| With the in-depth implementation of the strategy of "strong transportation",a large number of transportation infrastructures,such as Qinghai-Tibet Railway and QinghaiTibet Highway,have been built in the alpine and high-altitude permafrost areas in China.The influence of human activities and engineering has led to global warming,increased warming and increased rainfall,which in turn has led to permafrost degradation and permafrost thawing and sinking,slurrying,uneven foundation settlement and other hazards,and China’s Qinghai-Tibet Plateau region permafrost engineering is facing huge challenges.In order to cope with these challenges,special roadbed structure has been a hot area of research in China,experts and scholars have successively researched block stone roadbed can effectively reduce the temperature of permafrost and slow down the trend of permafrost degradation,block stone roadbed structure mainly uses air convection heat exchange,the natural convection of air in the roadbed to become strong convection,heat insulation in the warm season and cooling in the cold season to reduce the temperature of permafrost,the effectiveness and practicality has been confirmed in engineering practice The effectiveness and practicality have been proven in engineering practice.The warming of the climate is accompanied by the intensification of the phenomenon of wetting,after the rainfall,a small part of the rainwater adheres to the surface of the block,and the rest infiltrates into the bottom of the roadbed permafrost,resulting in an increase in water content and specific heat capacity of permafrost,and the above phenomenon has a greater impact on the convective heat transfer characteristics of the block layer,the cooling effect and the lower permafrost,and the long-term service performance of the block roadbed in the case of rising temperatures and increased rainfall needs to be urgently answered.The paper is based on indoor model tests,and different thicknesses of block and soil layers,different rainfall amounts,and different boundary models are selected for the study of block and stone roadbeds in permafrost areas.Firstly,we monitor the temperature and heat flux in the block layer,and the permafrost ground temperature,heat flux and water content data;then we classify,summarize,compare and graph the above monitored data to obtain four data maps corresponding to each time point in the model;then we analyze the current situation and changes of the block roadbed model in the permafrost area according to the four data maps at different time points.A comprehensive evaluation and analysis of the cooling effect of blockstone roadbed and permafrost temperature after rainfall under different influencing factors is carried out,so as to provide reference for structural design and selection of anti-drainage measures for blockstone roadbed in the permafrost area,and the thesis obtains the following research conclusions:(1)The temperature amplitude at each depth of the open boundary block layer has a significant abrupt change during the rainfall time,with a maximum lift of 2.13℃,and the temperature of the closed boundary block layer changes slowly after the rainfall;the summer rainfall can reduce the surface temperature of the block rubble layer,and the temperature response of the block rubble layer with smaller thickness is more significant to the rainfall;the size of natural convection in the closed block layer after the rainfall is related to the rainfall amount and the thickness of the block layer.With the increase of rainfall,the natural convection gradually decreases in the enclosed block layer with larger thickness,while the Rayleigh number gradually increases in the enclosed block layer with smaller thickness,and the natural convection is enhanced.(2)The heat carried by rain infiltration during rainfall has a warming effect on the subsoil layer,and compared with the future warming rate of 0.052℃-a-1 on the Tibetan plateau,rainfall infiltration raises the temperature of the subsoil layer and accelerates the warming rate of the subsoil layer,with a maximum warming of 1.12℃ compared with that without rainfall,while increasing the warm season amplitude temperature duration and decreasing the cold season temperature amplitude duration;the average temperature of the subsoil layer under the effect of concentrated rainfall is basically warming.The average temperature of the subsoil layer under the effect of concentrated rainfall basically showed a warming change,and the active cooling effect of natural convection of the0.6m-thick closed block layer could offset the heat input of rainfall and maintain the cooling effect on the subsoil layer.(3)The open block gravel layer can slowly cool down the lower permafrost by using its strong convection,and the cooling rate and cooling magnitude of the subsoil layer after rainfall are smaller than those of the non-rainfall condition;the natural convection is mainly in the closed block gravel layer,and the subsoil layer is warmed up under the test conditions in this paper,but the warming rate of the rainfall condition is slightly higher than that of the blank condition.The rainfall will weaken the cooling effect of the block gravel layer on the subsoil layer and accelerate the degradation of the permafrost.If the natural convection effect is weak,the permafrost will warm up slowly,while rainfall infiltration will raise the soil temperature for a short period of time,and will lead to increased permafrost warming for a long period of time.(4)During the warm season rainfall,the volumetric water content of the shallow substrate layer increased significantly,which showed that the volumetric water content first rose sharply and then decreased slowly,and the response of the soil surface to temperature lagged behind the moisture change.With the increase of rainfall,the volumetric water content and the change of soil layer showed an overall increasing trend,and mainly affected the shallow soil layer at 20 cm depth.The maximum water content of the soil layer after rainfall is increasing,and the long time high water content is not conducive to the stability of the roadbed structure,and in the warm season in the frozen state of the subgrade soil layer,precipitation is difficult to infiltration will form water accumulation,for the thickness of small block gravel layer,the subgrade soil layer melts with the increase in temperature and deepens the heat brought into the infiltration of rainfall,it is recommended that the interface between the block gravel layer and the soil base should be designed as a water barrier in the rainy areas It is recommended that water barrier design should be carried out at the interface between block gravel layer and soil base in rainy areas.(5)There is a temperature difference between the rainwater and the model interior,and the heat flux between the surface of the block gravel and the subsoil layer in each cycle during the rainfall period has a sudden change and the overall change increases with the increase of rainfall.During the cooling process,the release of latent heat from the water phase to ice increases as the volumetric water content of the soil layer rises,and the heat absorption and heat release of the subsoil layer increase with the rainfall. |
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