| When building railways and highways in permafrost regions existing for a long time,the main problems encountered were the damage of frost heaving and thaw collapse.The bridge project along the Qinghai-Tibet Railway was also plagued by these two problems.It carried out thermal and mechanical research on bridge pile foundations in permafrost regions,to comprehensively grasp the frost heaving of pile foundations in permafrost regions and the frost heave characteristics and the characteristics of pile-soil interface of the frozen soil around the pile,which provided reference and basis for the operation and maintenance of bridge pile foundation in permafrost regions,so as to be conducive to the safety and long-term stable development of the Qinghai-Tibet Railway project.Based on the actual project of Qinghai-Tibet Railway,it relied on the bridge project of the wetland section in the permafrost region of Qingshui River and used the theoretical analysis method as the main research method to establish the thermo-mechanical coupling numerical model of pile-soil system by using COMSOL Multiphysics finite element software.Change situations in the temperature field of the frozen soil around the pile in the bridge project of the permafrost region during the 30-year period after measuring the ground temperature,the frost heaving deformation of the frozen soil,the related interface characteristics between the pile soil and the freezing temperature and time,and the force form of the pile foundation and the anti-freezing mechanism and other aspect were studied,and the main conclusions were as follows:(1)In the process of studying in-situ water freezing based on elastic mechanics,it was concluded that not only the temperature variable had an effect on the frost heaving coefficient,but also the Poisson ratio of the soil had an important effect on it.It combined with the ice-water phase transition rate of different soils in the frozen state,to derive the relational formula between the frost heaving rate of the soil and the frost heaving coefficient during the frost heaving process.(2)Within 2m below the surface,seasonal changes had a greater effect on the temperature of pile-soil interface.Affected by factors such as global warming and solar radiation,the absorbed heat was transferred to the frozen soil layer by the pile foundation,so that the temperature of the pile-soil interface had a rising trend with the increase of the time,And leads to the temperature of the soil around the pile was higher than the temperature of the undisturbed soil at the same depth.The effect of atmospheric temperature on soil temperature would decrease with the increase of soil depth.After reaching a certain depth,the atmospheric temperature would no longer interfere with soil temperature.And the temperature change of deep soil was lagging behind that of surface soil.(3)The frost heaving amount of the soil around the pile was smaller than the frost heaving amount of the soil away from the pile side,The difference increased from 1.74 centimeters in November to 5.47 centimeters in February.The maximum frost heaving in February appeared 5.1m away from the pile side.Compared with November,the distance from the pile side increased by 3.1m.It is indicated that with the freezing time,the pile foundation has a greater and greater effect on the frost heaving constraint of the surrounding soil and the influence range is wider and wider.The normal stress between the pile-soil interfaces at surface was the largest.With the increase of freezing time,the distribution form of normal stress between the pile-soil interfaces did not change,but the normal stress value continued to increase.Contact normal stress is mainly concentrated in the range of 1.5m below the ground.November to January is the growth phase of tangential stress between piles and soils,and its maximum value appears near the surface.With the decrease of ground temperature and the degree of frost heaving deepens,the tangential stress increases significantly,and the vertical displacement of pile foundation increases significantly.From January to February,the tangential stress between piles and soils is stable.The tangential stress increases slowly with the development of frozen depth and reaches its maximum gradually.in the process of frost heaving,the positive and negative critical points of the tangential stress value between the pile-soil interfaces gradually moved downward.(4)The relative sliding displacement between the pile-soil interfaces at surface was the largest,And the relative sliding displacement is mainly generated within a range of about 2.5 m below the ground.With the increase of freezing time,the displacement of the pile foundation changed slowly,but the frost heaving amount of the soil around the pile was increasing continuously,resulting in the relative sliding displacement between the piles and soils increased continuously.(5)Due to the load applied to the top of the pile,the axial force of the pile is the pressure at the beginning of the freezing.As the passage of freezing time,the axial force of the pile becomes tensile force and the value of tensile force increases continuously.And the position of the maximum tensile force gradually moved downward due to the deepening of the degree of frost heaving,indicating that the deeper the degree of frost heaving,the greater the effect of soil frost heaving phenomenon on the pile foundation. |
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