A Study Of Temperature Field Evolution During The Construction Of Connecting Passage In The Dong-Bin Intersection Of Nanning Subway Using Artificial Ground Freezing Method

The artificial ground freezing technology is not limited by the scope and the depth of support.Moreover,the construction area is small and it does not cause secondary pollution Therefore,this method has been used in underground engineering such as urban excavation,coal mines,tunnels,and subways.As the frozen soil is a material with extremely poor thermal stability,its physical and hydraulic properties will be greatly changed by temperature changes.And the incomplete development of frozen walls is the main cause of common engineering accidents during freezing.Therefore,an artificial ground freezing method can be used to strengthen the connecting passage,where the distribution and evolution law of the temperature field of the surrounding soil has important guiding significance for the actual project.This paper relies on the construction of the freezing method for the connecting passage in the section of Dongge Road Station-Binhu Road Station of Nanning Metro Line 3.In this study,the underlying theory and related parameters of the temperature field distribution and evolution during heat transfer were firstly introduced.Secondly,through the investigation of on-site data,the process of the freezing process,the design of the frozen station and the study of freezing hole layout were carried out.Based on the temperature monitoring data during the active freezing and maintenance freezing process and the ideal soil thermophysical parameters in the Nanning area,a three-dimensional numerical model was established using the temperature field module of the ADINA finite element software.The temperature distribution of each temperature measurement point in the simulated temperature field model was compared with the field data to verify the accuracy of the three-dimensional numerical model.Finally,the thermal field evolution law of the soil around the frozen pipe and the development of the frozen wall were discussed under the conditions of thermal conductivity,latent heat of phase change,soil mater brine cooling plan and the change of the original ground temperatureThe main conclusions are as follows:(1)In the initial stage of freezing,the temperature of the soil is cooled rapidly.The temperature of the soil layer slows down until it freezes to the lowest freezing temperature,when it is freezing to 0 C.And the trend and extent of numerical results are consistent with those of the measured data.Furthermore,the freezing and cooling process can be approximated with a quadratic curve.(2)The temperature on the characteristic surfaces meets the design requirements as it was froze to the 40th day.The vulnerable sections in the construction process are at the top and the two sides of the connecting passage,where spare freezing pipes are needed to ensure the construction quality.Moreover,the isotherm of-1? can completely enclose the connecting passage and the collecting well at the dangerous section,so constructions can be approached under this freezing effect.Under the influence of environmental factors,the freezing effect of the soil also shows that the better the thermal conductivity,and the lower the original ground temperature,then the faster the soil layer cooling rate.For instance,the thermal conductivity fluctuates in 60%,the days of the temperature of the studied soil takes another 15 days to drop to 0 ?.And the latent heat of phase change has a minimal effect on the cooling of the soil.Moreover,the original ground temperature has a significant impact on the cooling effect since the soil layer freezes to 0?,and that gradually decreases since the temperature drops down to minus.Correspondingly,the difference between the brine cooling plan will significantly affect the final freezing temperature of the soil layer.As temperature of the soil drops to 0 C,the temperature development under different saltwater cooling plans remain similar.Furthermore,the freezing effect obtained by soil materials is different,for instance,that for the sand is better than that for the clay,and the soil after cement improvement has a faster cooling rate.