Research On Vertical Freezing Effect Of Shield Tunnel At Water-rich Sand Layer Based On Temperature Field Characteristics

As one of the key processes in the construction of shield tunnels,the initiation of shield tunnels is easy to cause safety accidents.Before the start of shield construction,the water-bearing stratum around the head end must be reinforced in advance according to the design requirements to ensure that the reinforced stratum has a certain strength and sealing water.In this way,the shield machine can avoid problems such as sand gushing,water outflow and collapse when waiting for the ground to be drilled.The artificial freezing method is widely used in the end reinforcement because of its advantages of strong adaptability of the stratum,good water barrier performance and no environmental pollution.In the construction of the artificial freezing method,the analysis of the main parameters of the mechanical properties of the frozen soil,the thickness and strength of the frozen wall all depend on the distribution law of the frozen temperature field.At the same time,the distribution law of the freezing temperature field is also the main basis for guiding the freezing construction process and judging the freezing effect,and is the basis of the theoretical research on the artificial freezing method.Therefore,it is of great practical significance to study the law of temperature field evolution during the freezing method construction.The application of freezing method in Taiyuan subway excavation is still in its infancy.In actual construction,it is more based on the experience of engineering in other regions.In addition,the Taiyuan Metro Line 2 mainly crosses the stratum as a water-rich sand layer,which has strong permeability and poor self-stability.Therefore,it is necessary to conduct an in-depth study on the temperature field strengthened by the freezing method at the origin of shield.Based on the research background of the vertical freezing reinforcement project at the beginning of shield construction of Shuangta Xijie Station,this paper studies the change and development of the freezing temperature field during the active freezing period by combining field measurement and numerical simulation.The main research contents and achievements of this article are as follows:(1)This paper introduces the application of freezing method in the initialreinforcement of Taiyuan subway shield tunneling,and introduces the process flow,freezing parameter design and freezing hole layout of freezing method by investigating the field data.According to the salt water temperature changes of the outgoing and return circuits monitored on the spot and the temperature data of the measuring points of the soil,the detailed analysis of the changes in the freezing temperature field during the active freezing period is carried out.It is concluded that: The temperature changes of different temperature measuring holes are similar during the freezing process.The temperature difference between the soil and the brine temperature is large at the initial stage of freezing,and the heat exchange is violent,which causes the temperature to fall faster,and then the temperature rises briefly due to the presence of the phase change latent heat.After the release of the phase change latent heat is completed,the temperature continues to drop rapidly,maintains stability after falling to the design freezing temperature,and the freezing ends.Then use the measured data to calculate the thickness of the frozen wall and the average temperature,and use this as the basis to determine the time of the removal of the tunnel wall and the departure of the shield machine.(2)The finite element software was used to construct a three-dimensional model,and the freezing temperature field change process under typical parameters was calculated.According to the calculation results,the freezing temperature field change rule of soil and the development of the freezing wall during the active freezing period were analyzed.It is concluded that under this freezing design scheme,the time of the freezing wall intersection is 7 days,that is,the time of forming the closed freezing wall is 7 days,and after 25 days of freezing,the vertical freezing wall with thickness greater than 3m can be formed to meet the requirements of the reinforcement range.The results of the finite element calculation are compared with the temperature data on site to verify the reliability of the finite element calculation.At the same time,the spatial distribution of the temperature of each path is compared,and it is concluded that the end of the freezing pipe is the weakest part of the construction,and it should be fully considered when designing the freezing.(3)Considering the initial ground temperature,brine temperature,freezing pipe diameter,the change of the coefficient of thermal conductivity of four factors on the effect of freezing temperature field,calculated by finite elementsoftware analyzed,under the influence of single factor and multi-factor orthogonal actively the development of the freeze freezing temperature field change rule,analyzes the different freezing time on the impact of various factors on temperature field size and its significance.The results show that the higher the thermal conductivity,the larger the diameter of the freezing tube and the lower the initial ground temperature are,the faster the cooling speed of the soil layer will be.The change of brine temperature will affect the final freezing temperature,but there is no difference in the cooling effect of different brine temperature before the soil temperature drops to the initial freezing temperature.The research results can provide some guidance and reference for the design and research of freezing reinforcement engineering,and provide some references for the temperature field research of freezing method construction and the freezing design and construction of actual engineering in Taiyuan area.