| Artificial ground freezing(AGF)has been widely used in municipal engineering due to its advantages such as strong controllability,pollution-free and wide application range.It has become a mature construction method in soft clay areas.With the increasing awareness of groundwater resources protection and the increasingly strict restrictions on the dewatering of urban foundation pit excavation,many areas with abundant coarse-grained soil layer in China have begun to use AGF for construction.However,coarse-grained soil has a large hydraulic conductivity which makes it possible for large flow velocity provided with sufficient water,thus the frozen wall would be difficult to form.Understanding the temperature field changes of coarse-grained soil layers during the process of artificial freezing and the influence of related conditions on the development of frozen walls and surrounding soil are of great significance for the application of AGF in coarse-grained soil areas.Firstly,based on the mathematical model of seepage field and temperature field coupling,the saturated sand layer freezing model test under seepage condition was taken as the calculation prototype,and the finite element software COMSOL Multiphysics for multi-field coupling was used for numerical calculation.The results of model test and numerical simulation were compared to verify the accuracy of the calculation results during the freezing process of the coarse-grained soil.In order to make up for the shortcomings of the limited test conditions,numerical simulation was then used as the main research method.A numerical model of a horseshoe-shaped subway connection channel for a coarse-grained soil layer under the hydro-thermal-mechanical coupling was established to investigate the influence of three factors,i.e.the flow velocity,the spacing freezing pipes,and the arrangement of pipes on the formation temperature,surface deformation,and stress development.The main research results are as follows:(1)By comparing the test results with the simulation results,it was found that the data from the two methods were in good agreement,which verified that COMSOL software had a high accuracy in calculating the heat flow coupling problem of coarse-grained soil layers.The ratio of the thickness of the frozen wall upstream and downstream was defined as asymmetry degree.It was found that changes in the pipe spacing and the seepage velocity could affect the value of asymmetry degree.When the freezing pipe spacing was constant,the asymmetry degree decreased with the increase of seepage velocity.When the seepage velocity was constant,the asymmetry degree decreased with the increase of the pipe spacing.(2)The numerical calculation of the horseshoe-shaped subway connection channel in the coarse-grained soil layer found that when the flow velocity was below 3 m/d,it had little influence on the frozen wall shape;when the flow velocity was greater than 6 m/d,the asymmetry of the upstream and downstream of the frozen wall became more obvious as the flow velocity increased.Decreasing the freezing pipe spacing or increasing the number of turns of pipes could help increase the thickness of the frozen wall.The frozen wall closure time had an exponential relationship with the flow velocity and the pipe spacing.(3)Analyzing the changes of the surrounding soil during the freezing process,it was found that the stress growth process could be divided into three stages,i.e.,the initial maintenance stage,the fast growth stage and the slow growth stage.The surface deformation curve was similar to the normal curve.Under the influence of flow velocity,the surface deformation and stress both reached the maximum value at the flow velocity of 6 m/d.After exceeding 6 m/d,it decreased with the increase of the flow velocity.The position of the peak of the deformation curve also gradually shifted to the direction of water flow.Decreasing pipe spacing or increasing the circle number of pipes could increase surface deformation and stress after freezing. |
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