Research And Engineering Application Of Temperature Field Development Law Of Metro Connecting Passage Constructed By Freezing Method Under Seepage

The construction method of subway connecting passage in seepage strata often adopts freezing method.The construction method includes an active freezing stage before excavation and a maintenance freezing stage during the excavation.The active freezing stage will form a stable curtain of frozen soil to isolate the hydraulic connection between the frozen area and the unfrozen soil,so as to achieve the goal of no water or sand inrush during excavation.Therefore,it is of great significance to study the temperature field during active freezing under the action of seepage and its final freezing effect.The article takes the Taiyuan subway freezing method as the engineering background.The article uses four research methods including geotechnical test,theoretical analysis,numerical calculation and field measurement to study the following contents and obtain corresponding conclusions.Geotechnical test content: This part studies the changing law of the thermal conductivity of silty clay under different moisture content conditions in both frozen and unfrozen states;This part studies the calculation method of the specific heat capacity of frozen soil to provide references for numerical calculations;This part tests the freezing temperature of silty clay,which provides a freezing temperature reference for the analysis of the freezing effect after active freezing;In this part,a soil column freezing experiment was carried out,and the law of water migration,frost heave changes and temperature changes at different heights were studied during the freezing process.Theoretical analysis content: The temperature field control differential equation and the seepage field control differential equation as well as the coupling equations of the two are established using the micro-element volume analysis method to provide a theoretical basis for the numerical calculation.Construction monitoring content: During the construction of the connecting passage between Xuefu Street Station and Changfeng Street Station of Taiyuan Metro Line 2 using freezing method,this part monitored the return loop brine temperature monitoring,temperature monitoring at different locations,pressure relief hole monitoring,and the monitoring data were studied,to determine whether the freezing result meets the purpose of strengthening the ground.Numerical calculation content: A numerical model with the same conditions as the soil column freezing test was established and calculated.The calculation results are compared with the test results,and the correctness and reliability of the flow-heat coupling theory and the numerical simulation method are verified.A number of two-dimensional flow-thermal coupling numerical models was established,and the following contents was studied.(1)The coupling influence law of seepage field and temperature field is studied,and the influence law of different seepage velocity on temperature field and the influence law of temperature field and phase change on seepage field are obtained;(2)The influence of the freezing hole spacing and the number of freezing pipe rows on the freezing effect was studied.The results of the study provided construction suggestions for the layout of freezing pipes at different positions when the subway connecting passage was constructed by the freezing method under the action of seepage,which can reduce the construction cost.A three-dimensional fluid-heat coupling numerical model was established to study the freezing effect after active freezing for 45 d under actual freezing conditions.The analysis content is as follows:(1)The numerical calculation results of the two temperature measurement points and the on-site measured temperature results are compared and analyzed,and the change trends of the two are basically the same.It is concluded that the numerical simulation results have a certain degree of reliability;(2)Two longitudinal section temperature field cloud diagrams.One section is perpendicular to the axis of the connecting channel,and the other is parallel to the axis.This part analyzed the temperature field development of the two sections of the freezing process,and then judged the development of the freezing wall;(3)This part analyzes the development of frozen soil and seepage direction and velocity in different areas after 45 days of freezing;(4)By extracting the calculation results of different cross-sections and monitoring paths,the temperature change law,ice volume content change law and seepage velocity change law of each monitoring path in the freezing process are analyzed.After active freezing,the development of frozen soil thickness in different directions of the connecting passages of each monitoring path was summarized.In summary,the geotechnical test and theoretical analysis have laid the foundation for the numerical calculation;Through the calculation of multiple numerical models,the flow-heat coupling variation law and the design method of freezing pipes at different positions during the freezing construction of the connecting channel are obtained,and the rationality of the actual freezing construction design of the connecting channel of Taiyuan Metro is verified.The results of this study provide a reference for freezing construction design for similar freezing projects.