| The properties of frozen soil are closely related to temperature,and the change of temperature is bound to lead to the change of the state of water in the soil.Water in the frozen layer is a special hydrological phenomenon in the permafrost area,and it will undergo phase transition with the change of air temperature and surface temperature,which will lead to seasonal freeze-thaw of soil rocks in the active layer,and have a non-ignorable impact on the subgrade in the permafrost area.With the increase of subgrade operation time in permafrost area,hydrothermal erosion on the frozen layer will accelerate the degradation of permafrost,increase the melting area,increase the degree of highway disease and reduce the service time.Based on this,this paper constructs a two-dimensional hydrothermal coupling model from the subgrade temperature field in permafrost area,and explores the temperature field of subgrade in permafrost area under the condition of water migration.This paper analyzes and predicts the changes of subgrade temperature field in the next 50 years,studies the influence of different factors on subgrade temperature field,and combines with the actual investigation of subgrade retaining and drainage structure in permafrost area,so as to provide a reference scheme for highway subgrade construction in permafrost area.The conclusions are as follows:(1)With the heat absorption of pavement asphalt,the seepage of groundwater in permafrost layer has a great influence on the ground temperature under the subgrade.After 5 years,10 years,20 years and 50 years,the depth of the affected areas will be 9m,14m,16m and 20m respectively.The influence of seepage on surface temperature from large to small is the center line of subgrade,the foot of positive slope,the foot of negative slope and the natural ground.After 20 years of operation,the thickness increase rate of active layer with seepage at the foot of shady slope and sunny slope of subgrade is 0.25m/a and 0.28m/a respectively.After 50 years of subgrade operation,the ground temperature at a depth of 5m at the embankment center line is 3.3℃ when the water on the frozen layer percolates,which is 0.2℃ higher than that without percolation under the same conditions.Seepage intensifies the melting of permafrost and increases the thickness of permafrost active layer.(2)The influence of seepage on temperature in melting period is more obvious than that in freezing period.Under the condition of global warming,the downward rate of the upper limit of artificial frozen soil is 0.2m/a without considering seepage.The infiltration of water on the frozen layer of permafrost increases the downward rate of the upper limit of permafrost,and the downward rate of the upper limit of permafrost under the subgrade is 0.38m/a.During the freezing process of the active layer,the melting area of the section under the subgrade with percolation is 2m2 smaller than that under the subgrade without percolation,and the melting area of the section under the subgrade with percolation is 5m2 larger than that under the subgrade without percolation.(3)Four parameters,namely subgrade height,pavement width,water level and pavement slope,which can affect the temperature field of permafrost subgrade,are tested and calculated respectively.It is calculated that subgrade height has more influence on the temperature field of Qinghai-Tibet expressway subgrade than other engineering factors.Compared with narrow subgrade,wide subgrade has stronger heat-absorbing characteristics,and the upper permafrost limit,the lower permafrost limit decline rate and the warming range of permafrost ground temperature under wide subgrade are larger than those under narrow subgrade.Meanwhile,the separated subgrade can effectively weaken the heat accumulation effect caused by wide subgrade.The maximum melting depth of subgrade temperature field will produce lateral displacement with the variation of subgrade height,subgrade width,slope and water level.When the height of permafrost subgrade is 1.5m,the calculated lateral displacement velocity of the maximum melting depth in the first three years of operation is 4.02m/a,which is greater than the lateral displacement velocity of 1.01m/a when the height of permafrost subgrade is 4.5m.The wider the subgrade is,the larger the lateral displacement of the maximum melting depth is.When the subgrade width is 15.0m,22.5m and 30.0m,the lateral displacement rate of the maximum melting depth in 20 years is 0.5m/a,0.68m/a and 0.8m/a,respectively.The slower the slope is,the later the lateral displacement of the maximum melting depth occurs and the later the displacement reaches stability.The deeper the groundwater level is,the smaller the lateral displacement of the maximum melting depth of highway subgrade is.(4)The engineering site in Qinghai-Tibet region is relatively complicated.There are certain differences in the solar radiation received by subgrade,pavement and Yin and Yang slope,resulting in different upper boundary temperatures and inconsistent melting settlement.The settlement amount of highway subgrade varies with the direction of road and bridge.The results show that the settlement on the negative slope is the largest,and the total settlement is about 2800cm,while the minimum settlement on the positive slope is about 1700cm.The difference of settlement on the north and south sides of the north-south trend is small.To solve the problem of poor subgrade drainage in permafrost area is the key to reduce subgrade diseases.Subgrade drainage design in permafrost area is divided into water retention and drainage,and water retention is mainly divided into two parts,one is mainly for the road pavement and the water at the foot of the subgrade slope after rainfall,and the other is from the vertical runoff formed by melting snow on the top of the mountain and the upstream runoff along the road.The drainage mainly depends on the ditches on both sides of the roadbed,and prefabricated trapezoid and rectangular side ditches are selected respectively.According to the flow velocity and flow size of the runoff,rectangular drainage channels are used in the ditches near the mountain,and trapezoid channels are used in the ditches with large flow on the other side.With dredging and drainage as the main,water retaining as the auxiliary,retaining and drainage combined with the final formation of a complete hydraulic building system. |
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