Prediction Methods And Their Engineering Application Of Ground Frost Heave And Thawing Settlement In Subway Tunnel Horizontal Freezing Construction

In the period of subway tunnel construction with artificial freezing method, frost heave and thawing settlement of ground can produce serious influence on the surrounding environment, ranging from extending construction period and increasing project cost to endangering construction safety and cause significant economic losses. In the design phase of subway tunnel freezing engineering, reasonable method must be adopted to forecast possible frost heave and thawing settlement effect of ground according to the specific project, on this basis, suitable freezing implementing scheme, frost heave and thawing settlement prevention measures can determined, which can reduce the project cost and construction risk. For this reason, by means of theoretical analysis, numerical simulation and model test, prediction methods of frost heave and thawing settlement are researched in the period of subway tunnel construction with horizontal freezing method in the thesis. Aiming to practical engineering, the proposed prediction methods are used for in-depth analysis on the distribution law of frost heave and thawing settlement. Research contents and achievements are as follows:(1) Temperature field before freezing wall enclosing can be analyzed by single-pipe freezing theory, and temperature field after freezing wall enclosing can be analyzed by flat-panel freezing theory. Based on single-pipe freezing theory, flat-panel freezing theory and frost heaving rate calculation formula, considering the formation process of frozen wall, using stochastic medium theory, a duration prediction model of surface frost heave in the period of subway tunnel construction with horizontal freezing method is presented.(2) Temperature field under natural thawing conditions can be analyzed by flat-panel thawing theory, and Temperature field under mandatory thawing conditions can be analyzed by single-pipe thawing theory. Based on flat-panel thawing theory, single-pipe thawing theory and thawing settlement content calculation formula in the one-dimensional case, considering the thawing process of frozen wall, using stochastic medium theory, a duration prediction model of surface thawing settlement in the period of subway tunnel construction with horizontal freezing method is presented.(3) Considering various initial and boundary conditions such as ground temperature, surface convection and freezing and thawing phase change latent heat, etc, Mathematical model of freezing and thawing transient temperature field is presented. Defining frost heave rate and thawing settlement rate of frozen soil as instantaneous volume strain, considering orthotropic deformation characteristics of frozen soil, based on secondary development of ABAQUS finite element software, user subroutine used to calculate freezing and thawing deformation of frozen soil is compiled, and then coupled heat-stress numerical simulation prediction method of frost heave and thawing settlement is proposed in the period of subway tunnel construction with horizontal freezing method.(4) The stochastic medium theory and coupled heat-stress numerical simulation prediction methods of frost heave and thawing settlement are applied to circular tunnel full section horizontal freezing engineering and shallow large section tunnel horizontal freezing engineering, then freezing temperature field, frost heave displacement field, thawing temperature field and thawing settlement displacement field are presented, which are compared with field measured results to verify reliability of the prediction methods.(5) A large raw material model test system is firstly established, which can simulate two-lane tunnel horizontal freezing construction process. Ground temperature field, frost heave and thawing settlement variation are researched by model test in the case of two-lane tunnel sequentially freezing. Through numerical simulation analysis, the model test results are be verified. Frost heave displacement can be reduced when two-lane tunnel sequentially freezing compared with simultaneously freezing.The application of artificial freezing method will be more widely used in subway tunnel construction in the future. Achievements of the thesis not only can provide effective reference for such tunnel engineering construction, but also have great realistic engineering significance.