Experimental Study On Freezing Temperature Of Loess Polluted By Landfill Leachate Under Freeze-thaw Cycles

With the rapid development of economy and society in loess area,human activities and engineering construction make the special geotechnical engineering face new problems and challenges under the environment and climate change.At present,strict anti-seepage measures are taken in our country,but we know from the practical situation that landfill leachate still produces leakage of landfill and produces great hidden dangers.Considering the effect of landfill leachate displacement of original pore water and surface sewage in loess lining and overburden,pore gas,liquid and pollutant migration were changed by infiltration,the service performance and long-term deformation of landfill overburden in loess region were directly affected under freeze-thaw cycle.Soil freezing temperature is an important factor to determine whether soil layer is in frozen state.It affects the content of unfrozen water in soil and the development process of frozen edge.Therefore,the freezing temperature is very important to determine the phase transition process of soil mass,the development of frozen edge and the determination of frozen state.In this paper,the loess contaminated by landfill leachate was taken as the research object,and the mechanism was analyzed based on freezing temperature test and resistivity test.The results showed that:（1）According to the soil freezing temperature test,the soil freezing temperature increases with the increase of water content,but the freezing temperature is always lower than 0℃.The freezing temperature of soil samples varies greatly at low water content.At high water content,the freezing temperature of soil samples is similar to that of pure water with a small change range.Meanwhile,the duration of soil freezing platform increases with the increase of water content.The freezing temperature of soil decreases linearly with the increase of landfill leachate concentration.At the same time,the duration of soil freezing platform gradually increases and then decreases with the concentration of landfill leachate,and the decrease degree is obvious when the concentration is high.Under the action of freeze-thaw,the spatial structure of soil particles in the soil changes correspondingly,which leads to the soil freezing temperature and the soil freezing platform duration,which decreases first and then becomes stable with the increase of the number of freeze-thaw cycles.When N>10 times,the soil mass reaches a stable state and the change of both is not significant.The main decreasing range of soil freezing temperature and frozen platform duration is 2<N<10,and the most drastic change degree is in this range.The decreasing value can account for 80%of the total decreasing value.With the increase of the number of freeze-thaw cycles,the freezing front moves periodically and drives away the pore fluid in the soil body,thus reducing the freezing temperature of the soil.The total difference increases with the increase of landfill leachate concentration.（2）According to the resistivity test,the resistivity of soil decreases first and then becomes stable with the increase of voltage,and the voltage of 30V is the optimal voltage in this test.Contact resistance error correction,measured contact resistance R₀=3.24Ω.The resistivity of soil gradually increases and then decreases with the moisture content.The degree of change is significant at low moisture content,and the decreasing trend of resistivity decreases with the gradual increase of moisture content,showing a power function trend.The resistivity of soil with w=12%～24%decreases by about 50%.As the concentration of landfill leachate increases gradually,the soil resistivity decreases,showing a power function decline.There is a great difference between the samples contaminated by leachate and the samples not polluted by leachate,which can reach50%～60%.The resistivity of soil decreases significantly with increasing temperature,and the change range of soil resistivity under negative temperature condition is more obvious than that under positive temperature condition,showing a nearly linear correlation.In the process of soil resistivity increasing with temperature change,there is a significant change degree at a single node in the process of gentle growth of soil resistivity.The node with a greater change degree is the soil freezing temperature,which is consistent with the results of freezing temperature test.（3）Based on the test data,the empirical formula among soil freezing temperature,soil freezing platform duration,soil resistivity,landfill leachate concentration,water content,number of freeze-thaw cycles,and temperature is established.COMSOL Multiphysics was used to analyze the coupling of temperature field and water field in soil.It was found that the freezing front had a leading effect on the migration of pore liquid in soil,and the water content at the freezing front changed most obviously.The freezing depth of soil gradually increases and then decreases with the water content.The depth changes significantly at low water content,and the rate slows down at high water content.As the freezing temperature decreases gradually,the soil freezing depth increases gradually,and the soil freezing depth varies from 0.272m to 0.218m（the difference of freezing depth is 0.054m）.Meanwhile,when compared with the isothermal line of 0℃,it is found that there is a huge deviation（0.091～0.045m）.As the moisture content and the soil freezing temperature decrease,The line difference between freezing depth and 0℃increases gradually.