understand the changelaws of ground temperature underlying culvert. In order to analyse theinfulence of seepage on ground temperature, the governing differentialequations of the coupled problem of temperature and seepage fields arefirstly derived from the theory of thermodynamics and theory of seepage, the finite element formula of this program are obtained from Galerkin'smethod to simulate culvert's temperature fields in consideration ofseepage. Based on the frost and thaw calculating model, the numericalcalculation of the deformation in culvert's ground will be realized toanalyse the laws of frost heave and thaw settlement of different beds. theresearch are as follows:1. Based on the long-term monitoring and systemic analysis of theground temperature and air temperature taken from two culverts inTuotuo-River test section along Qinghai-Tibet Railway, the properconstruction technics and seasons suit to the special environment ofQinghai-Tibet plateau, the influence of culverts to ground temperature, the freezing and thawing features of the plateau permafrost under culvert, the change laws of air temperature inside and outside culvert and thevariation of permafrost table underlying culvert are studied. Thedisplacement features of culvert are also presented by frost heave andthawing settlement monitoring at the same time, these provide available reference for analyzing the culvert's adaptability to some kind frozen soiland frost damages prevention.2. Aiming at the characteristics that the permafrost culvert'stemperature field is a non-steady temperature field with phase changingand with conjugated heat transfer on fluid thermal boundary, and basedon the assumption of incompressible, fully developed laminar tube flowin the culvert, the laminar fluid flow and heat transfer model is putforward, the governing equation of the non-steady temperature field withphase changing of the permafrost based on the traditional equation is alsoformulated, then the thermo-fluid-solid interation mathematic model ofpermafrost culvert is given out. And then the dissertation solves thenon-linear partial differential control equation group in terms of Galerkinmethod, anlyses the equation with the finite element method by adoptingthe grid segmentation in space domain and format segmentation in timedomain by the mixture solution of the finite difference.3. According to the thermo-fluid-solid interation model under thefirst bound limitation, the temperature fields of culvert at differentflowing surface water conditions are calculated with complete couplingmethod. For the temperature of culvert with a small flow in warm season, the match between measured temperatures and simulated temperatures isshown to be very good, which demonstrate the accuracy of the numericalmodel.4. By means of comparing and analyzing the yearly change processof temperature field of newly-built culvert at different flowing conditionswith the passing of time when the culvert and embankment areconstructed, the paper discusses the yearly changing laws of temperaturefield of permafrost culvert, obtains the yearly limits with steady changesand with greater changes of temperature field of culverts at differentflowing conditions, finds out the maximum thaw depth of culvert groundbase and the time when the ground thaws to the depth, and also obtainsthe influence to temperature field of the time after the culvert's beingcompleted, the variation flowing conditions with the time and theculvert's ventilation.5. On the basis of summarizing theoretical results from scholars at home and abord, a finite element model is established based on thebasical seepage theory and heat transfer theory, the model takes intoaccount the coupling influences of seepage fields and temperature fields.Meanwhile, the temperature fields with consideration of seepage fieldsare calculated with sequence coupling method, and the influence ofseepage of surface water flowing through the culvert on the temperaturefield of culvert's ground base in permafrost regions are discussed. Theprediction results show that in case of seepage effect, the seepage willdeepen the maximum thaw depth of ground and enlarge the thawing coreunder culvert, so that the stabilities of culvert will obviously decreased.6. Based on the elasticity theory, the dissertation establishes athree-dimensinoal calculation model of deformation and stress of frozensiol. And then on the basis of summarizing theoretical results fromscholars at home and abord, a mathematical model of culvert ground basecoupled with moisture, heat, stress and displacement has been set up. Thebasic laws of the interaction among the temperature, moisture and stress, and displacement and their effects on frost heave and thaw settlementhave been opened out. The frost heave and thaw settlement of culvertground are calculated with FEM method and the compare of the real testvalues and the calculation proves the preciseness of the coupling model.In the mean time, both the effect of the dynamic chang of temperaturefield on the distribution law of frost heave and thaw settlement and thereason of the non-uniform deformation along culvert's axis are analysed.