| As a special reinforcement technology of water-rich soft stratum,artificial ground freezing method has been widely used the construction of subway tunnel in China.During the artificial freezing period of subway tunnel construction,the frozen wall develops continuously with decreasing ground temperature,resulting in different degrees of frost heave of the ground,which will inevitably have a negative impact on the surrounding environment of the engineering,ranging from extending the construction period to threatening the safety of surrounding structures.In view of this,this paper combines with indoor test,numerical simulation,theoretical analysis and field measurement to study the three-dimensional frost heave deformation mechanism of the stratum,the evolution law of the three-dimensional freezing temperature field and displacement field during the freezing period of the subway tunnel,and the main research contents and results are as follows:(1)Using a self-designed soil-freezing indoor experiment platform,the soil-freezing experiment research was carried out considering the experiment conditions such as coldend temperature of the sample,overlying load,and water-replenishment method.The temperature distribution law,moisture distribution law,and frost heave characteristics of the sample in the freezing process were systematically analyzed,and a temperature gradient was proposed to characterize the development status of frozen soil.Furthermore,statistical regression methods were used to derive the relationship between the freezing rate,frost heave rate,frost heave ratio and temperature gradient in the soil-freezing process,reveating the intrinsic mechanism of frost heave development in saturated permafrost.Based on the analysis results of different experimental conditions,an improved Takashi model for predicting the frost heave ratio of frozen soil is established.The research results indicate that the samples occur varying degrees of water migration under different experimental conditions,and the moisture content of the samples exhibit varying degrees of water redistribution along the depth direction after freezing due to the effect of temperature gradient and internal stress.The development law of frost heave deformation of the samples under different experimental conditions are consistent,but the change rate of frost heave deformation and the time required for deformation to reach stability are different.Also,there is a certain correlation between the instantaneous freezing rate,instantaneous frost heave rate,frost heave rate,and temperature gradient in the freezing process.(2)Considering the freezing temperature of the soil,the analytical solution of the radius of the freezing front and the outer radius of the freezing region are derived based on the improved prediction model of frost heave ratio.Furthermore,considering the spatial distribution of the freezing pipes and the development process of the frozen wall,the duration prediction model of three-dimensional frost heave deformation of ground surface during tunnel freezing construction period is established based on the stochastic medium theory.Then,a numerical calculation program is compiled by Maple mathematical software,and the analysis results of engineering cases show that after considering the freezing temperature of soil and the development process of frozen wall,the difference between the theoretical prediction value and the numerical simulation value and the on-site measured value after 150 days of freezing was 2.1 mm and 3.6 mm respectively in a single tunnel,which indicates that the theoretical prediction model has a better accuracy.In addition,compared with the "single-peak" surface frost heave deformation distribution form of the single-line tunnel,the distribution form of ground surface frost heave deformation of the double-line tunnel is of "double-peak" during the freezing period.(3)Combined with the characteristics of horizontal freezing construction of tunnels,a mathematical model of the ground temperature field during the freezing construction period of tunnels is established by considering the phase change latent heat of frozen soil.Then,a finite element model of the temperature field during the freezing construction period of tunnels is established by adopting the weighted residual method,and a model of thermodynamic coupling computation applicable to the freezing construction period of tunnels is established by considering orthotropic anisotropic deformation characteristics of frozen soil.Based on the user subroutine development platform of ABAQUS,the established prediction model of frost heave ratio is embedded into the application program,and the calculation subroutine applicable to the freezing deformation is compiled,and the numerical simulation prediction method of the ground freezing during the freezing construction period of subway tunnels is finally proposed.The established numerical simulation prediction method is applied to indoor experiments and shallowburied large-section subway tunnel freezing project.By comparing the results with the indoor experiments and field measurements,the accuracy and reliability of the numerical simulation method for predicting the three-dimensional frost heave deformation of soil of subway tunnels are verified,which can better reflect the actual situation in the field.(4)Taking the freezing construction method of the diagonal connecting passage between the Agriculture and Forestry University Station and Hongtang Station of Fuzhou Metro Line 5 as the engineering background,the construction technology,thermal physical and mechanical parameters of the soils during the freezing period are systematically analyzed through indoor experiments and filed monitoring.At the same time,the established numerical simulation method and theoretical prediction model for ground frost heave are used to study the evolution law of ground temperature field and frost heave displacement field during the freezing period of the diagonal connecting passage.The results indicate that the thickness of the frozen wall formed in the connecting passage reaches the design requirements after 45 days of freezing,and the maximum frost heave deformation of ground surface obtained by numerical simulation and theoretical prediction methods are 52.3 mm and 54.1 mm,respectively.Compared with the filed monitoring results,the difference is relatively small at 2.8 mm and 4.6 mm,respectively.This further proves that the proposed numerical simulation method has high reliability.The research results of this paper provide a theoretical basis and prediction method for investigating the three-dimensional frost heave deformation law of the ground during the freezing construction of subway tunnels,and at the same time,it can provide a reference for the design and construction of the subway tunnel freezing construction engineering.Figure 121 Table 27 Reference 192... |
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