Study On The Change Law Of Permafrost Table Under The Hydrothermal-salt Coupling

With global warming,the permafrost table is declining year by year,and causing a series of engineering construction problems such as unstable foundation of buildings,cracking of houses and sinking of roads,etc.Therefore,it is of great significance to determine the location of permafrost table and its variation law for the engineering design and construction of permafrost region.At present,the research on permafrost table is mostly carried out by insitu borehole measurement.However,this method is not only difficult to drill,but also has a long measurement period and the upper limit data obtained are locality and contingency.It is not conducive to the study of permafrost table.In view of the above problems,this paper proposes to determine the position of permafrost table by the change of temperature,moisture and salt content during the freezing-thawing cycle of the soil.According to the field conditions in Hulunbuir region,a permafrost model of "permafrost layer-seasonal thawing layer" was established based on similar theory,taking into account the transient temperature change and permafrost layer on seasonal thawing layer.Accelerated study on long-term variation of permafrost under natural conditions was carried out by self-designed numerical simulation and indoor test,the validity of the determination method of permafrost table was verified,the variation rule of permafrost table under the coupling effect of single factor and hydrothermal salt was obtained,and the influence degree of different factors on permafrost table under the coupling effect was determined.The main results and conclusions were as follows:(1)The changes of temperature,moisture content and salt content in frozen soil during freezing-thawing were analyzed.The results show that the temperature at the permafrost table was 0?,and the moisture and salt content were the largest.According to the transfer equations of temperature field,moisture field and salt field,the contents of soil temperature,moisture and salt at different heights at different times were obtained,and the mathematical method for determining the position of permafrost table under the coupled action of hydrothermal-salt was obtained by connecting the transfer equations of temperature,moisture and salt.(2)Based on the field conditions in Hulunbuir,a permafrost model under the coupling condition of hydrothermal-salt was established.The change of permafrost table were studied by numerical simulation and indoor test,and the value of permafrost table obtained by numerical simulation and indoor test using different judgment indexes were compared with the measured value of field measurement under the same conditions,which verifies the correctness of the determination method of permafrost table position.(3)Through numerical simulation and indoor test,the variation law of permafrost table with temperature,moisture and salinity under single factor change condition were obtained: The permafrost table increases gradually with the increase of annual average temperature,and the increase rate gradually increased,showing an exponential curve;With the increase of initial moisture content,the value of permafrost table gradually increases,and the increase rate gradually decreases,showing a quadratic function curve;With the increase of initial salinity,the value of permafrost table increases gradually,and the rate increases first and then decreases,showing a power function curve.(4)Through numerical simulation and indoor test,the variation law of permafrost table under hydrothermal-salt coupling and the influence degree of different factors on permafrost table were obtained.With the increase of freezing-thawing cycles,the value of permafrost table increases gradually,and the acceleration first increases and then decreases,with the increase of temperature,the time to reach the maximum increase rate is advanced.With the increase of annual average temperature,the value of permafrost table increases gradually.With the increase of the initial moisture content,the value of permafrost table increases gradually.When the moisture content exceeds 30%,the value of permafrost table tends to be stable.With the increase of the initial salt content,the value of permafrost table increases gradually.When the salt content is between 1.0%-1.5%,the value of permafrost table has no obvious change.The influence degree of different factors on permafrost table under coupling condition was as follows: temperature > moisture > salt.Figure [53] table [29] reference[66]...