| Under the influence of global warming and man-made engineering construction disturbance,the permafrost thickness decreases and the upper limit of permafrost gradually moves down,which further aggravates the subgrade melt-settling deformation and reduces the stability and reliability of subgrade.At present,the influence of initial ice water content on subgrade is not considered in most numerical simulations of common subgrade in permafrost.The numerical simulation of permafrost block roadbed mainly simulates the influence of natural air convection on block roadbed by changing some physical parameters of roadbed.In order to solve the above problems,based on the Qinghai-Tibet Plateau,this paper established a water-heat-force coupling model of subgrade considering water migration and phase transition as well as initial ice and water content.In addition,the volume force in Brinkman equation was used to define the air buoyancy force and simulate the natural air convection within the block stone layer.Research has been carried out on temperature,moisture,deformation characteristics and long-term melt and settlement deformation prediction of subgrade with different structures in permafrost area.The main research results include the following aspects:(1)Taking common subgrade in Maduo test section of Gonghe Yushu Expressway on Qinghai-Tibet Plateau as the research object,a finite element numerical analysis model of water-heat-force coupling was established for common subgrade,and the temperature field,water field and deformation field of permafrost subgrade in 15 years and under different working conditions were simulated and analyzed.The results show that the shallow ground temperature of permafrost subgrade changes periodically with the atmospheric temperature and presents obvious asymmetry.In cold season,there is still high temperature melting core in subgrade soil,which affects the stability of subgrade.The maximum freeze-heave position of permafrost subgrade is in the left shoulder,and the maximum thawing settlement position is in the middle of subgrade,and the thawing settlement deformation is much larger than that of freeze-heave deformation.The downward velocity of permafrost upper limit and the velocity of thawing deformation decrease gradually with time,and the subgrade presents a stable trend.The reasonable height of the subgrade in this area should be controlled within 3-4m,the initial mass moisture content of the subgrade should be around 10%,and the slope slope can be considered according to the actual working conditions.(2)Taking the block stone roadbed in Maduo test section of Gonghe--Yushu Expressway as the research object,a three-field coupling model considering the natural convection of air in block stone layer was established by defining the uplift force in the non-isothermal seepage model in porous media.The temperature field,water field and deformation field of the block stone roadbed in permafrost in 15 years and under different working conditions were simulated and analyzed.The results show that compared with ordinary subgrade,the block subgrade can reduce the ground temperature,the content of unfrozen water in the lower part of soil,and the deformation of frost heave and thaw settlement.Increasing the thickness of the block stone layer can reduce the soil temperature and the subgrade melting deformation,but the excessive thickness of the block stone layer will cause waste.Through analysis,the reasonable thickness of the subgrade block stone layer in this area is determined to be 1.6m.Large or small particle size of the block is not conducive to reducing the soil temperature.The grain size of the roadbed block in this area should be controlled at 6-8cm.(3)Cellular automata and integral method are used to improve the background value of grey Verhulst model,and the improved model is used to predict the subgrade melt subsidence deformation.The results show that the improved model using cellular automata theory is superior to the improved model using integral method,and the improved model combining cellular automata theory and integral method has the highest accuracy.All the three models can accurately predict the deformation of subgrade with different structures,which is conducive to timely discovery of deformation causes and timely solution of problems,and has important engineering significance.Figure [39] table [23] reference [73]... |
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