Research On Creep Deformation Of Permafrost Block Stone Roadbed

Since the last century,with the development of economy,China has carried out large-scale projects in permafrost regions such as Daxinganling and Qinghai-Tibet Plateau.Due to the global climate change and large-scale cold-zone project construction,the original surface energy balance in permafrost regions is broken,resulting in an increase in ground temperature,an annual increase in the limit of frozen soil,and an increase in the thickness of hightemperature frozen soil layers.In the past decades,the engineering problems caused by submerged permafrost subsidence have been the focus of engineering studies in permafrost area in China.However,the thaw settlement is not the only reason for foundation settlement in cold regions.Studies have shown that the Qinghai-Tibet Railway subgrade creep deformation of high temperature frozen soil layer is also one of the major side effects.Therefore,studying the long-term creep deformation of the Qinghai-Tibet railway subgrade has important practical significance for the construction and maintenance of the cold area project and disaster prevention.The previous researchers have used the empirical methods for the settlement analysis of the engineering foundation in the cold area,such as the engineering creep theory.This method is simple and straightforward,and it is widely used in practical projects;however,its prediction accuracy is affected by many factors such as ice content,dry density,and temperature of soil.It is not suitable for practical projects and it is difficult to show the complex hydrothermal migration process involved in settlement.Numerical analysis methods provide new ideas for solving this problem.In order to better study the creep deformation of subgrade,this paper takes the study object of the block-stone subgrade,which is the most widely used active cooling methods for the Qinghai-Tibet Railway in permafrost regions.The main research contents include the following aspects:(1)The cross-section forms of several block-stone roadbeds were analyzed and summarized,and the cooling mechanism of block-stone layers were also summarized.the porous medium model and block stone model commonly used by predecessors were analyzed.They all have too many parameters and are involved in multi-field coupling numerical simulation.Therefore,this paper uses the transformation equivalent thermal conductivity method to comprehensively consider the heat transfer properties of the block rock ventilation area,making it more suitable for numerical simulation.(2)The typical road sections in the permafrost region of the Qinghai-Tibet Railway are selected to compare the natural ground temperature,the natural upper limit change of the original frozen soil,and the lower soil temperature between the center of the ordinary roadbed and the block-stone roadbed and the left and right shoulders.Compared to ordinary roadbed,we analyze block-stone roadbed’s cooling effect on the foundation.The temperature field in the permafrost block is calculated by the finite difference software FLAC 3D.The temperature conditions of subgrade after one year,two years and four years after operation were simulated respectively.The numerical simulation results agreed with the field ground temperature laws.After that,the temperature field predictions of 10 years,20 years and 30 years are provided and the analysis of temperature field changes is performed.(3)The creep mechanism of permafrost and the creep characteristics of permafrost in actual engineering were analyzed.Combined with the identification method of general soil mechanics creep creep model,the combination of elastic,plastic and viscous mechanical components was used,and temperature and time effects were considered.All of these are used to calculate the creep deformation process of block-stone roadbed.In other words,thermal analysis is first carried out before mechanical analysis.The creep model was embedded in the FLAC 3D numerical software to simulate the deformation of the subgrade after five years of operation.The numerical simulation results agree well with the field deformation laws.After that,the temperature field predictions of 10 years,20 years and 30 years are performed.Their creep situations are also analyzed.