Thermodynamic Coupling Study Of Thermal Pipe Foundation Of Power Tower On Permafrost Foundation Of Qinghai-Tibet Plateau

The biggest problem in building a power transmission tower in the permafrost foundation is how to solve the stability problem of the power tower foundation.Under the background of global warming,the thawing layer of permafrost becomes thicker year by year,and the permafrost upper table moves downward.Because the power tower foundation is generally shallow,the freezing force of the active layer on the tower foundation during freezing in the cold season,after several or long-term freeze-thaw cycles,the interaction between the foundation soil and the tower foundation resulted in the freezing-pulling phenomenon.The transmission rod(tower)is deformed or tipped,even cause the foundation to be pulled out and destroyed.In order to accurately calculate the long-term cooling effect of thermal pipe applied to the pile foundation of transmission tower and the freezing-pulling displacement and the distribution along the depths of pipe.Based on the field test of thermal pipe foundation and ordinary pile foundation(no thermal pipe foundation)of power tower in the Wonkhu-Budongquan section of Qinghai-Tibet Plateau,the following studies are carried out in this paper:(1)The field measured data is sorted to obtain the measured temperature and the displacement of the tower foundation of thermal pipe foundation and ordinary pile foundation in 2004 and 2005.From the measured data,the following can be obtained: the average ground temperature of the thermal pipe foundation in 2005 was lower than that in 2004,and the maximum drops of the average temperature at different depths was 0.77 ?.The average ground temperature at different depths of ordinary pile foundation is approximately the same or slightly increased for two years,which indicates that the effect of the thermal pipe on the decrease of ground temperature and increase the cold reserves of the foundation is obvious.It can be seen from the displacement of pile top that both the thermal pipe foundation and ordinary pile foundation all have freezing-pulling phenomenon,but the freezing-pulling phenomenon of the ordinary pile foundation is serious.(2)Temperature numerical calculation,A three-dimensional finite element model of thermal pile foundation was established in this paper,which based on the knowledge of frozen soil heat transfer and the field test of ground temperature of power tower foundation in the Wonkhu-Budongquan section of Qinghai-Tibet Plateau.Considering the global warming,frozen soil phase change,hydration heat release of concrete and the change of thermal pipe power,the thermal pipe power and the soil temperature around the pile are calculated by iterative method.The power of thermal pipe and the temperature of pile-soil system in 50 years were calculated and analyzed.The calculation results show that the calculated values are in good agreement with the measurement and can reasonably simulate the dynamic changeof the temperature of pile-soil system in this site.Within 30 years of the whole life cycle of the thermal pipe,the power of thermal pipe changes as discontinuous wave decreasing.the thermal pile foundation can effectively increase the cold reserves of the foundation and reduce the soil temperature,it also can shorten the soil back-freezing duration by 34%.in the fifth year,the soil temperature around the pile is the lowest,the melting depth is the least,it will increase the permafrost upper table by 48 cm in the thirtieth year.In order to maintain the thermal stability of the transmission tower foundation,it is recommended that new thermal pipe be replaced or other engineering measures be carried out in the second year after the end of the life of the thermal pipe.(3)Thermo-mechanical coupling numerical calculation,considering the influence factors such as phase transformation,relative ice content,frost heave ratio and Poisson's ratio of frozen soil,the pile-soil contact element is established.Based on the calculation results of the temperature,the freezing-pulling displacement and the size and distribution along the depths of thermal pipe foundation and ordinary pile foundation in permafrost area are calculated.