Research On The Defects And Measures Of Ground Temperature Field Of Artificial Ground Freezing At Cross Passage In Metro Section Tunnel

Since the 21st century,China’s urban subway construction has entered a period of rapid development.At the same time,there are many engineering accidents in the process of subway construction due to complex and changeable strata,construction management and technology.Among them,the construction of cross passage is one of the high-risk links with frequent accidents.Although the engineering quantity of the cross passage is small,it is easy to cause water gushing,collapse and even crack and collapse of the segments of the existing shield tunnel.Therefore,artificial ground freezing,pipe jacking method,pipe shed method and cement mixing method are mostly used for stratum reinforcement in the construction of cross channel.Among them,artificial ground freezing is the most widely used because of its advantages of high strength,good sealing,high safety and reliable technology.However,serious accidents frequently occur in the actual construction process of artificial ground freezing.so it is necessary and valuable to study the causes of these accidents.Based on the artificial freezing construction of the connecting channel between Shijiawan and tawan stations of Luoyang Metro Line 1,this paper uses ANSYS software to simulate the change of artificial freezing temperature field with time,and focuses on the analysis of freezing wall thickness,intersection time and average temperature in different parts and stages.The sensitivity analysis of formation parameters,design indexes and construction process parameters affecting the freezing effect of the frozen wall of the cross passage is carried out to fully understand the causes and Countermeasures of accidents in the construction of artificial ground freezing of cross passage.Studies have shown that when the construction of the artificial ground freezing of the cross passage adopts a single-sided refrigeration pump station plan,the development and evolution of the frozen wall along the longitudinal direction of the cross passage is uneven.The thickness of the frozen wall is the largest at 1m～3m outside the shield tunnel on the opposite side of the refrigeration pump station,reaching about 2.2m.Then as the distance from the right line of the shield tunnel on the refrigeration pump station side gradually decreases,the corresponding frozen wall thickness also decreases.At the excavation of the right line tunnel segment on the refrigeration pump station side,the minimum frozen wall thickness is about 1.7 m.The position of the vault on the side of the refrigeration pump station should be used as the control section for the thickness of the frozen wall.On the cross section perpendicular to the axial direction of the connecting channel,the thickness distribution of the frozen wall also has the characteristics of unevenness.Among them,the opposite side of the refrigerating pump station,located at the side wall of the area between refrigerating pipes D5(D6)and D7(D8),is the control position of the freezing cross-section of the entire cross passage.The main reason is that the distance between the freezing pipes in this area is relatively large(0.8～0.9m).At the same time,the frozen wall is also non-uniform in the development process over time.Generally,the freezing front line advances faster in the early stage and slower in the later stage.At the same time,the frozen wall is also non-uniform in the development process over time.Generally,the freezing front line advances faster in the early stage and slower in the later stage.The sensitivity analysis of the artificial ground freezing effect of the connecting channel shows that the sensitivity of the formation freezing effect with respect to the thermal conductivity,specific heat capacity and water content of the formation is 0.68,0.45,and 0.32,respectively,and the freezing effect(closure time of frozen wall,average temperature,width of frozen wall).Therefore,the freezing effect is simultaneously affected and controlled by thermal conductivity,specific heat capacity,formation water content and other formation thermophysical characteristic parameters.Among them,the thermal conductivity has the greatest influence.The result of sensitivity analysis of formation thermophysical parameters means that the clay content and sand content in the sand and gravel stratum will have a greater impact on the closure time of frozen wall.The sensitivity of the frozen wall thickness to the freezing pipe spacing is 0.59.Therefore,during the artificial ground freezing construction,the design parameter—the freezing pipe spacing has a great influence on the freezing effect,even exceeding the influence of the thermal conductivity of the formation.Therefore,the positioning of the freezing pipes Particular attention should be paid to the control of deflection and direction.