| The permafrost region in Northeast China is located on the fringe of the permafrost region in Eurasia.Since the 1980s,the average annual temperature increase has increased,especially in northern greater hinggan mountains and the lesser hinggan mountains.In this background,the permafrost in the Northeast China began to degrade,and human activities such as highway construction accelerated the degradation of permafrost.In turn,the degradation of permafrost has an extremely adverse effect on the stability of the upper structure.For expressway construction,subgrade uneven settlement and slope instability often occur.In order to explore the impact of permafrost degradation on the permafrost occurrence and subgrade of high-grade highways in northeast China under the background of climate change.Relying on the widening project of Beihei Expressway that crosses the lesser hinggan mountains,the research object is the island permafrost section of beihei expressway K161 + 200-k161 + 800.From September 2009 to October 2016,we conducted long-term observation of regional geological survey,ground temperature,subgrade settlement,underground resistivity,pore water pressure and other parameter changes.Based on the meteorological data of Sunwu county national weather station from 1953 to 2015,which is 10 kilometers away from the research area,the annual average temperature,thawing index,the upper limit of permafrost under the subgrade and the thickness of permafrost in the permafrost degraded section were analyzedCombining the characteristics of subgrade settlement in permafrost,this paper proposes a calculation method for subgrade settlement in permafrost regions in Northeast China and scientifically predicts post-construction settlement.In combination with the test of unfrozen water content by DSC method,the unfrozen water content of clay in the freezing-thawing process showed different variation rules,and the calculation model of unfrozen water content of clay in the freezing-thawing process was proposed.Combined with the characteristics of permafrost degradation and the stratified summation method of soft soil subgrade settlement,the calculation method of subgrade settlement in northeast permafrost region was proposed,and the final settlement of subgrade was predicted.The results of climate data analysis show that the annual mean temperature and thawing index of the island tundra area of Bei-Hei expressway increased gradually from 1 980,while the freezing index decreased gradually,and the freezing number decreased to less than 0.5 since 1988.It is concluded that the island permafrost in the study area has entered a stage of severe degradation.The results of ground temperature monitoring show that the temperature of permafrost under the foundation of the monitoring section is higher than-1?,which is obviously affected by seasonal changes.The results of ground temperature data analysis show that the permafrost at the left slope foot of the subgrade and the central reservation generally shows the permafrost table decline and the permafrost base rise,the permafrost degradation is severe.The permafrost table at the left slope foot is lower than that at the central reservation,and the permafrost base at the left slope foot is higher than that at the central reservation.Subgrade stratified settlement analysis shows that during the subgrade widening construction(2009.07?2011.08),the settlement and deformation of the new subgrade will be affected by both the subgrade loading and permafrost thawing.More than 80%of the settlement of subgrade(total settlement at the left slope foot of K161+440 is 7.75cm,and the settlement of permafrost is 6.35cm)comes from the thawing deformation of permafrost under the foundation.Permafrost thawing will cause considerable deformation in a long time,and the deformation will not stop until the permafrost thaw completely.The settlement and deformation of expressway subgrade in permafrost area is seasonal,and the settlement is large in thawing season.The deformation in the freezing season is only the drainage consolidation of the non-frozen soil,the amount of frost heave is less than the amount of settlement,and the phenomenon of frost heave is not obvious.The results of DSC test show that the unfrozen water content of clay shows different variation rules in the freezing and thawing process,there is undercooling temperature in the freezing process of soil,the undercooling temperature is not affected by the initial water content,the soil will not completely melt at 0? in the melting process.The calculation model of the unfrozen water content during the freezing and thawing process of clay is proposed.By comparing the results of the four unfrozen water tests with different initial moisture content and the calculation results,it can be seen that the model can better reflect the changing characteristics of the unfrozen water content in clay.The results of predicting the final settlement of subgrade show that the permafrost at the left slope foot of the subgrade will completely thaw in 2021,and at the central reservation of the subgrade will completely thaw in 2017.The final settlement amount of roadbed from the construction stage,the left foot of the slope reaches 0.97 meters,the left shoulder reached 1.291 meters,the central separation zone reaches 0.977 meters,the right shoulder reaches 0.297 meters.Due to the action of pore water pressure,it will take some time for the soil to be consolidated after the complete melting.By comparing with the subsidence monitoring data in 2015 and 2016,the calculated values of the model at the left shoulder and the central separation zone are in good agreement with the settlement values.This article is of great significance for exploring the degradation process of permafrost in Northeast China and the stability of frozen soil engineering,It provides strong evidence for various aspects of highway design and construction,and at the same time,it has accumulated valuable experience for the construction of highways in the northeast cold area. |
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