| As a brand-new construction method,the Freeze-Sealing Pipe Roof method is first applied to the Gongbei Tunnel project.It combines the technical advantages of the Pipe Roof method and Artificial Ground Freezing method,adopts large-diameter steel pipe closely arranged to form pipe roof as bearing structure,and carries out artificial freezing between pipes to form the frozen soil curtain as water-stop structure.Because the project has the characteristics of large section,shallow burial depth,complicated hydrological and engineering geological conditions,the construction is difficult,especially the freezing water stop between pipes and its impact on the surrounding environment has become the key problems of construction control,and the method has a new freezing concept,and there are still some practical problems to be solved and improved in the construction process.Therefore,based on the underground excavation section of Gongbei Tunnel,this paper studies the formation law of temperature field and mechanical characteristics of tunnel freeze-sealing pipe roof method.The theoretical analyses,model tests,and numerical simulations are combined to systematically study the theoretical solution of the steady-state temperature field of FSPR,the formation law of frozen soil curtain in the active freezing period of the tunnel,the variation characteristics of frost heaving displacement of stratum,the mechanical properties of "pipe-frozen soil" composite structure and the improved design of the profiled freezing tube.Based on the modification of the existing analytical solution of the steady-state temperature field of freezing,the calculation formula of the analytical solution of the steady-state temperature field in the form of the Freeze-Sealing Pipe Roof is deduced and verified by utilizing the conformal transformation method and the separation variable method,and the differences of the temperature field calculation results under different freezing parameters are discussed.The results show that the influence of different soil freezing temperatures on the calculation of frozen wall thickness should be considered in the calculation of freezing steady-state temperature field,and the theoretical formula is accurate enough to meet the needs of temperature calculation at different positions in the model.In practical engineering,the position of the middle line between the adjacent concrete pipe and hollow pipe is a critical area of "freezing water between pipes" in the Freeze-Sealing Pipe Roof method.Through theoretical calculation,the temperature range of this area within the size range of pipe is low and evenly distributed,indicating that a reliably frozen soil curtain can be formed between pipes to ensure the effect and safety of"freezing water between pipes".Taking tunnel engineering-geological conditions and freezing design scheme as the background,based on the mathematical model theory of freezing temperature field and frost heave displacement field,a quasi-coupled two-dimensional finite element calculation model of freezing and frost heave of the Freeze-Sealing Pipe Roof method is established.Combined with the field measured data,the formation law of frozen soil curtain and the change of ground frost heaving displacement during the active freezing period are analyzed.The calculation results show that the formation process of frozen soil curtain and the distribution characteristics of the freezing temperature field are closely related to the layout and the opening sequence of two types of freezing tubes in hollow and concrete pipes.When frozen to 90 days,the average thickness of frozen soil curtain ranges from 2.32 m to 2.58 m.The axial surface temperature between adjacent jacking pipes is low and the interface temperature changes evenly,which meets the requirements of freezing design.The calculation results of the frost heaving displacement field show that the distribution and variation law of ground frost heaving displacement are closely related to the formation process of frozen soil curtain and engineering geological conditions.The extreme values of vertical displacement and horizontal displacement of surface frost heaving at 90 d were 155.67 mm and 59.63 mm respectively.The numerical results are in good fit with the measured data,which verifies the accuracy of the numerical model and further discusses the frost heave displacement distribution characteristics of strata and surface.The simplified "pipe+frozen soil" composite structure beam model is selected from the engineering prototype as the research object,and the scaled model test is carried out.At the same time,the quasi-coupled three-dimensional contact finite element calculation model is established to analyze the mechanical properties of the composite structure.The results show that the hollow and the concrete pipe have different deformation and stress characteristics due to the stiffness difference and the uneven temperature distribution of frozen soil.In the freezing stage,the frost heaving force generated by soil freezing has the greatest impact on the horizontal force and deformation of the pipe,and the deformation of the hollow pipe is large.In the loading stage,the load mainly affects the vertical deformation and stress of pipe and frozen soil.The stress of concrete pipe is large and the deformation is small,while that of hollow pipe is opposite.The hollow pipe shows nonlinear deformation characteristics.when the load reaches 0.16 MPa,part of the upper and middle span of it has been "squashed".When the load increases to 0.28 MPa,the frozen soil between pipes is first destroyed due to insufficient strength,which leads to the failure of water seal of the structure.Compared with the actual working conditions,the composite structure can meet the bearing capacity requirements.Aiming at the practical problems in the construction,the improved design scheme of the profiled freezing tube in the hollow pipe is proposed,and three pipe combinations are designed.The improved design is verified by a scaled model test and numerical simulation,and the distribution characteristics and differences of freezing temperature field under different pipe combinations are studied.The results show that the installation method of cement mortar wrapped the double circular freezing tube in the hollow pipe can not only avoid a large number of welding operations and solve related problems,but also significantly increase the horizontal and vertical direction of the hollow pipe and the average thickness of the frozen wall between the two pipes within the same freezing time.The improved horizontal pipe roof combination shows a better freezing effect owing to the "group pipe effect",and the composite structure of "pipe roof+frozen soil curtain"with better bearing capacity and water sealing performance can be formed in a short time.In addition,the development law of the frozen wall between the pipes and the function of the limiting tube are further discussed.Figure 99 Table 34 Reference 181... |
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