| With the rapid development of urban construction in my country,the scale of urban rail projects under construction has ranked first in the world.When urban subway construction passes through water-rich and complex engineering geological formations,accidents of water gushing and sand bursting from the working face of subway tunnels occur from time to time.The liquid nitrogen freezing method has the advantages of fast cooling speed and high strength of frozen soil.It is one of the construction methods suitable for rapid water sealing and stratum emergency repair projects.Because there is little research on the design theory and construction technology of liqquid nitrogen freezing method at home and abroad,there is a big blindness in engineering application.For this reason,carrying out research on the temperature distribution law of liquid nitrogen freezing method has certain theoretical significance and high application value for improving the design theory of this method,guiding optimization of freezing schemes,and reasonable selection of freezing parameters.In this paper,taking the 12.4m～18.7m buried silt with sandy silt from Nantong Rail Transit Line 2 as the research object,the research method combining model test and numerical simulation is used to carry out the liquid nitrogen under different freezing conditions.Research on the formation law of temperature field of frozen soil,and obtain the following main research results:(1)Developed a liquid nitrogen freezing model test device.The device is composed of an adiabatic model box,a liquid nitrogen refrigeration system,and a temperature data acquisition system,which can realize the test and data acquisition of the formation process of the liquid nitrogen rapid freezing temperature field with different freezing pipe layout methods.It provides empirical reference for the next step of carrying out different freezing tube number,spacing,row number,and liquid nitrogen freezing model test under different freezing temperatures.(2)The temperature field gradient centered on the freezing tube shows a logarithmic function attenuation distribution;due to the superimposition of the temperature field of the double tube and the three tube freezing,the soil temperature at the intersection of the temperature field between the tube and the tube is significantly lower than the equidistant Other parts;the cooling rate of freezing in the early stage is higher than that in the later stage,the cooling rate of multi-tube freezing is higher than that of single tube freezing,and the cooling rate of the freezing tube near is higher than that in the distance.(3)The test shows that:the time for the single-tube frozen wall to reach the intersection range of the multi-tube freezing wall is 10.5h,the time for the double-tube and the three-tube freezing wall to intersect is 5.7h and 4.2h,respectively.The freezing cycle time of a single tube was shortened by 60%,indicating that the quincunx-shaped arrangement of freezing pipe holes can greatly shorten the freezing curtain cycle time.When frozen for 13 hours,compared with single-tube freezing,the area of the freezing curtain increased by about 4.9 times for three-tube freezing,and about 1.9 times for double-tube freezing compared with single-tube freezing.(4)The test shows that due to the heterogeneity of the thermophysical properties of the soil,the shape of the isotherm of the freezing temperature field in the early stage of freezing has a certain irregularity,and then with the increase of the freezing time,the shape of the frozen wall formed appears more Good symmetry.Namely:single-tube,double-tube,and three-tube freezing are round,dumbbell,and oval respectively.(5)The test shows that:the temperature of the inlet pipe,the pipe wall and the outlet pipe of the liquid nitrogen freezing has a big difference.When the freezing temperature is about-80℃,the maximum temperature difference between the inlet pipe and the outlet pipe is about 20℃,but the temperature change trend of the three pipes is basically the same during the freezing process.(6)Numerical analysis shows that when increasing the liquid nitrogen flow rate(decreasing the freezing temperature),shortening the freezing pipe spacing,and increasing the number of freezing pipes,the effects of freezing curtain area,intersecting circle time,and final freezing temperature will be improved.When the distance between the freezing pipes is increased,the area of the freezing curtain increases within a certain range,but it does not always increase with the increase of the freezing distance.Figure[62]Table[4]Reference[79]... |
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