| Longitudinal cracks are commonly generated in permafrost region subgrade(PRS),which are wide spread,large-scale and hazardous.PRS is always traversing the ice-rich and high-temperature areas,so the soil is very sensitive to freeze and thaw actions.For the impacts of the seasonal variation,sunny-shady slope effect,rainfall and loading conditions,cracks in the frozen soil are well-developed,which makes the problems about longitudinal cracks in PRS complicated.Therefore,to investigate the formation mechanism and the evolution law of the longitudinal cracks in PRS is significant for the secular stability of the Tibet Plateau and the prevention and reduction of natural disasters.With the background of longitudinal cracks in PRS in Tibet Plateau,mechanism of longitudinal cracks are built in this paper through experiments,theoretical analysis and numerical simulation.Based on freezing-thawing cyclic tests,digital image processing technology and CT scanning technology,through analyzing the evolution process of freeze-thawing growth in permafrost region of the Qinghai-Tibet Plateau,this research builds a conceptual model of the formation mechanism of freezing and thawing process cold raw cracks.A flexible sensor for one-dimensional tensile deformation test of permafrost roadbed and a model test system for simulating climate and environment are established.Through the model test,the dynamic evolution rules of subgrade potential crack development under different environmental factors.Based on the extended finite element method,the fracturing expansion model of permafrost roadbed is established,and the variation law of longitudinal cracks in permafrost subgrade is analyzed.The main research achievements and progresses are listed below:(1)Based on the digital image processing,CT scanning technique,the cryostructure mechanism of the Tibet Plateau under freeze-thaw actions are investigated,the cryostructure morphology,water migration and segregated ice evolution are systematically analyzed,the crack formation mechanism of the floury soil in Tibet Plateau is unraveled,and a conceptual model is built.(2)A flexible film sensor suitable for one-dimensional tensile deformation in frozen subgrade is developed.A physical modeling system for measuring space temperature field and crack propagation region is designed and made.Space temperature field and one-dimensional tensile deformation measurement of the ordinary subgrade,ventilation subgrade and gravel subgrade under seasonal cycle fluctuation,sunny-shady slope effect,rain fall and slope toe ponding.(3)By analyzing the environmental simulation test results,the temperature field distribution and crack propagation region have been unraveled,and the impacts of the seasonal cycles,sunny-shady slope effect,rainfall and slope toe ponding on the potential crack propagation region and the propagation morphology of the crack region are studied.(4)A temperature field calculation model of the sunny-shady slope in permafrost region is built,and the temperature field evolution law is unraveled by analyzing the temperature change in the sunny-shady slope.The relationship between the frozen up limit and the depth is analyzed,and the results show that the numerical simulation results is accordant with laboratory experiment results.(5)Based on the extended finite element method in subgrade engineering,the calculation model of the crack propagation law in PRS is established,the relationship between the crack propagation depth in frozen subgrade and sunny-shady slope effect,subgrade soil properties,subgrade height and pavement load are calculated.The results show that: sunny-shady slope effect,subgrade height,filling thaw settlement coefficient and pavement load all have important effect on the crack propagation.The relationship between the subgrade crack depth and the elastic modulus of the subgrade is exponential decay.The relationship between the subgrade crack depth and the subgrade filling height is logarithmic growth.The settlement caused by pavement load can be forecasting parameters of crack propagation depth. |
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